Here is o downlo-eorth, non-engineering level description of whotbosic wire ontennos ore, mefhods of coupling to them, ond some

Wire Antennas: A PrimerBY BOB SHRADER, W6BNB (SK)

interesting possibilities when using them.

Editoft note: The author, now a Silent Key, was a frequentCQ contributor and the author of Electronic Communi-cations, a standard engineering textbook published inmultiple editions between 1959 and 1991. We present thisarticle with the author's original hand-drawn illustrations.

I ll that is a dipole? Since the beginning of amateurItlt radio, one of the basic horizontal wire antennas! f has been the dioole. lt is a wire cut to a half-wave-

length (1/2-wave) for the middle of the band of frequencieson which it is to be used. A 1|2-wave wire allows electronsto oscillate back and forth along it most easily at its reso-nant frequency length.

r'[ipsls" can be considered to mean the shortest antennathat can have only two different maximum voltage polarities.When one end is at a maximum positive polarity, the otherend will be at a maximum negative, with zero polarity in themiddle. lf it is a little too long or too short, the true maximumvoltage cannot be developed on the wire. lf a radio frequen-cy (RF)AC voltage drives enough negative electrons to pro-duce a 500-volt negative charge at one end of a dipole, theotherend, having now lostthat numberof electrons, becomes500-volt oositive.

An important question for the radio amateur is: To whatlength should a dipole antenna be cut? The 160-meter band,which is 1,8 to 2.0 MHz, is our only medium frequency (MF)band. MF means 0.3 to 3 MHz, Our other 10 high frequen-cy (HF) bands are between 3 and 30 MHz. The length of adipole in feet for any of these bands can be determined if thedesired frequency of operation is known in megahertz (MHz).The formula is:

Dipole length in feet = 468/MHz

This formula includes the required shortening to t95% ofa dipole's length factor because of its two capacitive "endeffects." When there are two or more dipole wires connect-ed in series, any added dipoles should all be computed by ano-end-effects-shortening formula, or:

Second or other dipoles in feet = 492lMHz

All of the answers obtained will be the operating length ofthe wire between the holes in the two end insulators. Add 3or 4 inches of wire at each end to go through the insulatorholes and come back to be twisted back around the dipolewire. lf an antenna has to be low or erected near metalobjects, shortening it a little might help its operation.

Let's assume a dipole should be a 1|4-wave above its"effective ground level." And what is that? A good effective

ground level would be the surface of a salt-water-soaked soil,or marsh, extending out many wavelengths in all directionsunder the antenna. lf the soil is dry and possibly sandy orrocky, the effective ground level may be one to several feetbelow the surface. The effective ground level at a station usu-ally depends on how wet or dry the soil happens to be thatday. You can see we may be playing with rubbery numberswhen talking about antennas.

Small length variations will probably not affect how well anantenna seems to radiate and receive. Antenna theory can bean exact science for any given f requency, but just getting closeto the correct length for a mid-band dipole for our relativelywide amateur bands may be all that a ham needs to worryabout. lf he (or she) uses an antenna tuner, that can correctfor most slightly incorrect dipole lengths, heights, etc.

It is generally agreed that doubling the height of a dipoleabove ground level almost doubles its effectiveness for bothtransmitting and receiving. So tall poles, towers, and treeslook pretty good to radio amateurs - but unfortunately notto the people who make the laws on allowable antennaheights and antenna placements in cities and other places.

A Dipole AntennaThe antenna in Figure 7 is an 8O-meter band, 1i2-wavedipole, cut for some frequency in the "CW" (Continuous-strength Wave) part of that band, let's say for 3.550 MHz. Byusing the formula:

length = 468 /3.55 = t131.8 ft long

lf the antenna happens to be cut a foot long or short, itshould stil l work fine, even if an antenna tuner is not used.Most transceivers today have an internal output "Tune" but-ton that corrects smallvalues of incorrect inductive or caoac-itive reactance (detuning) exhibited by the antenna.

A dipole for the 80-m "radiotelephone" ('phone),band, let'ssay 3.900 MHz, should be:

fength = 468/3.9 = tl20ft long

Itls interesting that either of these dipoles will work quitewell over the whole CW and phone parts of the bands ifopen-wire feeders and an antenna tuner are used, asdescribed later.

A dipole wire cut apart in the center becomes two 1l4-wavewires. lf such a dipole is fed RF at its center by some kind ofa 2-wire RF "transmission" or "feed" line, it will be a "bal-anced" dipole. The open middle ends can be said to have a"radiation resistance," or a "center feed-point impedance" of+73 ohms to RF AC. This "feeder impedance" is abbreviat-ed as "Z1." ("2" for i mpedance and "f" f or f eeder or f eedpoi nt).

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Figure 1. Basic dipole lengths and impedances. The high Zs,or feedpoint impedance, is at the ends of the dipole. (All

illustrations are hand-drawn originals by the author)

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Figure 2. Connecting a coaxialfeeder to a dipole.

The center of a dipole has a low 21, bul the ends alwayshave a high 21, a value of let's say +2,500 ohms, but it mayvary considerably due to length, wire thickness, ground mate-rial, etc.

High Z points are also points of high RF vottages and tit-tle current. Low Z points carry higher currents because alloscillating electrons have to go through these points andvoltage values will be low. So a dipole antenna wire couldtheoretically be thinner at its ends and thicker at its center,but it is usually a simple bare copper wire having a wiregauge of #1 O to #1 6. (A thinner lacquer insutated #22 gaugewire might be used if it is supposed to be "hidden"!) lf thewire is insulated, it works fine, but if the insulation is solidplastic, in a few years it will dry out, begin to peel, hangdown, and some of it may drop off, making the antenna'sappearance pretty crummy.

Rotary dipoles for higher frequency bands, or the elementsof beam antennas, usually use aluminum oi some othermetal tubing. The wider diameter and greater surface areamay suggest slightly shorter computed lengths. This is usu-ally disregarded, but the diameters are usually reduced asthe ends are approached to reduce tubing weight and phys-icaldrooping.

lf the two middle ends of a dipole are connected to a 2-par-allel-wire transmission line having an inductance-to-capaci-tance ratio that produces a 73-ohm 21the antenna and thetransmission line impedances will match at the antenna feedpoint. This allows RF energy to transfer between feedlineand the antenna with essentially no loss.

But amateurs often use a 50-ohm transmission line ratherthan 73 ohms. Why? The modern manufacturers of amateur

radio equipment usually design their outpuVinput circuits tohave a t5O-ohm impedance, for good reasons. A horizontaldipole has a center Zl of r73 ohms; the common vertical 1/4-wave antenna has a base Zl of +36.5 ohms; and beam anten-nas usually have 50-ohm Zf inputs. A transceiver's 50-ohm21 outpuVinput is about halfway between horizontaland ver-tical antenna center impedances, and is a match for beamantennas. The difference in operation between 73 and 50ohms, or between 50 and 36.5 ohms, is not too much, par-ticularly with modern transceivers having an internal reac-tive correcting circuit that tunes the antenna to operate witha minimum SWR (below).

Coaxial CablesTrying to produce a 2-parallel-wire 50- or 73-ohm transmis-sion line with its wires held apart the required small fractionof an inch to allow them to provide the desired impedancealong the whole line's length is not too practical, particular-ly in wet weather. This why 50-ohm Zl"coaxial" cable trans-mission lines are usually used. The center wire of a coaxialcable would be connected to one of the two 1/4-wave wiresof a dipole, as shown in Figure 2. The outer metallic con-ductor of coax is a tubing-like copper braid around the thick-insulation covered center wire. This outer braid would be con-nected to the other 1|4-wave wire. A small strain insulatorshould be used between the two dipole wires.

f f RF AC power feeds f rom a transceiver's 50-ohm Z+ oul-put fitting into the end of a SO-ohm coaxial cable and'theninto the 73-ohm antenna, energy will couple transceiver-to-transmission line-to-antenna with some loss of radiatedenergy from the dipole, but now some of the RF energy willbe radiated from the more or less vertical coax cable'souter braid.

Long coaxial lines have greater losses due to the energyabsorbed by the resistance present in the internal insulatingmaterial. Also, the higherthefrequencyof the RF, the greaterthe losses in coaxial cables. However, they can be cut to anylength, assuming the impedances at both ends match rea-sonably well. The open end of a coax cable where it is con-nected to the antenna must be coated with some type ofwaterproofing and insulating substance.

The arrows shown on the dipole's RF radiating flat top inFigure 2 indicate that at this particular moment, RF currentis flowing in the same direction in both of the 1/4-wave wires,but it will be running in the opposite direction in the feedline(Remember, RF is an alternating current), A half-cycle later,the flat top RF currents will both be flowing in the oppositedirection, reversing all polarities. Since the two 1/4-wave flat-top currents are flowing in the same direction, they radiateas a single 1|2-wave dipole.

lf the 8O-meter antenna in Figure / is used on 4O-meters,each half of the dipole is now one 1/2-wave in length, pro-viding high Zlto any low 2150-ohm coaxial feeder. The two1|2-wave wires will now accept very little power to be radi-ated as a transmitted signal, although the outer braid of thecoaxialcable will now be radiating some RF energy,

SWRRF feedline energy recognizes any mismatched antennaimpedance connection when it comes up to it. Some or mostof its energy will be reflected back down the transmissionline to the source (all of it in the case of a short circuit at theantenna feedpoint end) and "standing waves" of voltage orcurrent are developed on the transmission line.

The ratio of the highto-low voltages (or currents) alongthe line develops a "standing-wave ratio" (SWR) value that

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Figure 3. One form of a balun involves wrapping several turns of both the bal-anced antenna wire and the unbalanced coaxial feedline through a ferrite core.

Figure 4. A half-wave open-wire feed line coupled to the end of a dipole. See textfor discussion of the LC tuning circuit and the two 100k resistors to ground shown

in dashed lines.

can be shown on a "reflectometer" con-nected in the transmission line. Thegreater the impedance mismatch, thehigher the SWR and the less trans-mission of energy to the antenna. Anexample would be trying to feed thehigh-Z1of 12,500 ohms at the end of adipole with a 50-ohm coax l ine, a+2500/50 mismatch, and an SWR of*50:1. Al though an SWR ot 2:1 mayreduce energy transmission and an-tenna radiation somewhat, asthe SWRincreases over 3:1, the antenna nolonger wants to accept much RFpower, so RF radiation decreases asthe SWR increases.

In the cases of matching a 50-ohmtransceiver and coax line to the centerof a dipole, the differences in imped-ances may be 73150 for an SWR oft1.46:1. When a vertical antenna baseis coupled to a 50-ohm line, the SWRwill be 50/36 or +1.4:1. Neither of theseis near the perfect 1:1 SWR ratio, butboth are close enough. Note that the

greater value is always divided by thelesser. An internal reactance-correctingcircui t can br ing the SWR to 1:1.

No matter how high the SWR, therewill always be some radiation andreception with any antenna. The greaterthe SWR in a coax cable, the greaterthe transmitting loss will be becausesignals will be reflecting back and forthin the coax due to the mismatch, losingenergy on each reflection, lt is interest-ing that the reception of RF signals witha high SWR will be much betterthan willbe its transmission of RF power. Tuninga transceiver's antenna improperlymakes this quite evident - with almostno transmission of RF there are usual-ly fairly strong received signals.

The BalunWhen using a coaxial-fed dipole, therecan never be equal capacitancesbetween the right and left halves of thedipole to both the inner and outer coax-ial conductors. Simple coaxial coupling

to a dipole, while it works well, producesan unbalanced antenna feed system.The outer coaxial braid, from transceiv-er to the antenna, will emit paft of thetotal RF radiated energy, more or lessvertically. To correct this, a smallferrite-ring with two small coils on il forms anRF transformer. lt can be used to cou-ple the BAlanced dipole wire halves tothe UNbalanced coaxial cable, asshown in Figure 3. lf such a "balun" isused with a coax-fed antenna, the sys-tem becomes balanced. The dipoleradiates horizontally and all of its feed-line RF waves are kept inside its coaxcable. There are several different typesof baluns, and some may be more fre-quency-sensitive than others.

Dipole HeightsThe center impedance of a dipole at dif-ferent heights is interesting, lf lying onthe ground, it has a center Zl ol only afew ohms. As it is raised, its center Z+continually increases up to its first 73ohms value at a height of 1/4-wave,Continuing up in height, it goes througha 21 peak of about 97 ohms beforedecreasing to 73 ohms again al a 112-wave height, Continuing on up to its 3/4-wave height, it dips down to about 58-ohms 21 before rising back to 73 ohmsagain. From there on, its impedancepeaks and dips vary less and less aboveand below 73 ohms every 1/4-wave inuntil above about 3 wavelengths inheight, its 21 remains essentially con-stant at 73 ohms. All this can makematching the Zl of a dipole to a feedlinevery interesting.

Harmonic TransmissionCenter-fed dipoles can also operate onthet odd harmonic frequencies, Forexample, a 7-MHz band dipole alsoworks on its third harmonic, in the 21-MHz amateur band. The 7-MHz dioole isnow working as three 1|2-wave dipolesconnected in seriqs. The low-Zr centerpoint of the middle dipole may now be alittle higher than 73 ohms, but the mis-match may not be allthat important, par-ticularly if a balun is used. (Other har-monics of frequencies in the 3.5-to-4MHz and the 7-to 7.3-MHz bands alsofall into other HF ham bands.)

Coaxialfeed can be used to any pointthat is 1/4-wave from either end of anymultiple-1/2-wave long antenna wire,or to the center of any of its 1/2-wavesections.

Open-Wire Transmission Linesl{ow can a dipole be made to acceptpowerefficiently if it is fed at its high-impedance (high-voltage) end? An-

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swer: Match its high-Z1(t2,500 ohms)end with a high-Z transmission line.

A transmission line using two parallel#14 wires held an equal distance apartby tS-inch-long thin insulator spacerrods every 3 to 5 feet along the line, canproduce a very low loss feedline oft600 ohms Z+. While 600 ohms match-es the end Zl of a dipole better than a50-ohm coaxial cable does (SWR =12500/600 - *4:1), this is stil l not good.With a 4:1 SWR, the dipole accepts lit-tle RF with considerable vertical radia-tion from the feedline because line cur-rent maximums do not line up to cancel.

Cutting such an "open-wire transmis-sion line" to a 1/2-wavelengfh produceshigh-Z1at both ends. Whenculto a 1/4-wavelength, if it sees a high Zl at oneof its ends, it will have low Zl at its otherend. When operating as atuned 112- or1/4-wave open-wire line, exactly equalspacing between the wires is no longertoo important.

Larger antenna wires have less RF"skin resistance" because RF AC trav-els only on or near the surface of wiresresulting in less RF loss with larger-sizeantenna wires, lf resonant feedlines area little too long, they are "inductivelyreactive." Small capacitors can beadded in series with them to balanceout the inductive reactance. lf too short,they are "capacitively reactive" andsmall tapped coils can be added inseries with them to balance out thecapacitive reactance.

Cqaxial cables can also be used astuned lines. They must be cut to 10.66of the 468/MHz or 492/MHz values,making them more or less single-banddipoles.

Coupling Open-Wire LinesSuppose the high-21 end of a dipole isconnected to one end of a l/4-waveopen-wire transmission line, The trans-mission line sees the dipole's high-Zend so it assumes a high-Z value there.A 1/4-wave down the line, its imped-ance will now be a low-Z value and pro-vides a reasonable match to a 50-ohm21 transceiver antenna fitting.

It a 1/2-wave long open-wire trans-mission line is attached to the high-21end of a dipole (see Figure 4), it willrepeat its high-Z value at the trans-ceiver end. This length requires a high-Z antenna tuner between the feeder'shigh-21 value and the low-21 transceiv-er antenna fitting. What makes up ahigh-Z circuit? A parallel coil and capac-itor (LC) circuit tuned to the operatingfrequency is one example of a high-Zresonant circuit. A high-21 transmissionline's wires can be connected directly

across such a high-Z tuned LC circuit.Antenna tuners usually have parallelresonant circuit LC circuits as their out-put circuit. The antenna tuner must alsoprovide a reasonably good match to thelow-21 transceiver antenna fitting. Thiscould be by using a few-turns low-impedance "link coupling" coil coupledinto or coiled around the center turns ofthe tuner's LC circuit, as indicated.When the tuner is tuned to the operat-ing frequency, the transmission line willsee high-Z matches at both of its endsand the dipole will radiate RF energyefficiently. Antenna tuners use moreinvolved 50-ohm impedance couplingcircuits, The impedance values areshown in capital letters.

When center-feeding a dipole, themaximum radiation is at 90 degreesf rom the wire direction. When end{eed-ing a dipole, the maximum radiationdirection is slightly more in line with thewire and away from the feedpoint. Thelongeli the antenna, the more exagger-ated this effect becomes.

lf a1/2-wave open:wire tuned line cancouple an end-fed dipole to a high-Zantenna tuner, couldn't a 1/2-wave-longsingle wire (high Zl at both ends) coupleenergy to the end of the dipole? lt could.But with a 2-wire tuned open transmis-sion line, the currents in the two parallelwires are equ albut opposite in direction,canceling RF radiation from them. Theonly radiation is from the dipole. With asingle 1/2-wave wire feeding the end ofa dipole, both the 1-wire feeder and the

antenna will now radiate energy. Half ofthe power will be radiated by the hori-zontal dipole and half by lhe essentiallyvertical one-wire 1 /2-wave teedline. Twosuch series connected 1/2-wave reso-nant wires form a "full-wave" antenna.An interesting thing about this antennais that when RF current is going outwardfrom the antenna tuner, at the sametime, the RF current will be going inwardon the horizontal dipole. A half cyclelater, both currents reverse.

Different Types of DipolesThere are several different antennadesigns used by amateurs that areeach variations on the basic 1/2-wavedipole. We'll take a look at the mostcommon ones, starting with one that -at first glance - doesn't seem to eitherbe a 1/2-wave antenna or a dipole"

The Quarter-Wave \d{calAntennaA common amateur antenna is a metalpole or wire 1/4-wavelength long, withits bottom end grounded (see Figure 51.It is actually half of a vertical dipole, withthe earth operating as the other 1/4-wave element to make it a resonant 1/2-wave circuit. Actually, the earth oper-ates as an almost infinite number of1/4-, 314-, 514- elc. wave elements ex-tending outward in alldirections.

Making a glood RF ground connectionfor a vertical antenna can be difficult.An 8-foot, copper-clad iron pipe is often-

Figure 5.A quarter-wavevertical.Dashed linesrepresentradials.

Figure 6. How an antenna grounding switch is connected. The DPDT switchassures that both antenna leads are grounded. You need to be sure that the

switch you choose can handle the maximum power you will be transmitting.

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driven into the ground to form theground connection, but where is theeffective ground level? To provide a bet-ter ground system, four or more 114-wave wires may be laid out in differentdirections from the base of the antennato form a "radial system." The radialwires may be laid on the ground, beburied a few inches under the ground,or may even be on top of a building ifthe antenna is also up there. lf only oneradial wire is used and it is going out-ward toward the north, the antenna willradiate more of its RF northward.

When a 1/4-wave vertical antenna isopened at its base, it has t36.5-ohmfeedpoints. These are usually fed byamateurs with 50-ohm coaxial lines.Maximum radiation is only a few degreesabove the horizon with vertical anten-nas, making them very desirable as DXantennas. They have zero RF radiationdirectly above them, making them use-ful as aviation markers.

Marconi AntennasAny antenna using the earth or groundto make it resonate properly is knownas a "Marconi" antenna. lt may be a 1/4-wave vertical or a horizontal antennasome odd multiple of a 114-wave long,although an exact length may not be tooimportant if an antenna tuner is used atthe transmitter end. Shipboard radio-telegraph low-frequency (90-160 kHz)Marconi antennas were usually madeas long as possible with loading coilsadded to them. A 90-kHz 11 -wave shipstation antenna should be: 46812, or2941.09 = t2600 ft, or t0.5 mile long- on a ship? A whole lot of loading coilinductance was needed between thetransmitter and a ship's t250-footantenna wire to make it work like a 1/4-wave Marconi antenna.

An old-time amateur method of cou-pling the near end ol a1l4-wave, single-wire Marconi antenna to a transmitteroutput was to push a two- or three-turnlink coupling coil into the nearest-to-ground-potential turns of a transmitter'soutput LC circuit. The farther this link waspushed into the coil, the tighter the cou-pling was to the antenna circuit. The f reeend of the link coil might be eithergrounded or be connected to a variablecapacitor or inductor to ground for bestantenna tuning. Today, antenna tunersare usually used to couple Marconiantennas to transceivers.

Hertz AntennasAny antenna that does not depend onthe ground or eafth to make it resonant,such as a dipole or any multiple ol a112-

wave wire, is known as a "Hertz" anten-na. But a Hertz antenna should alwaysbe resistively grounded in some way todischarge any possible high DC staticelectric charges that may build up on it.Very high-voltage DC charges can buildup on a large ungrounded antenna sys-tem just by its being in the atmosphere.For the Hertzian antenna in Figure 4, a1 - or2-watt, 1 00,000-ohm resistorcouldbe used from both open-wire transmis-sion line wires to ground (see dashedlines) to assure the antenna wires wouldalways remain DC discharged.

Coaxial transmission lines to Hertzantennas are normally groundedthrough the output circuitry of the trans-ceiver. But this is no protection againstnearby lightning strikes. The best pro-tection from lightning strikes is to com-pletely disconnect the antenna from therig and connect it to a metal pipe driveninto the ground, or to any other goodground connection outside the building.An antenna grounding switch is shownin Figure 6.

Windom DipolesA single #14 copper wire alone in air issaid to have a self-impedance of t500ohms. Such a single wire can be usedas a transmission line if it is connectedto a +500-ohm point on a dipole, usual-ly 114% out from its center (or to a sim-ilar point on a vertical antenna) and thento a similar 21 point above ground on anantenna tuner. This is known as a "win-dom" antenna. How much vertical radi-ation it also puts out depends on thelength of the feeder wire. One thingabout this antenna, at all frequenciesother than the computed one, it proba-bly tunes as a top-loaded vertical anten-na on any band with an antenna tuner,The operator may never know how it isworking. Windom antennas can also

use a parallel 2-wire feeder to cancelradiation from the feedline if the off-cen-ter impedance opened point equals theimpedance of the feedline.

Zepp AntennasA dipole fed at one end with a tunedopen-wire transmission line is known asa"Zepp" antenna. In the early 1900s,when gas{illed, lighter-than-air dirigi-bles flew the skies of the world, suchzeppelins (named after Ferdinand vonZeppelin) dropped this type of antennaout of the radio-operating cabin while inflight. A fairly heavy insulator attachedto its end kept the antenna and its trans-mission line taut and more-or-lessstraight horizontally. Your author com-municated with the Graf Zeppelin byCW on 500 kHz f rom his passenger shipwhile the Graf Zeppelin was flying nearSpain in the mid 1930s. Incidentally, hisfirst ham antenna in 1931 was a 40-meter Zepp (but is now a 4O-meter dou-ble Zepp). Today, blimps seen flyingabove football fields use small VHF orUHF antennas to send TV and audiosignals to their local ground stations.

A vertical VH F dipole with a 1 /4-waveopen-wire transmission line attachedto the bottom end of the dipole formsa "J antenna," The two bottom open-wire transmission l ine ends can beconnected directly to a coaxial cableand to a transceiver. Or, the two open-wire line ends and a coax cable shieldmay be connected together with thecable's internal lead going up a shortdistance to a 50-ohm point above theshorted parts on either of the trans-mission wires, usually through a smallcapacitor. J-antennas are popular"omnidirectional" (all directions) VHFor UHFvert icalZepp antennas with thetransmission line coupled to the trans-ceiver through a long coaxial cable,

Figure 7. Possibly the best ham wire antenna, says the author, is an 1}-meterdipole fed with open-wire line and an antenna tuner. See text for discussion of

the dotted letters and the arrows indicating direction of current flow.

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allowing the little antenna to be mounted high enough forgood radiation and reception.

A Multi-Band AntennaPossibly the best wire antenna for all HF amateur bands isshown in Figure Z. lt does require an antenna tuner. lt is a cen-ter-fed (CF) 1i2-wave 80-meter dipole. lt might be 1130 feetlong with a1l4-wave t65-foot long open-wire transmission line.On 80 meters, it has low Zl at its center. Moving down the 1/4-wave feeder, the open-wire line presents a high-Z to the anten-na tuner. Coupling the antenna tuner to the transceiver mightbe by link coupling or some other low-Z coupling system.

The tuner's resonant LC circuit acts electrically like a 1/2-wave dipole. Every 1/2 wave along an antenna or feed line,the RF voltages reverse polarity, so they do the same acrossthe tuner's LC circuit. The solid arrows show the 8O-meterRF currents present at some particular time. High and lowvoltage andZl points are indicated by the solid H and L let-ters. The currents in both of the 1/4-wave horizontal wires ofthe dipole are in the same direction, producing one complete1|2-wave dioole radiator on the 8O-meter band.

On the 4O-meter band, the flat top is now two end-fed 1/2-wave dipoles coupled to the transmitter by a 1/2-wave tunedfeedline. The dipoles, feeders, and the antenna tuner all havehigh-Zl points at their connections. The high and low voltageand Zl points are now indicated by dashed H and L letters.On the 40-meter band, this is two separate Zepp-fed dipoles,or a "double-Zepp."

As a 4O-meter double-Zepp, the current in the left dipolemight be going left, as shown by lhe dashedarrow, The cur-rent then reverses and goes downward in the left-hand feed-er. lt reverses again across the resonant LC circuit. lt revers-es yet again to go upward on the right-hand feedline, andfinally reverses to leftward on the right-hand dipole. So, thetwo 1i2-wave dipole flat-top currents are going in the samedirection, or are radiating "in phase." They are now radiatingtwice as much power at exactly right angles to the wires ofboth dipoles. This is a power gain of two times, or 3 dB, inthis direction. This is actually a 2-element wire-type beamantenna. Because the currents in the two feeder wires are inopposite directions, radiation from them is zero.

The next higher frequency amateur band is the 30 meters,a U.S. no- 'phone band from 10.1 to 10.15 MHz. Here, eachantenna radiating section is + 0.75-waves long, or three 112-waves long. An antenna tuner will tune it to a 1:1 SWR. lt isnow an "extended double-Zepp." lt radiates 1/3 of its powerat 90o from the antenna wire and 2/3 of its power in two otherdirections at about 50o from the antenna wire.

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Figure 8a. Circular lobe of the horizontal radiation of a dipoleantenna, looking at it from the end of the wire.

On all other higher bands, starting with 20 meters, the anten-na can be brought to an SWR of 1:'l by an antenna tuner andoperate quite well in all directions, horizontally polarized.

lf the antenna tuner tunes to 160 meters, it can bring this80-mipole dipole circuit into resonance on Top Band as well,but the antenna will only be radiating as two 1/8-wave flat-top wires in series. lt is not very efficient, but it willtune to 1:1SWR and radiates about half power. lf there is enough realestate to double the lengths of the 8O-meter dipole and thefeedline to produce a 160-meter dipole, it will do much bet-ter on 160. On the other hand, if you can only put up a 66-foot 40-meter dipole with a 33-fool1/4-wave open-wire feed-line, it will also work on higher frequency amateur bands, butnot very efficiently on 80 meters.

lf the antenna tuner output circuit is not grounded , a2-inchdiameter, 20-turn, coil can be added in series with one of theopen-wire transmission lines in case the antenna and feedlines are nol quite long enough. lf the antenna/feedline com-bination is too long, a 200-pF variable capacitor can be addedin series with one of the feedlines. Bend the tip of one endrotor plate so it touches a stator plate to short the capacitorout of the circuit when it is not needed.

Horizontal Radiation LobesThe RF "radiation pattern" or "lobe" of a north/south dipolewire in outer space, seen end-on, is outward in all directions(Figure 8a), and is a circular lobe. lt is picturing the relativeradiation in all oossible directions.

When looking down on a N/S dipole, the radiation lobes toboth its E and W sides appear as slightly elongated circles(Figure 8b), with maximum horizontal radiation at 90" fromthe wire. This is picturing the total radiation as RF currentflows from one end of a dioole to the other end,

lf near earth, a horizontal dipole radiates equal RF in alldirections at right angles to the wire. Some of the downwardradiation may warm the earth a bit, but much of it may bereflected upward at many angles by the earth and may thenbe re-reflected downward by the ionosphere.

With the 40-meter double-Zepp, the radiation lobe at 90"from the wire would be twice as long as when it was used asa single 80-meter dipole. Since there is no more power being

Figure 8b. Horizontal lobes of a dipole with a north/southorientation, looking down from above.

www.cq-amateur-radio.com February 2017 o CQ o 55

transmitted, its radiation lobe width (a picturing of its totalradiated energy)would be reduced from the nearly 90'of asingle dipole to about 45". Actual lobe shapes may bechanged by antenna heights, nearby structures, trees, near-by wires, metal poles, etc.

The radiation pattern of a full-wave N/S antenna (twodipoles long) looking down on it in outer space is shown asfour major lobes, all roughly 50'from the antenna, with max-imum lobe widths of about 40" (Figure 8c). Note that the twodipole lobes cancel each other because the currents in themwill always be in opposite directions, resulting in zero 90'radiation. With a 100-watt transmitter, the total power radi-ated by a dipole, a double Zepp, afull-wave antenna, etc. willall be the same, but different amounts of the RF power willbe radiated in the different lobe directions. In between lobesare directions of theoretically zero radiation, called "nulls."

The radiation from an antenna that is 1.5 wavelengths long(3 dipoles long, such as the 30-meter extended double-Zeppantenna) in outer space is shown in Figure 8d. lts middle 1i2-wave section is developing two lobes at 90'from the anten-na. The other two dipoles produce four lobes like a full-waveantenna, but at about 40" from the antenna, with six lobesand six nul ls.

The radiation of two full-waves in phase (as an 8O-meterdipole on 2O-meters) is shown in Figure 8e.lts beaming effectdevelops four longer and slimmer main lobes at about 35"from the wire. Since four currents are cancelling each other,there is no lobe at 90o from the wire.

lf a receiver is in a null direction between two lobeS or offthe end of an antenna there should be no RF transmitted tothe receiver, However, the ionosphere, up 200 or so miles,is in constant motion and provides many constantly billow-ing reflective surfaces. Any RF signals striking them arereflected or refracted in many directions. So there will alwaysbe RF energy reflected or refracted to points on earth in unex-pected places. This is why signals are heard in null directionsand why signals may sometimes be heard in the expectedskio zone of a station.

Looking Ahead in CQ,..Here ore some of the orlicles we're working on

for upcoming issues of CQ:

o Schemotix: Tronsmit Circuit Diogroms Withouto Comouter

o Results: 20.16 CQWW RITY DX Contesto Phontom of the Attico DX History: Sponish Sohoro

Upcoming Speciol lssues

. June: Toke it to the Field

. Oclobet: Emergency Communicotions

. December: Technology

Do you hove o hobby rodio story to tell? Something forone of our speciols? CQ now covers the entire rodiohobby. See our writers' guidelines on the CQ website ot<http : //www, cq-o moteu r-rod io. com/cq_wriiers_guide/cq_wriiers_g uide. html>

Figure 8c. Lobes of a full-wave antenna, also orientednorth-south.

Figure 8d. Lobes of an antenna that is three half-wave-lengths long.

Figure 8e. Lobes of a two-wavelength long dipole. All ofthese may exist on the same physical antenna (see Figure7), depending on the frequency band in use at the time.

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The nulls of a handheld 1/2-wave VHF receiving dipole ora loop antenna can be used as relatively sharp directionalindicators of hidden transmitters a short distance away. Lobepeaks are never very sharp directional indicators.

A horizontal N/S dipole wire always radiates horizontallypolarized signals at right angles to the wire, or E and W. lt alsohas vertical radiation lobes. For heights up to almost 1/8-wave,most of the radiation is upward. At 1/4-wave height, the radi-ation is some upward but more outward. When 1/2-wave high,the lobes are all outward at t30o with nothing upward. Al3l4-wave, its upward lobe is greater than the outward lobes. At 1-wave high, there is only outward with nothing upward. For all1/2-wave heights above on full wavelength, there is nothinggoing upward. In between is both upward and outward, withthe upwards lessening as height increases.

The EAff lobes of a horizontal N/S antenna, being partiallyoutward, will be radiating to some extent in both northerly andsoutherly directions. The ionosphere can reflect and refractthese waves down to distant stations in both N and S areasand actually provide reasonable strength signals in these direc-tions - off the ends of the antenna - and in supposedly nulldirections! A full-wave N-S antenna wire radiates even moreRF in line with the antenna wire, producing quite strong N-Sdistant received signals as well as in the expected major lobedirections. The ionosphere reflects or refracts some addition-al signals down to earth in these directions.

Theoretically, at a short distance, a vertical antenna shouldpick up no signal from a horizontal antenna and vice versa.Actually, a horizontally radiated wave begins to lose its radi-ated angle. After a few miles a vertical antenna may pick uphorizontally transmitted waves quite well due to reflected sig-nals from the ionosohere.

Erecting DipolesA dipole operated ata1l4-wave height above effective groundlevel, or any multiple of 1|4-wave, provides a 73-ohm centerimpedance value to its transmission line. lf not at exactly 1/4-wavelength in height, it may have some other center imped-ance value. Actually, a dipole at about 0.2 wave and again atabout 0.6-wave heights has a center impedance of close to50 ohms and matches a 50-ohm coaxial cable nicely.

lf operating at less than a 1|4-wave height, much of its RFdownward radiated energy is reflected upward by the groundand may be re-reflected right back down again by the ionos-phere. As a result, the higher a horizontal antenna is the bet-ter it will be for long distance DX. lf lowered, its reflected sig-nals may provide better and stronger shorter distancereflected signals.

A wire dipole is fairly simple to put up and work with. lt canbe hung between two reasonably equal height trees, build-ings, or poles. Or its center may be fed at the top of a singletall pole, with its two flat-top wires dropping down to shorterpoles, making it a non-rhombic "inverted V" or a "droopingdipole" antenna. Dipoles usually have shorter skip-distancesthan verticalantennas. While categorized as bidirectional, aspointed out above, lowerdipoles are often more or less "omni-directional" or all-direction radiators.

For RF powers up to perhaps 500 watts, glass or other typesof end insulators need not be more than about three inchesin length. lt is usually better not to put too much tension onantenna wires or guy wires. A little slack helps ceramic orglass insulators in high winds.

Although more expensive and difficult to build, multi-ele-ment unidirectional rotary beam antennas provide strongertransmitted and received signals in the direction they arepointed. But that's another storv.

whot's newARRL Handbook and Operating ManualThe 2017 ARRL Handbook for Radio Communications andthe 11th edition of theABRLOperating Manualare nowavail-able. As we have said in the past, these two referencesshould be in every ham's library and ref reshed every so oftenas technology and operating modes progress.

The ARRL HandbookAs always, the Handbook covers the basics of RF com-

munication technology, including the fundamentals of elec-tronic theory, design principles and more. New additions for2017 include "A Revised Approach to Measuring CrystalParameters" and "Updated Details on the Placement of FilterStubs" forthe engineers amongus, plus more practical seg-ments on "Decoding Fox-1Satellite Telemetry," "A 30, 17and 12-Meter Antenna Project"and "A Raspberry Pi NetworkServeriOlient for Antenna Ro-tators." The book includes a CDwith allthe text and illustrationsfrom the print edition as well assoftware, PC board templatesand more. The hardcoverHandbook sells for $59.95; thesoftcover edition is $49.95.

ARRL Operating ManualThe first thing we noticed about the 1 1 th edition otthe ARRL

Operating Manualfor Radio Amateurswas that it appearedto be thinner than the 1Oth edition, and indeed, it is nearly100 pages shorter, The contents have been completely reor-ganized, with 14 specifically-focused chapters in the 2012edition changed to four broadly-focused chapters in thenew book:

. Basic Station and Operating Techniques

. Radio Clubs and Public Service

. On-Air Activities and Radiosoort

. Resources for the Active Ham

Most of the subjects covered inthe previous edition are includedwithin those four broad subjectareas. lt is significant, though, thatthe 1Oth edition chapter on traffic-handling has been reduced toabout three pages within "RadioClubs and Public Service"; thechapter on remote station controlover the internet has been cut toabout three paragraphs in twowidely-separated areas of "On-Air Activities and Radiosport,"and the previous chapter on "FCCRules and You" has been eliminated entirely. We find thosechanges somewhal curious.

Nonetheless, the Operating Manual continues to be anexcellent resource, and has actually come down in price by$10 from the previous edition to $24.95.

Both books are available from ARRL, 225 Main St.,Newington, CT 061 1 1, <www.arrl.org>, or from any numberof ham radio retailers.

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