fm notes

of this chapter to provide sufficient technical informa-tion for the broadcaster to achieve this.

The height of the antenna over the service area,distances to areas of population, the ERP and the eco-nomics are items that must be considered.

Antennas currently available in the United Statesdiffer considerably from those to be found in Europe.The various American types are discussed so that theengineer will be informed on the subject. Considerableadvances have been made in recent years in the designand fabrication of FM antennas. These improvementsprovide greater penetration of signals into automobileFM radios as well as popular small FM transistor radiosof all kinds.3 The newer FM broadcasting antennasmust meet the more stringent requirements for FMstereo and subcarrier broadcasting. Most FM stationsin the United States are using CP antennas.

PROPAGATION

FM broadcasting has some distinct advantages overAM (medium wave) broadcast service. These advan-tages stem from the propagation characteristics of FMfrequencies as well as the modulation system.

There is essentially no difference between day andnight FM propagation conditions. FM stations haverelatively uniform day and night service areas. FMpropagation loss includes everything that can happento the energy radiated from the transmitting antennaduring its journey to the receiving antennas. It includesthe free space path attenuation of the wave and suchfactors as refraction, reflection, depolarization, diffrac-tion, absorption, scattering, Fresnel zone clearances,grazing, and Brewster angle problems.

Propagation is dependent upon all these propertiesout to approximately 40 miles (65 km). Some addi-tional factors enter the picture at greater distances.Radio wave propagation is further complicated be-cause some of these propagation variables are func-tions of frequency, polarization, or both, and manyhave location and time variations.

The technical intent of the broadcaster is to puta signal into FM receivers of sufficient strength toovercome noise and to provide at least 20 dB carrier-to-noise ratio, which will provide at least 30 dB ofstereo separation. The required RF signal level variesfrom about 2 mV/m (microvolt per meter) for highsensitivity FM stereo tuners in the suburbs to about500 mV/m for less sensitive transistorized portables.

4.12FM BROADCAST ANTENNAS

PETER K. ONNIGIAN, P.E.ERIC DYE

INTRODUCTION

This chapter is for broadcast engineers, techniciansand station managers who must make importantdecisions regarding FM transmitting antennas. Toensure the best possible signal strength in the stationsservice areas, the site location, antenna height, an-tenna type and propagation conditions must all beconsidered.

FM broadcasting was first authorized in the UnitedStates in 1940 by the Federal Communications Com-mission (FCC). The first FM station began operationin 1941. In 1945, the FM service was assigned to the88 to 108 MHz band and divided into 100 channels,each 200 kHz wide. In 1991, there were over 5,400commercial and 1,700 educational FM stations.

Most FM antennas are nonsymmetrical, that is theyare mounted on one side of a steel supporting toweror pole. FM antennas outside the western hemisphereon the other hand are usually symmetrical, that isinstalled on all faces of a tower. However, both meth-ods are capable of providing excellent omnidirectionalazimuth patterns.

Antennas for FM broadcasting use horizontal polar-ization (H-pol), vertical polarization (V-pol), or circu-lar polarization (CP).1 Cross polarization is used asa means to prevent co-channel interference in someEuropean countries but not in the western hemisphere.CP, together with its special form, elliptical polariza-tion (E-pol), was introduced in the United States inthe early 1960s as a means to provide greater signalpenetration into the many different forms of FM receiv-ing antennas, which are now found in the service area.H-pol is the standard in the United States, CP orE-pol may be used if desired. V-pol only is permittedfor noncommercial FM stations seeking to limit inter-ference to TV channel 6.

Between 1980 and 1990 there were over 300 millionradios sold in the United States. Over 140 million wereautomobile AM/FM radios.2 FM radio receivers use avariety of antennas including extendable monopoles(whips), dipoles and capacitive coupling to power linesand headphone leads.

Antennas for FM broadcasting must be chosen care-fully in order to cover the service areas properly withadequate level and quality signals. For economic andtechnical reasons, the desired effective radiated power(ERP) should be produced with a balance betweenantenna gain and transmitter power. It is the purpose

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National Assoc. of Broadcasters (NJ) (PS8295) PKF 01-06-99 09:34:51 CH4x12 Page 739

SECTION 4: RADIO TRANSMISSION FACILITIES

Automobile receivers have wide ranged sensitivityvalues.

FM antenna manufacturers do not guarantee cover-age. They supply antennas which meet certain radiationpattern requirements and gain. Many of the antennasassume an omnidirectional pattern. Although reason-able in free space, it is never fully achieved in practicedue to sources of distortion (support structure, feedlines, etc.)

Some manufacturers in the United States provideazimuth pattern adjustment service to insure a hori-zontal plane pattern circularity of 63 dB whenmounted on the side of a specific tower or pole. Itmust be pointed out that this radiation pattern andgain are for free-space conditions and may not relatedirectly to signal strengths measured at or near groundlevel, well away from the antenna.

Radiation pattern and propagation are two distinctlyseparate conditions. The pattern is the radiation whichis transmitted by a given antenna in any given direc-tion, without any propagation limitations, as measuredon a good antenna test range. Propagation depends onpath and environmental conditions existing betweenthe transmitting antenna and the receivers.

The actual service area signal strength contours arebased upon two probability factors. Contours are notsolid signal areas. For example, the FCC signal cover-age charts referred to as the F(50/50) curves, are basedupon a probability of occurrence of certain voltagelevels at 50% of the locations, 50% of the time. Thismeans that at any given location, 30 ft above ground,the signal has a 50% chance to measure up to thepredicted contour level. Furthermore, half the time atthat location, it may reach or exceed the level predictedwhile at other times it may be lower in strength.

These FCC charts (FCC Rules, Section 73.333) arebased upon the assumption that average propagationconditions exist. One or more of the conditions men-tioned in the second paragraph under this heading mayreduce the measured signal strength from the predictedvalues substantially.

Propagation LossThe power radiated from a FM transmitting station

is spread over a relatively large area, somewhat likean outdoor, bare light bulb on top of a tall pole. Thepower reaching the receiving antenna is a very smallpercentage of the total radiated power.

At 100 MHz and a distance of 30 miles (48 km)the figures indicate the free-space path loss to be 106dB.4 The formula used to compute free-space loss is:

FSL 4 36.6 ` 20 log D(miles) ` 20 log F(MHz)Doubling the distance increases the space loss by 6

dB. The path loss does not attenuate the signal withdistance as much as some other factors. Path loss be-tween an earth station and a satellite is a classic textbook example of a 6 dB loss every time the distanceis doubled. But a typical FM station signal travelsthrough a nearly perfect dielectric (air) and over the

imperfect earths surface (ground). Herein lies the FMradio propagation loss problem.

Refraction, diffraction, and reflection from scoresof objects such as hills and buildings may occur inthe propagation path between the transmitting and thereceiving antennas. These, along with absorption, scat-tering, lack of Fresnel zone clearances, and other fac-tors, all reduce the signal strengths.

Signal loss due to foliage has been well known toUHF TV broadcasters for many years.5 This samecondition exists to a lesser degree for FM broadcasting.Trees, shrubs, and other foliage on hills or smoothterrain affect the reflected as well as the lateral signalloss with distance. With average values of permittivityand conductivity in both foliage and ground, a loss ofabout 2.5 dB was found to exist in a ten-mile path, atFM frequencies.6 The height gain factor is increasedwith heights above the foliage.

Considerable depolarization takes place because thetransmission through or reflections from ground foli-age is a diffracted field contribution.

Multipath ProblemsThe ideal reception condition is a strong direct single

source signal. When energy from two or more pathsreaches the receiver, (due to reflections) a conditioncalled multipath reception occurs. Poor reception isexperienced when there is insufficient strength differ-ence between the direct and the reflected signals, be-cause they can cancel each other where the geometryplaces them out of phase.

Nothing is more important in the way of broadcast-ing facilities than the location of the transmitting an-tenna. Great care must be exercised to find a suitablesite. Poor selection of the transmitter point can resultin unfavorable signal propagation and negate the entireproject. One very serious result of poor site selectionis multipath propagation in some directions.

As an example, the transmitter should not be locatedso that strong reflections take place from nearby hillsor mountains. This can happen when the transmitteris placed on one side of a large city and the other sideof the city has a high mountain range. Radiation intothe city directly from the transmitting antenna, as wellas reflections from the nearby hills and mountains willcreate two or more signal paths to many receivers.These reflections can be so strong that only a 10 dBdifference may exist between the direct and the re-flected condition which causes severe multipathproblems.

A TV station at this same location would experienceunusable signals due to heavy ghosting, even withdirectional receiving antennas, which exhibit moderatesignal pickup from their back. This is illustrated inFigure 4.12-1 where a mountain range causes reflec-tions back into a large city.

The multipath example shown in the sketch was anactual case. The site was chosen by the FM broadcaster,without proper engineering guidance, simply becausethe hill had a tower, building, power, and a road wasin place. Later, the broadcaster learned why the original

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FM BROADCAST ANTENNAS

Figure 4.12-1. Example of poor station location causing severemultipath conditions due to delayed reflections from mountainson the right. Not to any scale.

owner, a television station, had abandoned the site:the TV station had failed in part due to extremelyheavy ghosting into the principal city.

A much better FM transmitting site was located onthe hills between the high mountain range and the city.Using a directional transmitting antenna with very littleradiation towards the high mountains, reflections weresatisfactorily reduced, and the FM station is now oper-ating successfully.

Multipath reflections are easy to identify. On anautomobile radio, the signal will drop out, sometimesabruptly, as the car moves. This effect may be rhythmicwith distance while traveling slowly. It is sometimescalled picketing as it acts like a picket fence alternatelyblocking and letting the signal pass. A field strengthmeter will usually reveal great variations of signalwhen moving, say, 100 ft (30 m) in a line with thetransmitter. Cyclic variations over quite uniformlyspaced intervals on the ground as great as 40 dB havebeen observed by the author.

This variation in signal levels is caused by the re-flections adding and subtracting from direct and re-flected signals caused by propagation problems ex-isting in the path between transmitter and receiver. Itusually has nothing to do with the qualities of thetransmitting antenna. It is a function of site selection.This should not be confused with a similar effect ob-served near the base of the tower supporting a high-gain antenna. Nulls produced by stacking bays for gainare found near the antenna and may be filled-in ifneeded. (See Beam Tilt and Null Fill in this chapter.)Ground Reflections

In the elevation plane between transmitter and re-ceiver, nearly all FM signal coverage lies between thehorizon and 10 below. Called the grazing angle, it liesbetween the horizontal plane and the earths surface.Generally the higher the transmitting antenna abovethe service area, the greater this angle will be.

The angle of incidence and reflection are not thesame, as shown in Figure 4.12-2. The depression angleand the grazing angle are not equal as would be thecase for a flat earth. Reflections from these angles playan important part in the strength and the quality of thesignal in FM broadcasting with circular polarization.

The ground, which causes reflections at these graz-ing angles does not treat H-pol and V-pol in the samemanner. The V-pol is attenuated considerably morethan the H-pol as shown in Figure 4.12-3. The phaseof the V-pol changes substantially with angle, whileH-pol remains nearly the same. At these useful low

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Figure 4.12-2. Beam tilt to radiate maximum ERP at the horizon.Not to any scale and exaggerated for illustration.

Figure 4.12-3. Magnitude of reflection coefficients showing differ-ences for H-pol and V-pol, and the Brewster angle.

propagation angles, there is considerably less V-polsignal reflected than H-pol, when grazing takes place.Field measurements confirm this fact.7 For this reason,it is impossible to measure accurately axial ratios inthe service area. To be meaningful, the H-pol andV-pol ratios must be measured on a good antennatest range.

It is quite difficult to predict accurately the reflectioncoefficient (efficiency), which varies considerably asa function of polarization, frequency, grazing angle,surface roughness, soil type, moisture content, vegeta-tion growth, weather and the season. There are complex

formulas for predicting the ground conductivity at thefrequency of interest. For 100 MHz, a value of 10millimhos per meter ground conductivity is often used,with a permittivity of 25, as being about the averagefor the continental United States.6

Brewster AngleFor polarization with the electric field normal to the

plane of incidence, there is no angle that will yieldan equality of impedances for earth materials withdifferent dielectric constants but like permeabilities.An incident wave with both polarizations present willhave some of the one polarization component but littleof the other reflected. The reflected wave at this angleis thus plane polarized with the electric field normalto the plane of incidence, and the angle is the polariz-ing angle.

Notice that in Figure 4.12-3 the minimum reflectioncoefficient occurs at a grazing angle of about 2. Belowthis angle, the reflection coefficient rapidly increasesto unity. The angle at which the minimum reflectioncoefficient occurs is called the Brewster or polarizingangle, after the Englishman who first discovered thisphenomenon.

For ground reflections occurring near the Brewsterangle, the reflection coefficient is much smaller forV-pol than the H-pol. Therefore, the reflected V-polsignal component of CP are attenuated considerably.The greatest attenuation for V-pol from ground reflec-tion occurs at this angle.

Field measurement of V-pol signals will usuallyshow a significant variability of H-pol to V-pol ratiosdue to this Brewster angle phenomenon. The Brewsterangle is a function of soil conductivity and maychange from place to place, as well as from seasonto season.8

It is important, then, that the antenna height abovethe service area results in grazing angles which areless than the Brewster angle. Otherwise the V-pol willbe reduced and the radiation will be much more ellip-tical than circular in polarization.

Fresnel Zone ClearanceA much neglected consideration in FM transmitting

antenna location and height is Fresnel zone radiusclearance in the path to the service area. Microwaveengineers always make certain that their signal pathshave this important clearance.

The effect of clearance above ground or other obsta-cles was studied by August Jean Fresnel, a Frenchscientist who first discovered this phenomenon in op-tics. Fresnel zones are circular areas surrounding thedirect line-of-sight path of a radius such that the differ-ence between the direct and the indirect path lengthto the zone perimeter is a multiple of half-wavelengthlonger than the direct path. This is illustrated in Figure4.12-4. The zone diameter varies with frequency andpath length. The greater the path length, the larger therequired mid-path clearance required for full signal.

Fresnel also discovered that the entire first zone

radius is not required for full signal strength. Six-tenthsof the first zone would suffice, which is fortunate sincethe radius is quite large at the FM frequencies. Theequation for determining the first Fresnel zone radiusfor 4/3rd earth curvature is:

R 4 1140 d/fwhere d is the path length in miles, f is in MHz andR is in feet for the first radius.

In Table 4.12-1 the required 0.6 first Fresnel zoneradii clearances at the middle of the path are shownfor 98 MHz and service areas up to 52 miles (92 km)from the transmitter. The idea is to raise the height ofthe transmitting antenna so that the mid-path height isas high as or higher than shown in the table. Due tothe geometry of the Fresnel zone, if the terrain isrelatively flat, the mid-path radius will control and belarger than that required elsewhere along the path. Ifthe mid-path clearance is less than the values shown,the FM signal will be attenuated in accordance withthe curve shown in Figure 4.12-5, presuming idealreflection off the ground or obstructions.

The center-of-radiation heights of the antennas inTable 4.12-1 are actual and not height above averageterrain (HAAT). Some of these recommended heightswill reduce the allowable ERP in accordance with FCC73.211 (b), depending on the class of station and thezone. However, it is better to have the Fresnel clearancethan the maximum low height ERP values, as thehigher heights will produce stronger signals.

It is a well known propagation axiom that greaterheights are more useful in producing higher signalstrengths far from the antenna than ERP levels, every-thing else being equal.

Without the first Fresnel clearance of 60% the signallevel at the distant point may suffer. This reduction willfollow the curve shown in Figure 4.12-5 for differentvalues of clearance and worst case reflection condi-tions.

In order for the FCC prediction curves to be valid,the recommended minimum antenna heights should beachieved. These heights not only provide line-of-sightconditions to the service limits but also proper Fresnelclearances. Both conditions are required for the FCCF(50,50) curves to be valid.

The values in Table 4.12-1 are for relatively flat

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Figure 4.12-4. First Fresnel zone clearance occurs as shownabove, but only six-tenths is required for full free space signallevel. Not to any scale.



terrain, but take into consideration the FCC suggestedroughness factor of up to 6150 ft (50 m). Wherethe tower height is limited by HAAT values or otherlimitations, the signal strength will suffer due tothose factors.

Table 4.12-1Recommended minimum antenna heights (for flat terrain and 98 HMz)

Service Area Fresnel Zone RecommendedRadius Min. AntennaSix-tenths ProbableClearance HeightRequired FCC 8090

Miles Km Feet Meters Feet Meters Class5 8 155 47 310 95 A71/2 12 189 58 378 115 A

10 16 218 66 426 130 A15 24 267 81 534 167 A, B, C-220 32 309 94 618 188 B-1, C-225 40 346 105 700 213 B-130 48 378 115 756 230 B35 56 409 125 818 250 B40 64 437 133 875 267 C-145 72 463 141 925 282 C-150 80 488 149 975 297 C-157 92 522 159 1,043 318 C, C-1

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Figure 4.12-5. Attenuation of FM propagation when the path be-tween transmitter and receiver lacks Fresnel zone clearance inthe ratios shown.

Soil ConductivityThe conductivity and permittivity of the soil, to-

gether with the vegetation on it, play a small part inthe attenuation of FM signal strength. Average soilhas a dielectric constant of about 15 millisiemens permeter at 100 MHz.9

Linear Height Gain EffectBy raising the receiving antenna above the immedi-

ate effects of the soil, the signal level will be increased.Actual field measurements have proven a 9 dB increasein signal when the dipole was raised from 3.28 ft(1 m) to a level of 30 ft (9.1 m). This is due to reflectionphenomena in the foreground of the receiver, notground conductivity.

FCC Service ContoursFrom the FCC coverage prediction charts, it is possi-

ble to draw contours of the various grades of servicefor a given ERP and antenna height above averageterrain. These predictions, at 50% of the locations,50% of the time, constitute the basis for the servicecontours. The city grade contour is 70 dBm (3.16 milli-volts per meter) and primary service contour is 60dBm (1.0 millivolts per meter).

The FCC Rules, Section 73.333 charts for thesepredictions have a built-in terrain roughness factor, asexplained above.

GENERAL COVERAGE STANDARDS

There are certain height and power levels fixed by theFCC for various classes of stations. The United Stateshas been divided into three different geographical areasbased on population density as well as propagationrefractive index levels. These ERP and height valueshave been set to prevent co-channel and adjacent chan-nel interference.

Zone I, generally speaking, is the northeastern partof the United States. Zone I-A includes Puerto Rico,


talk problems with SCA channels. Outside FM receiv-ing antennas generally provide good reception.

Field strength measurements should not be used todetermine the transmitting antenna radiation pattern orefficiency except under carefully controlled conditions.The propagation factors discussed previously camou-flage the true antenna performance. The only techni-cally acceptable way to determine the antennas char-acteristics is on an antenna test range.

See Chapter 4.15, AM and FM Field Strength Mea-surements for more information. This information maybe used to determine the actual quality of service andthe areas where useable signal levels in fact exist.Predicted contours may be considerably different fromactual measured values.

Required Signal StrengthWhat is the minimum satisfactory signal strength?

What is the maximum above which it is wasteful?The history of FCC proceedings provides some of thefollowing levels:

34 dBu 4 0.05 mV/m For rural areas60 dBu 4 1.00 mV/m Suburban areas70 dBu 4 3.16 mV/m Principal community82 dBu 4 12.64 mV/m Highest useful level

The first three levels were set by the FCC in theearly 1950s when tube receivers and H-pol antennaswere popular. Modern day transistor radios have muchgreater sensitivity. CP has added greater signal pene-trating power than H-pol when the levels were firstestablished.

The FCC defines two grades of signal contours onits applications. The first is based on the 70 dBu con-tour (3.16 mV/m) required to cover the principal com-munity of license. The second is the 60 dBu contour(1 mV/m) which defines the primary service area.

The FCC also stated that, in rural areas, levels aslow as 50 mV/m were useful. Indeed current homestereo tuners and FM auto radios operate very wellwith only 25 mV/m. In practice, 50 mV/m (0.05mV/m) provides good quieting in nearly all automobileand transistor radios receiving a stereo signal from aCP station antenna. Therefore 50 mV/m should beconsidered the minimum useful signal level.

If the highest level of 3.16 mV/m is quadrupled, itwill be 12.64 mV/m. This is a 12 dB increase, equalto increasing the FCC power level by more than 15

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Virgin Islands, and that portion of California lyingbelow the 40th parallel. Zone II includes Alaska,Hawaii, and the remainder of the United States not inthe above two zones. This is more fully described inFCC Rules Section 73.205.

Under the FCC Rules which resulted from Docket8090: Modification of FM Broadcast Station Rules toIncrease the Availability of Commercial FM BroadcastAssignments, in 1983, new ERP levels and additionalclasses of stations were created. The distance to the60 dBu (1 mV/m) signal contour is the controllingfactor so that the ERP based on the HAAT is adjustedto produce that level and no more at a specific distancefor a particular class station.

Table 4.12-2 shows for each FM class station, thezone, the maximum ERP, the maximum HAAT, andthe distance to the 60 dBu contour calculated by usingthe maximum ERP and HAAT, and then rounding tothe nearest kilometer and mile.

Under the Rules, Class C stations are required tohave at least 100 kW ERP and an antenna height ofmore than 984 ft (300 m) above average terrain. ClassC-1 stations are now permitted a maximum of 100 kWERP with an antenna maximum HAAT of 984 (300m), while C-2 stations may go to 50 kW at a maximumHAAT of 492 ft (150 m). Higher HAAT may be usedwith reduced ERP values, in accordance with equiva-lent 60 dBu coverage. Class C and C-1 stations maythus share the same antenna and tower.

Stations may be upgraded using the easiest method,which is to increase existing location tower height.Such factors as local zoning laws and aircraft flightpatterns may preclude this approach, however.

FM Signal MeasurementsThe signal strength received at 5 ft (1.5 m) above

ground, which is about average for auto whip antennas,is several times lower in level than at the standardFCC measurement height of 30 ft (9.1 m). This factshould be taken into consideration when comparinglow height measurements with the FCC Rules Section73.333 prediction charts, which are based on a 30 ftreceiving height, where signals are considerablystronger.10

Signal levels inside houses, apartments, offices, andother structures vary greatly. Levels depend on thetype of building construction, but in nearly all caseswill be lower than those outdoors. Reflections insidethe building reduce stereo separation, and cause cross-

Table 4.12-2FM station classes, zones, and ERP

FM MAX. ERP MAXIMUM HAAT DISTANCE TO 60 dBuCLASS ZONE In kW Feet Meters Miles km

A I, I-A 3 328 100 15 24B I, I-A 50 492 150 32 52B-1 I, I-A 25 328 100 24 39C II 100 1,969 600 57 92C-1 II 100 984 300 45 72C-2 II 50 492 150 32 52


tion mode is used by the Commission. The verticallypolarized energy must not exceed the H-pol (exceptfor noncommercial, educational FM facilities at-tempting to minimize interference to TV channel 6 re-ception).

The power gain of an antenna is used with the trans-mitter gain and transmission line loss when deter-mining the ERP. Consider for example a 10 kW trans-mitter and an antenna power gain of 5. Neglectingtransmission line loss, the ERP is 10 kW 2 5 4 50kW ERP. If the antenna gain were 10 and the transmit-ter power was 5 kW, we would have the same ERPof 50 kW. (5 kW 2 10 4 50 kW ERP)

The FCC defines EPR to mean the product of theantenna input power (transmitter output power lesstransmission line loss) times the antenna power gain.Where circular polarization is used, the term ERP isapplied separately to the H-pol and V-pol of radiation.For allocation purposes, the ERP is the H-pol compo-nent of radiation only. The V-pol component powernormally must not exceed the H-pol power.

Beam TiltFM broadcasting antennas are normally mounted on

towers which are plumb, so the peak power beam inthe elevation pattern is perpendicular to the tower axis.A standard FM antenna without any beam tilt radiatesmore than one half of the total radiated power abovethe horizon. All this power is lost.

The higher the antenna is above its average terrain,the larger the predicted coverage area. Since the earthis curved, the service horizon is bent lower than aperpendicular angle from the earths surface. Thus thestrongest portion of the signal is aimed above the hori-zon. It also follows that the higher the antenna abovethe terrain, the greater the elevation angle down to theearths horizon.

In order to strike the farthest service area from ahigh HAAT, the beam may need to be tilted downtowards the earth. Electrical beam tilt lowers the beamangle equally in all azimuth headings and is chosenmore frequently than mechanical tilting, which ex-hibits different effects in different directions. Chooseenough tilt to position the center of the main beam onthe furthest edge of the desired coverage area or justbelow the horizon, whichever is closer.

For low gain antennas (two to four bays), the mainbeam is very broad, and if the antenna HAAT is lessthan 500 ft there is little to be gained with beam tilt.On the other hand, beam tilt makes a large differenceon high-gain antennas mounted on towers with ahigh HAAT.

Figure 4.12-6 shows the comparison between eleva-tion angle path and coverage distance. It incorporatesthe curvature of the earth. This chart can be used todetermine the optimum beam tilt. Follow the curvewhich is closest to actual HAAT, and mark the pointwhere it intersects the horizon or crosses the distanceof furthest desired coverage area (vertical axis). Readthe beam tilt on the horizontal axis. Round this valueup to the nearest 14.

times. It can be safely said that this level of 12.64mV/m is considerably more signal than necessary byany present day working FM radio. Any signal levelhigher than this at the receiving antenna has not provento be of significant value.

BlanketingExcessive RF signals can overload the front end of

receivers and make satisfactory reception impossible.The FCC in Section 73.318 defines the 115 dBu (562mV/m) level as the blanketing contour, and adoptedthe free-space prediction method to predict how farthis contour extends.

New or modified FM stations have the responsibilityto satisfy all complaints at no cost to the complainant,of blanketing-related interference inside this contourwithin one year of commencement of operations.

The distance to the 115 dBu contour is determinedusing the following equation:

d (in kilometers) 4 0.394 PD (in miles) 4 0.245 P

where P is the maximum ERP, measured in kilowattsof the maximum radiated lobe, irrespective of verticaldirectivity. For directional antennas, the horizontal di-rectivity shall be used.

ANTENNA CHARACTERISTICS

Antenna gain can be increased by adding additionalradiating elements (bays) to the antenna at the costof narrowing the radiated beam. High-gain antennasconcentrate the energy into such a narrow beam thatoften null fill must be employed to achieve the desiredsignal strength within the first few miles to the tower.

Directional antennas achieve increased gain overnondirectional antennas by limiting the radiated energyin various directions. Directional antennas are usefulwhen the tower is located near a large body of water,mountain range, or other areas where energy radiatedin those directions is otherwise wasted. They are alsoemployed to avoid interference where stations are in-sufficient distances apart.

Antenna gain is expressed in power ratio or in dB.For example, an antenna with a power gain of 2 isalso said to have a gain of 3.0 dB.

FCC Rules, Section 73.310(a) defines antenna gainas the inverse of the square of the root mean squarevalue of the free-space field strength produced at onemile in the horizontal plane, in millivolts per meterfor 1 kW antenna input power to 137.6 mV/m. (Inmetric units, 1 km and 221.4 mV/m).

Notice that this gain is in reference to a horizontallypolarized half-wave dipole. For a CP antenna, the gainis half for the same input power.

A two-bay H-pol antenna has a power gain of ap-proximately two. But a two-bay CP antenna in FCCterminology has a gain of about one because the otherhalf of the power is V-pol and is not consideredin the gain calculations. Only the horizontal polariza-

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Figure 4.12-6. Twelve bay Cpol antenna with 0.3 beam tilt show-ing ERP distribution with coverage from 492 foot (150 m) towerover flat land. Degrees below the horizon are based on 4/3rdearths curvature. Horizon is 10.341 at 32 miles (52 km).

Consulting engineers, familiar with this problem,can easily work out the required amount of beam tilt,if it is necessary. Typical values are one-half to onedegree of tilt, depending on the antenna height, dis-tance to the far service area, and the antenna eleva-tion pattern.

Beam tilt is usually accomplished electrically, bydelaying the currents to the lower bays, and advancingthe phase of the upper bay currents during the designand construction of the antenna at the factory.

Null FillWhile the beam tilt puts more signal into the far

reaches of the service area, it does not solve the prob-lem sometimes caused by high-gain antennas withinseveral miles of the transmitter. Elevation angle nullscommon to all antennas with two or more bays appearfarther and farther away from the antenna as its gainis increased with more bays.

When multiple bay arrays are employed, lobes andnulls occur in the elevation pattern. As the number ofbays increases, the main beam narrows and the firstnull radius increases. The advantage of beam tilt andnull fill varies depending on factors such as towerheight, site elevation, number of bays, and relativelocations of communities to be served.

A simple rule-of-thumb is that null fill is beneficialwhen there is desired service area within the radius ofthe first null. The equation below gives the approxi-mate radius of the first null for multiple bay antennas.

Null Radius (miles) > H(ft)Number of Bays/5280 ft /mi

where H is the height (ft) of antenna above radius ring.In most FM applications, the null is very close to

the antenna, thus a small amount of null fill (510%)takes care of the problem. Larger amounts of null fillare unnecessary and reduce the gain of the antenna.Note that null fill has no effect on distant coverage.

VSWR BandwidthAccording to theory, the bandwidth of an FM signal

is infinite if all the sidebands are taken into account.Also, at certain modulation indices, the carrier ampli-tude goes to zero and all the transmitted power is onfrequencies (sidebands) other than the carrier fre-quency. Practical considerations in the transmitter andreceiver circuitry make it necessary to restrict the RFbandwidth to less than infinity.

Prior to 1984 the maximum deviation for FM sta-tions was 75 kHz, representing 100% modulation. Inthat year the FCC changed the maximum deviation to82.5 kHz (110%) for those stations with 10% injectionof subcarrier channels. This additional deviation re-quires greater antenna system bandwidth than previ-ously needed.

System bandwidth is measured at the point in theantenna system where the transmitter is connected.This usually includes the harmonic filter, the maincoaxial transmission line, and the antenna.

The significant sidebands are usually considered tobe those whose amplitude exceeds one percent of theunmodulated carrier. With 110% modulation (82.5kHz deviation) these side bands produce a bandwidthof 260 kHz.11,12

The VSWR bandwidth is the range over which thesystem under consideration has a reflection coefficientof less than five percent; a VSWR of 1.1:1.

Checking System VSWRFrom time to time, the VSWR of the narrow-band

antenna system should be checked and adjusted. If theexciter has thumb wheel exciter frequency adjustabilityin 10 kHz or 50 kHz steps, it can be used to changethe frequency to check VSWR on different frequencies(with the transmitter operating at low power duringthe overnight experimental period). The reflectometermay be used as the indicator.

Alternately one of several methods for checkingVSWR in coaxial line systems using test equipmentmay be used. These include a signal generator testsetup, an impedance test set, or a network analyzer.

The VSWR should be measured to ensure that thereflection response is balanced to 130 kHz on eachside of the carrier frequency. With transmission lineslonger than 300 ft (100 m) it is suggested that theVSWR bandwidth be under 1.08:1 all the way out to6130 kHz. The additional delay due to increasing linelength becomes more of a problem, so the amplitudeof the reflection must be reduced, for best opera-tional results.

Importance of Low VSWRThe VSWR shown by the transmitter reflectometer

does not increase or decrease the range of the signal.It has nothing to do with coverage. But VSWR valuesabove 1.1:1 may decrease the final amplifier efficiency.Other definite negative effects of VSWR are increasedintermodulation products and AM synchronous noise.Stereo separation is also degraded with increasedVSWR.13

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DH and DV are the directivities of the H-pol andV-pol azimuth patterns, while GA is the gain if theH-pol over the V-pol pattern.

Due to the gain of its azimuth patterns, directionalantennas have gains which are typically 2 dB to 6 dBhigher than their nondirectional counterparts of equalnumber of bays.

ANTENNA POLARIZATION

Radio waves are composed of electric and magneticfields at right angles to each other and to the directionof propagation. When the electric component (E) ishorizontal, the wave is said to be horizontally polar-ized, as shown in Figures 4.12-7(b) and 4.12-7(d).Such a wave is radiated from a horizontal dipole. Ref-erences are with respect to the earth plane. If thedesired electric component is vertical as in Figures4.12-7(a) and 4.12-7(e), a vertical dipole could be usedto produce the vertically polarized wave.

Circular PolarizationWhen the two plane waves are equal in magnitude,

and if one plane wave lags or leads the other by 90electrical degrees, the field will rotate as shown inFigure 4.12-7, at the speed of the carrier frequencyand will be polarized circularly.

Only in the special case where the horizontal andvertical components are equal in strength with a 90phase difference is the radiation said to be CP.

The direction of rotation shown by the vector arrowsin Figure 4.12-7 depends on the relative phase of thetwo components. Thus the polarization of the wave willappear to have either clockwise or counter-clockwiserotation, as shown. The FCC has set clockwise rotationas the technical standard, in order that similar senseof rotation antennas may be used for reception inthe future.

Notice that in Figure 4.12-7 the polarization rotatesas the field propagates in time and space. Importantly,vertical and horizontal components are in quadraturephase. It is this rotation which enhances the signalpenetrating qualities of CP, so useful in FM broad-casting.

The axial ratio as shown in Figure 4.12-8 is thatbetween the maximum and minimum voltage compo-nent at any orientation of the reference measuring test

Intermodulation and SAM DistortionIntermodulation distortion and synchronous AM

(SAM) noise can be caused by narrow VSWR band-width in the antenna system, as well as by final ampli-fier circuitry all the way to and including the antenna.14

SAM is extremely important in FM transmitter facil-ities employing subcarriers. SAM is AM modulationof the carrier caused by frequency modulation of thecarrier frequency in the VSWR notch. At the notchthe reflected energy is the lowest. As the deviationtakes place, the greater the frequency swing, the greaterwill be the reflections, due to the VSWR notch. Witha flat VSWR curve, SAM does not take place. If theVSWR curve is skewed, SAM will occur and intermodand stereo crosstalk will increase.

Directional AntennasThe FCC sometimes requires that the azimuth radia-

tion pattern be directionalized to reduce normally allo-cated ERP towards a given short-spaced station, or forother reasons. (See the FCC Rules Section 73.213, 215,and 316(b) and (c).) To conform to these specifications,most broadcasters order antennas which are patternadjusted, measured, and certified to the Commis-sions requirements.

Directional antennas are licensed for peak ERP val-ues based on the azimuth pattern. The V-pol gain maynot exceed the H-pol gain in a CP directional arraynor may V-pol exceed the H-pol in the protectiondirection (except in the case of FM protection to TVChannel 6, mentioned elsewhere). The amplitude awayfrom the null cannot climb more than 2 dB per 10of azimuth.

Directional antennas are usually mounted on polesalthough some have been tower mounted. Since thesupport affects the pattern, they are specified and mea-sured with the pole or tower on which they aremounted. Most firms will make the antenna meet thespecific pattern requirements.

Directionalizing is a combination of the natural pat-tern resulting from sidemounting and the use of para-sitic elements. Using the two factors, virtually anydirectional pattern can be produced.

Antenna gain is calculated differently for directionalantennas. The azimuth directivity increases the gainvalue to correspond to the pattern. If all elements/baysare the same (the typical case), pattern multiplicationcan be used to determine gain. For linearly polarizedantennas, the gain is simply the product of the azimuthdirectivity, the array factor, and the efficiency factor.The array factor is referenced to an ideal dipole.For directional CP antennas, the power distributionbetween polarizations must be taken into account.Antenna Gain (H-pol)

4 gH 2 array factor 2 efficiencyV-pol 4 gV 2 array factor 2 efficiency

where: gH 4DH 2 DV 2 GADH ` DV 2 GA

gV 4 gH / GA

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Figure 4.12-7. Circularly polarized wave propagation in one wave-length of travel, showing right hand rotation. Note vector rotationwith wave travel.


dipole which is placed perpendicular to the directionof propagation. An axial ratio of 1:1 (0 dB) is perfect.In practice, axial ratios of 2 dB or better are consideredto be excellent and commercially available. Axial ra-tios over 4.9 dB (1.75 to 1 voltage ratio) are consideredto be elliptically polarized, a hybrid form and not asgood in signal penetrating qualities as CP.

Since receiving antennas are linearly polarized, theintroduction of CP does not increase the net powerreceived since the vertical and horizontal componentsnever occur during the same instant. Thus, CP doesnot necessarily mean an increase in coverage. How-ever, the introduction of CP eliminates the requirementof the receiving antenna to have a specific polarization.Thus, CP allows more consistent coverage within thecontour. Its rotating vector can penetrate areas where

Table 4.12-3Transmitter power versus antenna gainClass A 3 kW ERP Zones 1 and 1-A

Maximum HAAT 328 Ft (100 m)SIGNAL LEVEL IN mV/m

Service Distance Vertical 7.5 kW Transmitter 1 kW TransmitterMiles Km Angle 1 Bay Antenna 6 Bay Antenna

1 1.6 3.58 275 2102 3.2 1.80 88 813 4.8 1.21 42 404 6.4 .92 24 225 8.0 .74 16 166 9.6 .63 11 117 11.3 .55 8.5 8.58 12.9 .49 6.2 6.29 14.5 .44 5.0 5.0

10 16.1 .41 3.7 3.712 19.3 .36 2.5 2.514 22.5 .33 1.8 1.816 25.7 .31 1.4 1.418 28.9 .29 1.1 1.120 32.2 .28 .85 .8522 35.4 .28 .70 .7024 38.6 .27 .55 .5526 41.8 .27 .40 .40

linear polarization is stopped, shadowed, or canceleddue to out-of-phase reflections.

MATCHING COVERAGE AND ANTENNAS

Table 4.12-3 shows the FCC predicted signal strengthsfor a typical Class A facility on a relatively flat plane,with the antenna center 328 ft (100 m) HAAT. Apower of 3 kW is used. The first two columns showthe distances, with the farthest being the horizon fromthis height. The third column indicates the true earthangle from the antenna to the distances shown. Fromthe elevation information the ERP from each antennawas determined at each vertical angle. This ERP valuewas used to find the signal strength from the FCCF(50,50) FM prediction chart, FCC Rules Section73.333, Figure 1.

Under the signal level in millivolts-per-meter (mV/m) column, the predicted field strengths shown in thisTable are based on the above procedure. From 5 miles(8 km) to the horizon, the signal strengths are identical.This is due to the shape of the antenna elevation patternnear the maximum.

Departure occurs as the depression angle to the re-ceiver becomes larger. Beyond 4 miles (6.4 km), theone-bay antenna and the six-bay antenna producenearly the same signal level.

Going towards the transmitter from 4 miles (6.4km), the field increases in favor of the one-bay antenna.In this example, the table clearly indicates that thehigh-power transmitter low-gain antenna does not im-prove the signal strength available to the receiversbeyond about 4.5 miles (7.25 km). The signal levelstarts to increase between 4 miles and 5 miles (6.4and 8 km). Any increase above this level is uselessbecause full limiting has certainly taken place in even

748


Figure 4.12-8. Axial ratio expressed in dB is the ratio of the largerpolarized component divided by the smaller at any referencedipole orientation, where the maximum ratio occurs.


lines. It is compared with a 10 kW transmitter feedinga 10-bay CP antenna. The terrain flatness is assumednot to exceed 6150 ft (50 m).

Table 4.12-4 indicates that the signal levels are thesame from 4.6 miles (7.4 km) out to 35 miles (56km) under similar columns as for the Class A stationcomparisons. The FCC uses a receiving height of 30ft (10 m) so the horizon is a bit further away, at 31.3miles (50.5 km).

From the transmitter out to about 2 miles (3.2 km),the signal rises much more rapidly in the 2-bay antennathan in the 10-bay, the latter being somewhat similarto a cosecant curve. There is surplus signal close inand more than is needed or can be tolerated.

Therein lies one of several problems with high trans-mitter power and low-gain antennas as seen in Table4.12-4. With the 2-bay antenna there is 900 mV/m atone mile (1.6 km) and 562 mV/m as far out as 1.55miles (2.5 km). This is above, or at, the blanketinglevel of 562 m/Vm. (See Blanketing in this chapter.)The high-gain antenna does not cause this type ofproblem under identical conditions.

The signals from both combinations are much morethan necessary for present day FM receivers out toabout 10 miles (16 km). There is no practical differencetechnically in useable signal strengths presented toreceivers in the entire market area, from either antenna.There is, however, a great deal of savings in capitalcosts as well as operating expenses between the twocombinations.

One antenna factor is not clearly indicated in Table4.12-4. The two antennas have elevation pattern nulls.The 2-bay antenna null at 130 falls 852 ft (260 m)from the base of the tower and can be disregarded.The ten-bay antenna nulls can be filled to as little as212% field, which will not affect its gain. This wouldrepresent a minimum ERP at the nulls of 31 W. Al-though seemingly very small, it is very effective asshown in Table 4.12-5.

It is obvious that the 10-bay antenna nulls can easily

the poorest FM receiver. (See Required Signal Strengthin this chapter.)

In Table 4.12-3 the same signal strength of (16mV/m) at 5 miles (8 km) comes from either transmitter-antenna combination. This is due to the fact that theERP power at the vertical angle of 10.74 is aboutthe same from both antennas. The ERP at 0.0 elevationpattern will of course be exactly the same for bothcombinations. The field does not change measurablyuntil observation is made beyond 1.5 from the peak100% value in a six-bay antenna.

The signal strengths in this table were based onrelatively flat terrain for an antenna 328 ft (100 m)HAAT. The true earth curvature distance to the horizonis 25.56 miles (41.23 km). Therefore the outer reachesof useful signal drop off very rapidly beyond this pointin the typical Class A station.

There are no nulls in a one-bay antenna pattern. Ina six-bay antenna the first null occurs at about 110,approximately 0.37 miles (0.6 km) from the tower.Antenna arrays are never perfect so the null is neverzero power. With a minimum radiation of 5 W in thefirst null, the predicted signal would be 31 mV/m. Thesecond null is closer to the tower and with the same5 W ERP would be even stronger in this example. Soin practice there may be no need to fill in the nulls ofthe six-bay antenna.

Another consideration is that the nulls may fall veryclose to the tower and the number of people occupyingthe null areas may be small. Thus problems resultingfrom these close-in nulls would be minor.

Typical Class B and C-2 Station CoverageThe same comparisons of transmitter-antenna com-

binations can be made for Class B and the new ClassC-2 stations, operating with a HAAT of 492 ft (150m) with 50 kW ERP. This is shown in Table 4.12-4.A 55 kW transmitter with a 2-bay CP antenna wouldprovide the 50 kW ERP, with high efficiency coaxial

Table 4.12-4Transmitter power versus antenna gain

Class B, C-2 50 kW EFPZone 1, 1-A & C-2

SIGNAL LEVEL IN mV/mService Distance Vertical 55 kW Transmitter 10 kW TransmitterMiles Km Angle 2 Bay Antenna 10 Bay Antenna1 1.6 4.97 900 1401.55 2.5 3.25 562 1652 3.2 2.49 310 2303 4.8 1.67 153 1354 6.4 1.26 92 884.6 7.4 1.15 71 715 8.0 1.02 57 577.5 12 .70 22 22

10 16 .55 13 1315 24 .41 6.5 6.520 32 .36 3.1 3.125 40 .33 1.9 1.930 48 .328 1.1 1.135 56 .332 .7 .7

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Table 4.12-521/2% Null fill in 10 bay antenna

Null Angle ERP Distance FieldFirst 15.75 31 w 4,800 Ft 31 mV/mSecond 111.50 31 w 2,240 Ft 70 mV/mThird 117.25 31 w 1,512 Ft 109 mV/m

be filled to produce signal levels in excess of thoserequired. If the transmitter is located in a populatedarea, these high levels prevent the loss of stereo separa-tion and noise in the SCA (if there are reflections fromhigh level lobes in the built up areas). This problemis common to TV transmitters which produce ghostsfrom high signal level lobe areas reflecting into nullareas. This problem is greatly and satisfactorily re-duced with null fill as shown in Table 4.12-5.

MATCHING TRANSMITTER POWER ANDANTENNASSeveral available combinations of antenna gain andtransmitter power will provide the necessary ERP. Butwhich combination is the best? The choice is furthercomplicated by the nature of the terrain in the servicearea. Is it all flat, some rolling hills, mountainous ora large valley? What are the regulatory limitations onthe antenna supporting tower height?

Important considerations when choosing the trans-mitter power and the gain combination to produce agiven ERP are as follows:x Transmitterx Feed systemx Antennax Final amplifierx Towerx ac power consumptionThe transmitter, antenna, tower, and coaxial feed lineare one-time capital costs for the station. Tube costsand commercial power use, however, are a continuinghour-by-hour cost factor. From the above it is apparentthat a low-power transmitter is much more economicalthan a high-power transmitter. But is there a differencein signal strength?

The ERP is the product of the antenna power gainand the antenna input power. Many different combina-tions of power gain and input power will yield thesame ERP. The azimuth pattern will be quite similarfor many different antenna power gains.

The only difference in various combinations is theelevation pattern. As discussed previously, there is nomomentous or important difference in serving listenersfrom very different transmitter/antenna ratios.

The signal strength at any given location is a directfunction of the ERP from the antenna elevation patternangle to that location, the height of the antenna, andthe propagation path. The ERP at the pertinent angleis the product of the elevation pattern relative ampli-tude at that angle squared, times the maximum ERP.

In practice there is no significant difference betweena 3 kW ERP Class A station using a 7.5 kW transmitterand a 1-bay CP antenna, or, one using a 1 kW transmit-ter and a 6-bay CP antenna, all other factors beingequal.

Normally, all the power radiated above the antennaelevation pattern to the horizon is wasted. It is theradiated power below the angle to the horizon thatstrikes the earth with all its FM receivers. Thereforeonly the radiated power towards the earth should beconsidered useful.

The ideal antenna system would put the same signallevel from the base of the tower all the way out tothe horizon. This requires an antenna whose elevationpattern is a cosecant curve, the normalized reciprocalof sine. It would be the most efficient antenna elevationpattern. Although this curve is impossible to achieve,it is approached as the antenna gain becomes greater.

ANTENNA SITE SELECTION

The transmitter location must be carefully chosen. Siteeconomics should be secondary to the technical advan-tages of a particular site. Fresnel zone clearances andother factors outlined in this chapter should be consid-ered. A site with an operating FM or VHF TV stationmakes an excellent source of signals to check propaga-tion for a new station. If the existing station is FM,make certain that its antenna pattern has been opti-mized to provide as much circularity as possible.

A good field strength meter should be used to mea-sure the actual signal from the existing station. Relativereadings are important, not the absolute. Check forreflections as well as level changes within a shortwalking area of about 100 ft (30 m). Check for stereoseparation. Using this information, the operation of anew station near the one being checked can be com-pared before moving or submitting the FCC applica-tion. The consulting engineer may find it useful toconsider this information to evaluate the suitability ofthe new site.

High-Gain Antenna ContradictionsThe many advantages of high-gain, low-power

transmitter combinations to produce the ERP havebeen shown. Their superiority in relatively flat landapplications cannot be disputed.

There is, however, the matter of unusual height overaverage terrain to be considered. As examples, if thetransmitter is located on Mt. Wilson, in California, ona very tall building in Chicago, or New York, theelevation pattern problem can become serious. This istrue particularly when there are listeners near the sitesas is the case for these three locations.

Mt. Wilson which serves the greater Los Angelesmetropolitan area is more than 1 mile (1.6 km) abovemost of its listeners. In fact coverage is required from11 miles (17.75 km) out to the horizon which is 10.57at 105 miles (168 km). Pasadena, the nearest city, is13 below the horizon. A high gain antenna tilted down

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separation from the channel 6 aural frequency of 87.75MHz. In any event, the interference area should nothave more than 3,000 people living in it. (See FCCRules Section 73.525(c) and (e).)

Cross PolarizationSeveral organizations have made discrimination

tests in the United States and in Europe with cross-polarized antennas. It has thus been well establishedthat discrimination of 16 dB is to be expected in ruralareas and 10 dB in urban areas between two stationsone using the V-pol and the other using H-pol, andthe receiving antenna being similarly polarized. Thisis sufficient in most cases to resolve the FM-Channel6 problem.

While technically, cross polarization will help solvethe problem, the Commissions Rules do not requireit. This is left as an option for the FM applicant touse. Most TV channel 6 receiving antennas will remainH-pol, while automobile FM antennas will stay V-pol.So if the TV station remains H-pol, this interferenceproblem may be cleared up if the FM station willswitch to V-pol. (See FCC Rules Section73.525(e)(4).)

Rejection FiltersThe FCC believes that rejection filters installed at

the TV receiver would be helpful, while others thinkthis is not a satisfactory solution. Unfortunately, manyviewers do not sufficiently understand this problemand are thus not motivated to have the necessary fil-ter installed.

It is further complicated by the fact that many ofthe existing TV receivers still have balanced antennainputs (300 V) and filters designed for them that donot usually provide the necessary amount of rejection.As more TV receivers with coaxial inputs are pur-chased by the TV viewing public, this situation couldchange. Coaxial (75 V) filters with 20 dB of attenuationof the FM signal are readily available.

COMMERCIALLY AVAILABLE ANTENNAS

There are several basic classes of antennas availablefor FM broadcasting. These and variations of them aremade by several manufacturers in different models,gains, and input power ratings. They may be brokendown into the following classes:x Ring stub and twisted ringx Shunt and series fed slanted dipolex Multi-arm short helixx Panel with crossed dipoles

These antennas have many things in common. Forexample, nonsymmetrical antennas are designed forsidemounting to a steel tower or pole. Radiating ele-ments are shunted across a common rigid coax line.This has eliminated the problems associated with theolder corporate feed system using semi-flexible soliddielectric low-power cables.

0.5 would serve the far reaches well, but would notlay down a moderate signal at 113.

Section 73.211 of the FCCs Rules limits the ERPfor overheight antennas such as those on Mt. Wilsonwith 2,900 ft (884 m) HAAT. New stations usingthat height must reduce ERP in accordance with theequivalence calculation, so that the predicted signal atthe 1 mV/m contour does not extend beyond 32 miles(52 km) for Class B stations.

In these situations a moderate gain antenna shouldbe considered. From Mt. Wilson several existing fourand 5-bay antennas now provide excellent service.

TV CHANNEL 6/FM ANTENNA PROBLEM

Television Channel 6 occupies the band from 82 MHzto 88 MHz. The FM broadcast band extends from 88MHz to 108 MHz. Noncommercial educational FMstations are assigned from 88 MHz to 92 MHz. Interfer-ence can exist between the two, with the TV stationviewers receiving interference from the FM stations.The FM receiver is relatively selective with a responseto about 200 kHz, but the TV receiver has a bandwidthof at least 6 MHz. However, more than TV receiverselectivity is involved in this interference problem.(See FCC Rules Section 73.525.)

Three principal techniques can be employed to mini-mize Channel 6 interference from FM stations: (1)collocation, (2) where collocation is not feasible, loca-tion of the FM station in an area of low populationdensity, and (3) antenna cross polarization.

Co-locationThe purpose of co-location (that is, placing the FM

transmitter at the Channel 6 transmitter site) is toachieve the same propagation path for both TV and FMstations, thus maintaining a nearly constant desired-to-undesired signal ratio in the service area. If possible,both antennas should be mounted on the same tower.If not, a maximum separation of 0.25 miles (400 m)between the two is still considered as co-location.

The horizontal and vertical plane radiation patternsof both antennas should be similar because the objec-tive is to maintain a near constant desired-to-undesiredsignal ratio. The HAAT should be similar, thus thedesirability of collocating on the same tower. The max-imum ERP of the FM stations operating on this basisis in Section 73.525(d) of the FCC Rules.

Alternate LocationsThe FM station may not be intended to serve the

same community as the TV station, or collation maynot be possible. In this event, the FM broadcastershould locate in an area of relatively low populationdensity by imposing a limit on the population whichmay be included within that area where a particularundesired-to-desired protection ratio is exceeded.

Two ratios were proposed by a committee whichstudied this problem in 1983.15 Their recommendationvaried according to the educational station frequency

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Shunting elements every one wavelength across atransmission line makes impedance matching simple.Bandwidth is limited by the VSWR of the individualelements and the use of an internal transformer.

With more than about 7 bays, the first three ofthe above antennas have a more difficult task beingmatched and there is undesirable beam squint, sincethe elevation beam angle changes with frequency devi-ation by the transmitter. Antennas with more than 7bays are fed from or near the center, thus dividing thephase change in one-half and effectively eliminatedthe beam squint. Center feeding also simplifies theVSWR matching.

A means for tuning out reactance after the antennashave been installed on the tower is also common withall the antennas. Located at the input to the antenna,the VSWR tuner consists of adjustable location dielec-tric or metal slugs on the inner conductor of the maincoax line. Several fixed-position variable capacitors,spaced 18 wavelength along the main feeder near theantenna input are also used on some sidemounted an-tennas to adjust the VSWR to very small values.

Another variety of antenna has curved radiating ele-ments around a circumference whose diameter is deter-mined by the number of element arms. Each radiatorconsists of two, three, or four such circular arms, de-pending on the model. Each element is fed through ashunt arrangement, and then shunted across the verticalrigid feed coaxial line.

Wideband panel antennas are becoming popularwhere high buildings, favorable mountain sites or hightowers are available. Several firms make widebandpanel antennas. Some have very wideband VSWR fea-tures in each radiator. Others with not so broad VSWR,use phase impedance compensation similar to theEuropean scheme, which uses 90 phase quadratureimpedance compensation.

Phase quadrature compensation makes it possibleto cover the entire 88 MHz to 108 MHz band with aVSWR under 1.1:1 while maintaining excellent eleva-tion, and azimuth patterns, together with very goodaxial ratios. Power ratings up to several hundred kilo-watts are offered so that many FM stations can bediplexed into one such antenna.

Only the wideband community FM antenna designnow uses a corporate feed system, while the othersare shunt fed from a common rigid coax line. Thiscorporate feed system, using air dielectric semi-flexibleline at the lower power levels, is very successful. Itsplits the input power to many different dipoles at thecorrect amplitude and phase.

Standard Sidemount AntennasStandard sidemount antennas come in a variety of

shapes and forms. They are currently used in the major-ity of applications. Their chief advantages are low cost,easy installation, availability of high gains, and lowtower constraints. They are available in linear polar-ized configurations (H-pol or V-pol) or circularly po-larized (CP).

Most sidemounts are comprised of a series of radiat-ing elements, or bays, which are fed via a rigid inner-bay feed line. The most typical feed lines used are158 in. O.D. for applications with less than 10 kWantenna input power, and 318 in. line for up to 40 kW.Most antenna elements come in high and low powerversions. These antennas are mounted directly to theside of a tower or pole. Leg and face mounts are typicalon tower structures.

Some manufacturers with test ranges offer side-mount antennas with custom directional patterns. Thepattern shaping is accomplished by optimizing itsmounting and adding parasitic reflectors which are onthe order of 12 wavelength. Repeated range tests haveshown that the sidemount antennas have largely dis-torted patterns due to feed lines, mounting structures,and other conductive media in the antenna aperture.

Sidemount antennas are inherently narrow banded.The bandwidth of a single element rarely exceeds 1MHz. Although diplexing two stations on a singlesidemount is occasionally done, the spread betweenstations must be small (a few MHz), and compensationtuning (such as long stubs must be used). In addition,none of these antennas are symmetrical, and each typehas uncontrolled radiation from booms and feed linesin the aperture. This distortion deteriorates the anten-nas axial ratio and circularity.

Series-Fed V-Dipole AntennasThis antenna has similar bandwidth to its shunt fed

counterpart, but the array is typically intentionallytuned high in frequency. The combination of this tun-ing scheme, and the internal protection of its feed allowthis antenna to be somewhat resistant to light icing.

This model is larger in size and heavier than othertypes of sidemount antennas. Care should be taken indetermining tower constraints. The antenna is typicallyfield tuned for an optimized match. Careful placementof ceramic slugs can produce a good VSWR over thestations useful bandwidth.

Ring RadiatorsThere are several antennas that are simple adapta-

tions of ring radiators and were designed and manufac-tured in the 1950s and 1960s for horizontal polariza-tion. By adding vertical stubs to the ends of the radiatoror twisting the ring, elliptical polarization (of sorts)is achieved.

Both the ring and the ring stub suffer from tempera-ture variations which tend to change the spacing be-tween the ring openings and thus the electrical capaci-tance and resonant frequency. The ring stub and thetwisted ring are not really circularly polarized becausethe axial ratio varies considerably with azimuth. Atbest they may be said to be elliptically polarized.

Over the years the design has improved by addinga second horizontal ring and improving the feed. Re-ducing bay spacing reduces high axial side lobes. Theantenna has very good circularity in free space, butlike other types of sidemount antennas, it is stronglyaffected by its support structure and feed line.

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Figure 4.12-9. Three bay non-symmetrical FM antenna mountedon the tower leg. The guy cables are insulated fiber glass rodsnear the tower legs.

The radiation patterns are strongly affected by thetower mounting environment. Being of relatively highQ design, they are more susceptible to detuning be-cause of icing. Radomes and electrical deicers areavailable to overcome this problem. While the icingproblems may be overcome, pattern optimization isnot offered for these antennas.

Ring-Stub AntennasThe H-pol radiation from these antennas comes from

the ring portion whose plane is parallel with the earth.There is a minor lobe from each radiator, which isstrengthened with vertical stacking for additionalpower gain. This nadir-zenith lobe is the result of 360stacking on the rigid coax feed line. It reduces thegain and presents a lobe at the tower base which isdetrimental to low level audio equipment and person-nel located in a building at the base of the tower.

In order to keep the cost down, like the twistedring antenna, the ring-stub is manufactured in severalradiator-to-radiator spacings across the FM band. Thisresults in some minor beam tilt up or down dependingon the frequency. Most higher priced slanted dipoleand helix antennas are spaced exactly 360 and areusually tested to assure this spacing during production.

Shunt Fed Slanted Dipole AntennasThe slanted dipole antenna in its present configura-

tion was developed and patented in 1970.16 It consistsof 2 half-wave dipoles bent 90, slanted and fed in-phase.

The slant angle is critical as it is the factor whichdetermines the ratio of vertically and horizontally po-larized radiated power. The phase point center is atthe feed insulator on the dipole support arm as shownin Figure 4.12-9. When fed through a vertical supportpole on which the antenna was mounted during initialdevelopment tests, the axial ratio varied less than 1 dB.

The commercial adaption uses a horizontal boomcontaining a step transformer. This boom supports twohalf-wave dipoles in which the included angle is 90.The two sets of dipoles are rotated at 22.5 from thehorizontal plane. Two opposite arms of the dipoles aredelta matched to provide a 50 V impedance at theradiator input flange. All four dipole arm lengths maybe adjusted to resonance by mechanical adjustment ofthe end fittings. Shunt feeding, when properly adjusted,provides equal currents in all four arms resulting inexcellent azimuth circularity.

Short Helix AntennasA relatively recent asymmetrical radiator is the four-

arm shunt fed helix. By using four dipoles, curved sothat their circumference is about one wavelength, aCP antenna is produced.17 Each dipole is about 12wavelength and is shunt fed. These are supported ona four arm structure, one end of which is tied to thesupporting structure. The dipoles overlap so that thecurrent flow around the circumference is circular. Thefour feed arms are connected in shunt and the feed

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impedance is quite low, but may be brought up touseful values with an internal step transformer.

The CP quality of the four-arm side-fire short helixis good. Three and two arm models are also available,but their axial ratio is not as good as the four arm.Pattern circularity is 61 dB for the four arm, togetherwith an axial ratio of about 3 dB.

These radiators are stacked about one wavelengthapart on a rigid coax feed line to obtain the necessarypower gain. Like other asymmetrical FM antennas itspatterns are strongly affected by the supporting struc-ture. See Pattern Optimization in this chapter for theneed and methods to circularize the azimuth pattern.

Electrical deicers using the stainless steel dipolearms as one half of the heating circuit are available.Heat is created by passing a large current at low voltagethrough each arm from voltage dropping transformersplaced at each bay level. Plastic radomes are alsoavailable to keep snow and ice off the sensitive VSWRparts of the antenna.

Twisted Ring AntennasThis type consists of one or more rings, which have

been partially twisted so that the open ends of the ringare about 10 in. (25 cm) apart. One semi-circular arm


of the ring is fed with a small loop or by a direct tapon that arm. A number of these rings are fed in thesame manner as the ring stubs, and have the samezenith-nadir lobe problem.

The mechanical twist is not the same when viewedin all the azimuth directions. Therefore the current isnot the same, with the end result that in some directionsthere is much more elliptical radiation than in others.

These antennas are very simple and relatively inex-pensive for single frequency use, but have some seriousoperational limitations for CP operation. They do nothave the same signal penetrating effect as the slantdipole, short helix, or the flat panel antenna type ofCP antennas.

Short HelixMulti-Arm AntennasThe number of arms may be increased to four instead

of the two in the slanted dipole variety. To provideCP, the arms are curved to form a 1 wavelength circum-ference. These short multi-arm helices are also stackedin the conventional manner, like the others in thisseries for power gains as desired. This design uses2 g feed straps to feed all the elements in phase.This antenna is shunt fed, and is arrayed and mountedsimilarly to the slanted vee dipole antenna.

The azimuth pattern of all these non-symmetricalantennas is affected by the supporting steel structure.With pattern optimization, the pattern can be madequite omnidirectional. (See Pattern Optimization inthis chapter.)

Series Fed AntennasA similar arrangement of arms supported by a T

arrangement may be series fed. That is, part of theouter end is insulated from the rest of the dipole andfed across the insulated break. To allow for adequatepower handling capacity and to increase the VSWRbandwidth, 3 in. (75 mm) diameter tubing is used.

The antenna has an VSWR bandwidth of about 1%,so it makes an excellent single channel FM antenna.The antennas are usually mounted on the side of asupporting tower or pole, and stacked vertically toachieve required power gain.

This antenna has greater wind loading than the shuntfed version due to its larger element diameters neces-sary to achieve useable VSWR bandwidth. It presentsconsiderable large amounts of ac power for electricaldeicing. Plastic radomes also present additionalwind loading.

Flat Panel AntennasPanel antennas for CP FM broadcasting are rela-

tively new in the Western Hemisphere, although H-pol and V-pol have been used in Europe since themid-1950s. This antenna was developed in Europeto provide a wide bandwidth for several collocatedgovernment stations without the need to change anten-nas when a new channel was added or the operatingchannels were changed from time to time.

Panels are from 7 to 8 ft (2,100 to 2,450 mm) square

in the flat configuration. In the cavity style they areabout 8 ft (2450 mm) in diameter and about 3 ft (1,000mm) deep.

A heavy metal frame is often used over which largediameter wire mesh has been welded. The wire meshscreen openings vary from 4 to 12 in. (100 mm to300 mm). Electrically they are considered nearly solidmetal. These openings produce relatively low windloads. The entire flat frame or cavity is strong enoughto support a man on its mesh openings. Some manufac-turers hot dip galvanize their steel after fabrication;others use stainless steel construction.

For FM use, two crossed dipoles are used as theilluminating source for each panel or cavity. Eachdipole is fed in phase quadrature, that is one dipolereceives its peak current 90 after the other, to produceCP. A typical set of electrical and mechanical specifi-cations for a CP 8-bay cavity community antenna isshown in Table 4.12-6.

Flat panel antennas are typically sidemounted onlarge face size towers. The screen panels greatly reduceinteraction and distortion between the antenna andtower. The panels are directional, thereby requiring 3or 4 panels to be mounted around the tower to achieveacceptable azimuthal circularity.

These antennas are usually branch fed, and oftenthe arrays top and bottom halves are fed separately.This allows operation of either half of the antennaseparately when it is necessary for maintenance orrepairs. Circular polarization is achieved on each panelby feeding two perpendicular dipoles 90 out-of-phase.This phase offset helps this antenna achieve usablebandwidths on the order of 10 MHz.

By pulling the dipole back on its feed support arms,the arrowhead shaped dipoles control both V-pol andH-pol azimuth patterns. Rotating the dipoles 45 withthe earth-ground reference improves the polarizationratios even further.

Round dipoles made of tubing as large as 618 in.(155 mm) in diameter are used along with a single linequadrature feed. This combined arrangement makes anexcellent wideband CP panel to cover the entire FMband. Power splitters, dividers, and cables, along witha number of these panels, completes the antenna.

Even on large face towers, circularity in the H-polcan be quite good, on the order of 62 dB. On standard

Table 4.12-6Typical measured community antenna performance

Operational frequency range . . . . . . . . . . . . . . . . . . . . . . .88 to 108 MHzSafe RMS input power rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 kWPower gain ratio, each polarization . . . . . . . . . . . . . . . . . . . 4.4 (6.43 dB)Maximum VSWR any frequency between 88108 MHz . . . . . . . . . .1.1:1Elevation pattern beam tilt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10.5Polarization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Right hand circularAxial ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Better than 2 dBAzimuth circularity Vpol or Hpol . . . . . . . . . . . . . . . . . . Better than 62 dBAntenna dead weight, less than. . . . . . . . . . . . . . . 7,000 Lbs (3,183 kgs)Active wind load, RS-222-C 50/53 PSF . . . . . . . . . 8,000 Lbs (3,636 kgs)Antenna input flanges, two, size . . . . . . . . . . . . . . . . . . . . . . . .6-1/8 inchNumber of bays (stacks) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EightRadiator type . . . . . . . . . . . . . . . . . . . . . . . . . .Circularly Polarized Cavity

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arms, which result in evenly radiated patterns in allpolarization planes. The bandwidth of a single cavitycan cover the full 20 MHz band with a VSWR betterthan 1.1:1. Therefore it is not necessary to skew mountthese antennas for bandwidth considerations.

This antenna has the advantage over some otherdesigns of greater VSWR bandwidth. It is consideredcloser to state-of-the-art due to better elevation andazimuth pattern control of both planes of polarizationsby the shape of the cavity.

Cavities and flat panels can be modeled using acomputer. Factors such as tower size and orientation,as well as the phase and skew of the elements, can bemodeled to determine optimum mounting and feeding.This is useful in projects which require a directionalpattern. A station should take pattern constraints anddesires to an antenna manufacturer to find out whatis feasible.

Crossed Dipole TheoryCommon to the flat panel and the cavity is the

operation of the dipoles which generate CP. The di-poles are fed currents in phase quadrature, through acoaxially balanced balun, which provides equal cur-rents to all four arms of the two dipoles. They excitethe entire cavity or flat panel with a rotating RF fieldin a plane parallel to the dipoles. The RF field is thusCP and may be ideally represented by a rotating vectorof constant magnitude revolving one revolution perwavelength of propagation distance. It is right handpolarized as the field rotation is clockwise as viewedfrom behind the screen, looking toward the directionof propagation, if the phasing between the two crosseddipoles is properly made.

Radiation patterns, associated beamwidth, and di-rectivity are determined to a large extent by the sizeof the cavity or flat panel. The geometry of the dipolehas less effect than the reflector size. The size andshape of the dipole controls the antenna impedanceand the VSWR. The screen panel, be it flat or a cavity,fulfills the following five important electrical func-tions:x Isolates the radiating elements from the tower or the

mounting structure, and reduces mutual couplingx Provides sharper beamwidth and more gain than

achievable with the dipoles alonex Furnishes pattern control so that the beamwidth is

nearly equal for both horizontal and vertical planepolarization

x With an effective balun feed system, the crosseddipole radiated pattern phase is very uniform as theamplitude changes normally with azimuth

x Computer aided designs are easily achieved in pro-duction for various width towers because the patternis simply pure electrical geometry.

Antenna Element Spacing and DownwardRadiation

Most FM antennas have elements that are spacedone wavelength apart (9 to 11 ft) for reasons such as

configurations, the V-pol pattern is quite different. Asa result, the axial ratio of this antenna ranges fromgood at some azimuth headings to rather poor at others.This is because the azimuth pattern of a H-pol dipoleis like a figure eight, or cosine function, while thepattern for a V-pol dipole is not directional in its azi-muth plane. Therefore, each polarization will reactquite differently when mounted in front of a panel.

Dipoles on these panels are often mounted at 45referenced to the ground. This has no effect on theaxial ratio or pattern performance, it instead is donefor tuning considerations to compensate for mutualcoupling.

Variations to this design have reduced the differ-ences between the patterns of the polarizations. Thesetechniques are effective for applications requiring onlya few MHz of bandwidth. One method optimizes theangle of the dipole bend as well as its distance to thepanel. This design requires three panels to be mountedaround a tower. Axial ratio and pattern circularity areimproved at the cost of system bandwidth.

Another method uses four dipoles forming a squareshape in front of the panel. By adjusting the spacingbetween the dipoles, the beamwidth of a panel canbe controlled. Over a small bandwidth, the patternperformance is greatly improved. It is necessary tomount four panels around a tower for a circular pattern.A large amount of panel interaction and leakage aresevere design limitations.

For projects that require wider bandwidth, skewmounting is often used. This physical configurationallows the panels to be fed in mod 1 (0, 90, 180,270 phase for 4 around) which can increase the band-width of the system at the input. Although skew mount-ing deteriorates pattern performance, the increase inbandwidth extends its applications.

Cavity Backed Panel AntennasThe use of a cavity screen instead of a flat panel

has greatly improved axial ratios. The cavity acts asa resonator with little leakage towards the tower. Theshape of the azimuth pattern in each plane becomesboth controllable and symmetrical. System bandwidthis improved over the flat screen design. By adjustingthe diameter of the cavity structure, beam widths canbe altered to meet specific requirements. Mountingthree cavities around a tower gives good pattern circu-larity. Axial ratios usually range from good to ex-cellent.

The cavity antenna uses the reflective propertiesof the flat screen panel. In the cavity however, theilluminating dipoles are flat instead of round and allfour arms are parallel to the plane of the cavity. Likethe flat panel with its round dipole supporting balun,the cavity also holds its flat dipoles with a doublecoaxial balun.

The dipoles in the cavity get their wide VSWRbandwidth through the sleeve dipole principle.18 Ca-pacity is provided by a metallic ring close to all fourdipole arms placed between them and the back of thecavity. Circulating surface currents flow on the dipole

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gain considerations, mutual coupling effects and easeof feed design. There are cases, however, which requirea different element spacing. High levels of downwardradiation is the most common reason, although consid-erations such as aperture constraints and beam shapingalso utilize nonwavelength spacing.

Radio frequency radiation (RFR) safety levels mustbe considered in nearly all site locations. Power radi-ated in the lowest sidelobe can cause a variety ofproblems, of which human exposure levels are mostcritical. See Chapter 9.4, Non-Ionizing Radiation, forfurther information.

When antenna elements are arrayed, the resultingelevation pattern contains lobes and nulls. The furthestsidelobe from the horizon typically peaks between 70and 90 below the horizon for full wave spacing. Thislobe occurs since the physical path results in no phasecancellation in that direction, and thus each elementsdownward radiation is additive. Shortening the spacingchanges the difference in the elements physical pathlength, and results in some phase cancellation.

Half wave spacing, for example, greatly suppressesthe levels of the side lobes, while increasing the widthof the main beam. Despite the lack of power in theside lobes, the extra width of the main beam causesthe pattern to be less directive. Thus, this exchangeresults in an overall gain reduction on the order ofa third. Spacing the elements 0.8 wavelengths apartimproves sidelobe suppression and the gain of theantenna is not greatly affected.

Short spaced antennas are fed either by a shunt orbranch feed system. For half-wave spaced antennas, ashunt line delivers each element 180 out-of-phasewith the next element. This phase distribution problemcan be overcome by flipping every other element up-side down, thus inducing a 180 phase shift in thefeed. Other spacings, such as 0.8 wavelengths, requirea branch feed, which can deliver equally phased signalsto each element independent of spacing.

Problems With Sidemounted AntennasSingle station FM antennas are typically side-

mounted on a pole or tower. Unlike panel antennas, thesupport structure greatly effects or distorts the radiationpattern. The resultant pattern may have large peaksand nulls that can result in coverage and receptionproblems.

In addition, the V-pol and H-pol patterns react quitedifferently to these distortions. Due to the geometriccomplexities of the CP radiating elements and towerstructure, computer modeling exists to accurately pre-dict pattern effects is a difficult proposition. Therefore,the use of test ranges are required to determine howan antenna behaves when mounted on a tower sectionsimilar to the one on which the antenna will eventuallybe used.

For nondirectional stations, a test range can deter-mine the proper mounting of an antenna to achieve anacceptable circularity. Depending on the tower size,the depths of nulls can be greater than 10 dB. Anoptimized mounting configuration can make the nulls

less significant and oriented in areas where service tothe primary coverage area is not hurt. Parasitic reflec-tors are often used to improve the circularity of nondi-rectional antennas.

When the top spot on a tower or structure is avail-able, pole mounting is often preferred. A pole providesa stable and symmetrical support structure which haslow interaction with the horizontally polarized compo-nent. In combination with the feed line, the pole typi-cally induces a null in the V-pol pattern, directly oppo-site of the elements. Proper orientation of the elementcan reduce the effects of pattern distortion.

Mounting an antenna on the side of a tower canproduce unpredictable results. The position of thepeaks and nulls vary greatly from the orientation ofthe elements. As the face size of the tower increases,the distorting effects magnify. To compensate, manystations use smaller sections of tower at the top, wherethey plan to install the FM antenna. Eighteen-inch andsmaller face towers tend to produce good results. Withcareful planning, the use of a 24 in. or larger facetower can also be successful.

Note that the patterns of each polarization reactdifferently, and thus axial ratios can be quite poor.

PATTERN OPTIMIZATIONSingle-station FM antennas are usually sidemounted ona pole or tower. This is economical and it frees the towertop for other possible uses. Unfortunately the pole ortower tends to distort the radiation pattern, seriously af-fecting station coverage in some directions.19

This problem can be serious if the FM antenna hasbeen randomly attached to a support tower. FM an-tenna makers do not manufacture and sell towers. Afew have made supporting poles on which the FMan

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