Antenna FundamentalsBy George V. Copland

Rt. 2, Box 12Duncan, OK 73533

AIRPLANE design and constructiontechniques in small airplanes hasshown remarkable change in the last10 years. Likewise new and better av-ionics are on the market covering newand improved services for aviation.Miniaturization of electronic equipmentallows installation of more complexunits in light aircraft or a variety of an-tennas are required due to the differentradio frequencies used. Frequency isinversely proportional to wave length ofthe radio wave. The higher the fre-quency, the shorter the wave length. Ametal conductor whose length is 1/4 ofthe wave length involved is the basicelement used in antennas. Table 1shows the relationship of aviation ser-vices parameters. The equation length(inches) = 2952/Frequency (MHz) de-fines the 1/4 length relationship.

Due to reciprocity, an antenna per-forms the same in either transmitting orreceiving mode. The pattern, impe-dance and efficiency are the same.Radio waves are polarized dependingon the structure that radiates theenergy. A vertical antenna will transmita vertically polarized wave and a verti-cal antenna is used to receive the radiofrequency wave. The key words areaperture and polarization. Since spaceis a limitation in small aircraft, antennaof 1 /4 wave length and shorter are usedwhere applicable. The 1/4 wave con-ductor is generally mounted perpen-dicularly on a ground plane by use ofan insulator at one end. A coaxial cableis used to electrically connect to the an-tenna. The outer shield is connected tothe ground plane and the center con-ductor connected to the end of the an-tenna at the insulator. This is referredto as an unbalanced feed. Note fromTable 1 that it is impractical to use a 1/4wave vertical element for the Loran Cof LF/ADF frequencies. More on thislater.

For reception of horizontal polarizedwaves, such as VOR, marker beaconand glideslope, dipole antennas areused. A dipole antenna is a 1/2 wavelength conductor, cut in the middle, andthe electrical connection made at thecenter. This antenna is a balanced an-tenna and may not be connected di-

rectly to a single conductor coaxialcable. A matching transformer or abalun is used. Two practical balun de-signs for single conductor coaxial cablewill be discussed later.

The radiation pattern for a 1/4 wavevertical antenna is different than thepattern obtained from a horizontal di-pole antenna. Since we are interestedin antennas mounted on an aircraft, adistinction must be made. We desire re-ception for communication and naviga-tion from all directions. This is called anomni-directional pattern. In the case ofhoming, such as glideslope, we are in-terested in forward directivity. Verticalantennas by nature are omni-direc-tional. Figure 1a shows the circular na-ture of the radiation pattern of a 1/4wave vertical, operating over a groundplane, as viewed from above. Figure 1 bshows a side view of the configuration.Notice that if the ground plane is infinite,the outside curve applies. Themaximum energy is almost maximumin a horizontal direction. As we reducethe diameter of the ground plane, thepattern is altered as shown by the innercurve. Now the directivity is more in anupwards direction. A vertical antennamounted on the lower half of theairplane would be more efficient loca-tion for communication with a groundstation for this reason. Notice how inef-ficient the antenna is in the vertical di-rection. Since we are trying to captureenergy as an aperture by the conductorit appears to reason that a vertical an-tenna pointed to the source would be ahighly inefficient method of receiving asignal.

Some vertical antennas are sweptback for streamlining and appearance.This modifies the forward portion of thepattern to a higher radiation angle.However, this small loss is acceptable.Any obstruction, such as landing gears,other antennas, tail fins and metal pro-pellers distort the pattern. It is goodpractice to keep antennas 1/4 wavelength or more away from metal con-ductors. This also includes control ca-bles, electrical wiring, drag and anti-drag bracing and streamline flyingwires. The shape of the aircraft highlymodifies the radiation pattern of an an-

tenna.The radiation pattern of a horizontal

dipole antenna is shown in Figure 2.Maximum directivity to a received signalis broadside to the antenna. The patternis in the shape of a torus or doughnutand, therefore, has a good down look-ing pattern. As before, very little signalis expected to be received oft the endsof the antenna. Using this style of anantenna for VOR reception one shouldexpect good forward and rearward re-ception and marginal reception from theside. This type of antenna is excellentfor glideslope reception as sidelobe in-terference is minimized while forwarddirectivity is at a maximum. This type ofantenna is also used for marker beaconreception where downward gain is im-portant. The marker beacon signal istransmitted upward and is not intendedas a homing signal. There is enoughsignal so as the null end effect of thereceiving antenna is not as apparent.Today, most marker beacon receivingantennas are miniaturized and not ofthe 1/2 wave dipole type.

VOR antennas are generally a Veeshape, that is, two 1/4 wave length ele-ments separated by approximately 90degrees as shown in Figure 3. The gainis not as high due to the aperture reduc-tion, but the pattern is almost omni-di-rectional. The relative signal strengthminimum to maximum varies no morethan 85%. Again, the aircraft modifiesthe radiation pattern. As before, it is im-portant to keep conductors away fromthe elements. This is the main reasonVOR antennas are mounted high on thevertical fin of a metal airplane with theVee looking forward. Reduced ice for-mation and lower static electrical dis-charge from the element tips is a plusin this mounting. This type of antennashould not be mounted close to the fu-selage or wing of a metal airplane asthe conducting surface of the aircraftand modifies the gain and pattern of theantenna. Commercial Vee antennasgenerally have a mast mounting built inand have been optimized in design. AU-shape or rams horn design is alsoused which approximates the pattern ofVee design.

Transponder and DME antennasSPORT AVIATION 55

both operate around 1,000 MHz asshown in Table 1. The transponder op-erates at 1,030 and 1,090 MHz whilethe DME operation is over a band offrequencies. Their physical dimensionsare essentially the same, but the ele-ment shapes are quite different. Practi-cal 1 /4 wave lengths of transponder andDME antennas are generally 2.25inches above the Teflon insulator. Thetransponder antenna is a small conduc-tor, 1/8 inch in diameter, with a smallball on the end to minimize static electri-cal discharge. In contrast DME anten-nas are generally 1/2 inch in diameterand no ball on top. The larger diameteris one way to increase the band widthof an antenna at the expense of smallloss in gain. The transponder antennawould be sharper in resonance at 1/4wave and a little better match to thetransmitter in the transponder. Bladeantennas are used for both services toreduce air drag.

Transponder antennas are mountedforward on the bottom of the metal fuse-lage with a clear line of sight. It is agood idea to have a ground plane atleast 4 wave lengths in diameter. Thisis not so bad as we are only talkingabout 36 inches in diameter. One othernote of importance is to keep the in-sulator clean. At the 1,000 MHz fre-quency, contamination will greatly af-fect signal strength. A top mountedtransponder antenna is not a goodchoice as the radiation pattern is up-ward and the closer you get to an ARSAor TCA the aircraft will shield the an-tenna's operation.

Loran C and LF/ADF antennas fallinto a similar category. Both radiowaves are long, 1/4 wave lengths inhundreds of feet, and do not dependupon line of sight for reception as VHFand above radio frequencies do. Bothservices are vertically polarized but dueto ground conduction and skywavemodification mainly due to the ionos-phere, almost any conductor will workfor reception. The longer the antennathe better is the rule. Easier said thandone on aircraft. Trailing wire antennashave long gone out of style due to mod-ern electronic circuit design. SinceLoran C operates on a single frequencyof 100 KHz high gain preamplifiers maybe used at the antenna. Most Loran Csystems are of this type. Couplers areused that utilize the vertical VHF anten-nas for receiving the Loran C signalwhile effectively blocking out the VHFtransmitted energy when in Comm use.This cuts down on the number of anten-nas involved.

LF/ADF system antennas are of aself-contained loop design matched tothe receiver. A long wire on the aircraftmay be required for the reference signaldepending on manufacturer.56 AUGUST 1990

ServiceLORAN CLF/ADFMARKER BEACONNAV/VOR/LOCCOMM/ELTGLIDE SLOPEDMETRANSPONDER

Table 1Frequency100KHz200KHz -400KHz75MHz108MHz -118MHz118MHz -137MHz329MHz - 335MHz962MHz -1213MHz1030MHz, 1090MHz

1/4 Wave Length2460 ft.900ft.

39.36 in.26.1 in.23.1 in.8.9 in.2.7 in.2.7 in.

PolarizationVert.Vert.Horz.Horz.Vert.Horz.Vert.Vert

Aircraft that use composite materialsfor construction, such as plastic foam,epoxy-fiberglass, carbon fiber andwood, must use special mountingtechniques for antennas. In the case of1 /4 wave antennas with their associatedground planes, the ground plane needsto be built into the structure. The groundplane should be a minimum of 40inches in diameter for VHP frequencies.Two copper foil strips, 1/2 to 1 inchwide, crossed at 90 degrees in form ofan X and bonded to the coaxial cablesheath at the center would be a practi-cal substitution. Two copper wires, 1/16inch diameter (16 gal.) used in a similarpattern could be used. It may not bepractical to install the conductors in aflat plane, but it is the ideal. For a trans-ponder antenna a solid disk is to be pre-ferred. A 10 inch diameter, .025 inchthick aluminum disk would be aminimum for use. The transponder an-tenna element needs to point down-ward.

The radio frequency cable used tocarry signals to and from the antennasis in practice determined by the connec-tors on the equipment and the routingof the cable. A 50 Ohm impedance co-axial cable is used. Two common ver-sions of this cable are the RG8/U andthe RG58/AU. 50 Ohm impedance is anelectrical term which relates the size ofthe center inner wire to the insidediameter of the outer conductor andbased on the dielectric mediumseparating the inner and outer conduc-tor. The RG8/U is approximately .4 inchoutside diameter and the RG58/AU isapproximately .2 inch outside diameter.The RG8/U is much more rigid anddoes not bend easily. RG58/AU is quiteflexible but is more lossy. The length ofcable on the Comm side of the trans-ceiver is usually short, so electricallythis is no problem. These cables havea stranded center conductor and are tobe preferred. Do not use RG58/U(notice the "A" is left out) because it hasa solid center conductor and is difficultto use. Extreme care must be taken notto scratch or nick the center conductorwhen assembling connectors. The wirewith the smallest scratch under vibra-

tion in time will break and cause an in-termittent connection. These breaks arehard to find, so don't consider buildingin this inherent problem. RG58/AU andRG58/CU both have stranded centerconnectors and are acceptable. The dif-ference between the AU and CU is thecable covering. The CU is noncon-taminating and slightly more expensive.A more preferred version of the RG58/AU is the RG58/AU Foam. This is muchpreferred in the installation to the trans-ponder antenna. Where as at 100 MHzboth RG58/AU (std.) and RG58/AUFoam have about the same loss, 4.9 dbvs. 4.5 db/100 ft., at 1000 MHz thelosses are 21.5 vs. 14.5 db/100 ft. Re-membering 3db is half power, this is sig-nificant. The RG58/AU Foam is BeldenPart No. 8219.

Many different types of connectorsare used with coaxial cable. Two of themost common found are the UHF83series and the BNC type. The UHF con-nectors are a little larger and easier toinstall on the coaxial cable. Usually theUHF style is used with RG8/U and theBNC is used with the RG58/AU. Am-phenol Part No. 83-1SP is used withRG8/U. An 83-185 adapter may beused with this connector when RG58/AU is desired. Amphenol Part No. 31-301 is a BNC style used with RG58/AU.This connector is an improved capti-vated contact style and is much pre-ferred over the 31 -002 model. Generallyone of these two connectors is used onthe Comm side of the transceiver. Onsome equipment the Nav receiver hasa receptacle for Motorola 13B coaxialconnector. This is the common styleused on automotive antenna systems.Using this style, antenna connectionsare not interchangeable on receivers ofNav/Com equipment which requireseparate antennas. This connector isnot used much any more.

The VOR antenna is made up of two(2) - 1/4 wave length rods separatelyinsulated and mounted horizontally in aVee configuration. The angle betweenthe rods is 90 degrees for an omni-di-rectional pattern. Length of the rods for113 MHz is 26 inches. This antenna isconsidered a balanced design since

both elements are insulated fromground. In order to connect a balanceddipole antenna to an unbalanced co-axial line, a balun is generally used.

The 1/2 wave length balun of coaxialcable between the two elements in Fig-ure 4a is equal to 5904/MHz times theVelocity of Propagation of the electricalwave in the coax. A value of 65% forV.P. is used for RG58/AU. Therefore,for a mid-range frequency of 113 MHz,the length would be 34 inches. The con-nection is shown in Figure 4a.

A 1/4 wave matching stub balun isshown in Figure 4b. The length of theshort section is 18 inches long. Leaveapproximately 1/4 to 1/2 inchs of centerpolyethylene insulation with center con-ductor inside sticking out of solderedarea to assure no shorting. A separateshort piece of shrink tubing or insulatingtape can be used in addition. Care mustbe taken when soldering the lower endto the coaxial cable so as not to damagethe inner insulation in the coax. Do notconnect the top shields together. Noconnection is made to the inside con-ductor of the shorting stub at the bot-tom. This second method is not as anefficient system above 50 MHz, but issometimes preferred because of easeof construction, durability and improve-ment in reducing static discharge noisesince both elements are effectively atground potential.

The current trend in composite mate-rials aircraft seems to be to conceal an-tennas wherever possible. Nav anten-nas for horizontal wave reception lendthemselves to be placed in the wings.For omni-directional receiving the Veestyle should be used. Mounting out-board in the wing gets the engine outof the way in the fore and aft patternalong centerline. Consideration must begiven not to have a conductor acrossthe open end of the Vee. The Vee maybe pointed either forward or rearwarddepending on wing design construction.

The VHF antenna with its associatedground plane offers a challenge for con-cealment. Height of the vertical elementcan be accommodated by bending itover after the first 6 to 8 inches. A log-ical location is aft of the baggage com-partment. The vertical element can belocated in the vertical fin with the groundplane elements built into the stabilizers.A difficult design and constructionchoice. It appears that a vertical orswept back element on the undersideof the aircraft, even though not con-cealed, is the best from constructionand performance. The ground planecould then easily be in the wing or wingroot area. The antenna does not haveto be on centerline.

One consideration which hasseemed to be ignored is that when

Fig. 1a

Fig. 2

Fig. 1b

Elements26"

Insulator

L = 34" for- RG58/AU

Fig. 4a

using foil or imbedded elements inepoxy-glass or foam structures, modifi-cation of element length must be made.The 1 /4 wave element length is a freespace determination based on thespeed of light. Electromagnetic wavestravel slower through dielectrics. Notethe effect given to the V.P. (velocity ofpropagation) constant in coaxial cable.For cellular polyethylene V. P. is 78%,that is, the elements of an antenna en-closed in that foam medium would beshortened by 78%. It would be expectedsimilar consideration should be takeninto account for foams used in compo-site structures. The same reasoningholds for the attachment of foils directlyon epoxy-glass surfaces. No direct fig-ures are available but it is a factor.

Antenna location in small aircraft is areal challenge to the designer. Perfor-

mance patterns are greatly modified bythe aircraft itself. What works for oneaircraft may not be directly applicablefor a similar design. This article shouldprove a guide for considerations in an-tenna applications of small aircraft.

References

Antennas for Small Aircraft, GeorgeCopland, EAA Forum Oshkosh 76.

Antennas, John Kraus, McGraw Hill1950.

Antenna Engineering Handbook,Henry Jasik, McGraw Hill 1961.

Radio Amateurs, VHF Manual,ARRL, 1972.

SPORT AVIATION 57