AntennaSv Dr. Dean M. Hall

(EAA Lifetime 77869)1637 W. Baker Ave.Fullerton, CA 92633

I,-N THE PROCESS of bui ldingmy Skybolt N52DH, I was deter-mined to keep the parasite drag at aminimum. Even biplanes, with alltheir inherent drag, can be helpedby cleaning them up. One of theproblems which presented itself forimprovement was the area of anten-nas. This was a particular problemwith my plane because it was to beIFR equipped which in my judge-ment meant two complete Nav-Comsystems, with its attendant antennas.

The local radio shops were rela-tively little help with suggestionssuch as "Yes, you can put the omniantenna in the wing, but it probablywon't work very well. Why don't youtry it and let me know how it works?"Then came the admonition. "Don'tlet either one of the rabbit ears get22 DECEMBER 1977

close to any metal." Well, that lastpart can't be done.

It appeared that nobody knew any-thing about antenna location exceptthat antennas seemed to work in cer-tain installations, and if you foundone that worked, you kept doing itthat way. I read every article onantennas that I could find, includingJim Weir's excellent article inSPORT AVIATION. While the phys-ics are well known with regard tothe antennas themselves, there areno hard and fast rules regardingplacement. Mostly it is a trial anderror business.

But now enters the picture, a flyer,homebuilder, antenna engineer andhobbyist by the name of Bob Archerof Torrance, California. I heard ofthe man and managed to trace himdown. When I first asked him aboutputting some sort of antenna in theSkybolt wing tip, his first answerwas, "it can't be done". As oftenhappens, the problem was one ofcommunication, and when the prob-lem was represented in a more in-tell igible mode, you could practi-cally hear the wheels clicking on theother end of the line as he mentallydsigned the antenna on the spot.

Bob has a wood wing Bellancawhich he uses as a test bed for vari-ous antennas and he plots the radia-tion patterns and response curveswith the result that he has a wealthof practical experience with regardto antennas in general, and woodwing airplanes in particular.

Among his observations, welldocumented, are the facts that somecommercially built planes have theirantennas in almost the worst pos-sible place. For instance, an omniantenna placed above and behindthe cabin may have a radiation pat-tern as determined both in the hori-zontal and fore-and-aft verticalplanes shown to be poorest directlyahead of and slightly below the air-plane, exactly where we want it towork best.

The antenna which Bob designedfor me has a number of advantages,not the least of which is that it workssuperbly, which after all is what an-tennas, as well as airplanes, are allabout. In addition, it is simple —even I can understand how to do it.It is dirt cheap — how about thatfor a change! It is light in weight.And most gratifying, it solves thedrag problem.

It consists of a 20" piece of 1/6"stainless steel welding rod inlaid inthe wooden wing tip bow and attach-ed to a BNC connector mounted onthe last nose rib. First, Bob Archerdisabused me of the notion that newantennas need two arms as in theclassical rabbit ears. In this system,

one arm is used with a ground plane.The chassis mount BNC connector(with single nut mount) is mountedon an aluminum plate which is madein the shape of a nose rib with theaddition of tabs along its free mar-gins (see Fig. 1). These tabs are allturned over the edge of the rib andunder the metal leading edge of thewing. The reason for the large numberof tabs is to increase the area of con-tact between the baseplate and theleading edge because radio fre-quency currents (which behave morecritically than ordinary currents) willset up interference if they are re-stricted to a narrow contact (so Bob

0 S

Figure 1

tells me). The entire leading edge ofthe wing then becomes a groundplane and it ideally should be iso-lated from the other metal in theplane. The BNC connector is mount-ed in the center of this plate with theinside end mating to the connectorof an RG 58 cable from the radio. Weput another disconnect at the wingroot. The welding rod was thensoldered to the center of the connec-tor on the outside. This rod wasmounted as shown in Fig. 2. The rodpasses through a hole in the tip bowand then is inlaid in a groove in theoutside of the bow. The wire isepoxied in place and covered as perusual. There is no need to skimp onthe silver paint in this area. Silverpaint is made up of particles whichdo not touch each other and thus donot shield an antenna.

In practice, my plane has two an-tennas, one in each lower wing tip,one hooked to a Radair 200 and theother to an Edo-Air 563S. Howwell do they really work? Well, Iwas pleasantly surprised when in theearly test flights over Flabob Air-port, I tuned in LAX which is 60miles away and got stable receptionon both navs. On the trip to Osh-kosh '77, the longest reception thatI could document precisely was 110miles with both needles centered.This was at the altitude of 9,500over eastern Colorado. On the entiretrip, I was rarely out of range of two

ANTENNA

LOCATION

By Bob Archer (EAA 48916)21818 Ocean Ave.

Torrance, CA 90503

After Dr. Hall had such excellentresults with the wing tip omni an-tennas he had installed in his Sky-bolt with my advice, he asked me tosay a few words about antennas forSPORT AVIATION.

For several years I had been in-tending to write an article aboutantennas, but with all of my pro-

jects I could never seem to find thetime. After Jim Wiers' excellent ar-ticle on antennas was published inSPORT AVIATION, in October of1976, there just wasn't much left tosay on the subject. With Dr. Hall'sencouragement, I decided to saysomething about a couple of thingsJim Wier didn't mention in his ar-ticle, or things he just touched onbriefly.

One of the first things is the an-tenna radiation pattern, which is thefield strength vs. direction. First, anairplane is a terrible shape on whichto mount an antenna and get a de-cent radiation pattern, especially asmall airplane. Ideally, we wouldlike to have a constant level of radia-tion in the horizontal plane with per-haps a plus or minus 30 degreeselevation angle. About the only wayone could get a pattern somethinglike this, would be to hang a dipolevertically in free space. If one wereto put an airplane in the picture,however, the R.F. energy bouncesoff and travels along the conducting

portions of the airframe distortingthe pattern all out of shape. Aboutall one can do then is to try to findthe least "worst" place to install theantennas. There are certain charac-teristics of antennas which need tobe known before one can make adecision as to the location of mount-ing antennas. To begin with the en-tire airframe of an all metal aircraftis the ground plane. R.F. energytends to radiate more energy in thedirection of the longest groundplane, also it tends to wrap aroundcurved surfaces and travel morestrongly down sharp edges. Thus, ifone places a communications anten-na in the middle of the wing fuselagejunction, one gets a horizontal pat-tern similar to Figure No. 1. If it isplaced on top of a round fuselage,the pattern in the vertical plane willbe similar to Figure No. 2. Likewisethe pattern would be stronger in theplane of sharp edges. Another thingis to not put an antenna anywherenear any conductor that is fromabout l/8th wavelength and longer.

stations at any one time. Wondersof wonders, while trying to cut downon drag, I had found an antennawhich gives a bonus in performance.

I believe that Bob Archer deservesthe recognition for his contributingthe solution to the navigation anten-na problem in this type of airplane.

Wing Tip Antenna — Dean M. HallFigure 2

E«D

PLANE: 9

FIGURE 1

118 Mhz.Vert. Pol.Horiz. Plane

Straight Stub©Wing-Fuselage

JunctionLow Wing

SPORT AVIATION 23

That is, anything within about onewavelength will effect and disturbthe field of the antenna. A quarterwavelength is probably the worstdistance to mount an antenna froman obstacle of some sort. If onemounts an antenna about a quarterwavelength from a surface or rodthat is longer than the antenna ele-ment, the energy is reflected fromthat surface or rod and the antennais then what the Ham operators calla "Bean Antenna." With this type ofantenna, one will get practically noradiation in the direction of the re-flector. Also, it is bad practice to puttwo antennas in close proximity asthis condition will also cause patterninterference.

From many years of antenna ex-perience, I have formed opinionsabout the best and worst locationsfor various antenna types. The bestlocation of the VOR antenna I be-lieve would be in the wing tips (seeFig. 3), with top of the vertical stabi-lizers (see Fig. 4), being second best.The worst VOR antenna positionwould be under the horizontal tail(see Fig. 5), with under and abovethe fuselage a close second (see Fig.6). The best communication anten-na location I believe is the top of thevertical tail (see Fig. 7), with secondbest being the wing fuselage junc-tion area (see Fig. 1). The worstcommunications antenna locationwould be on the belly in the middleof all the gear legs on a tricycle geartype aircraft. The basic rules boildown to this: on metal airplanesmount stub or unbalanced antennason the largest expanse of metal with-out nearby reflecting objects andon plastic or wooden airplanes mountbalanced or dipole antennas as faras possible from any reflecting ob-jects.

For flush or internal antennas, theideal installation would be for theaircraft designer to design them intothe structure. Since aircraft de-signers generally are not too knowl-edgeable in antenna design, weprobably will never have this condi-tion in homebuilt aircraft, but wecan provide suitable internal loca-tions for our antennas in homebuiltsduring construction.

For flush and internal type anten-nas, I have for years been usingmetal tape for the antenna ele-ments. I have used aluminium tape,copper tape and the tape they usearound windows for burglar alarms.I believe the burglar alarm tape islead. One can also use strips ofaluminium sheet metal. Using tapeand metal strips makes it easy to at-tach the antenna elements to theinside of plywood, fiber-glass, plexi-glass and even fabric, if one backs it

24 DECEMBER 1977

E6O E<£Q CPD

PLANE: H=

FIGURE 2

118 Mhz.Vert. Pol.Vert. Plane Straight Stub

Fuselage Back

FIGURE 3

113 Mhz.Horiz. Pol.Cut. Horiz. Wing Tip VOR

Swept Back Stub

up with some reinforcement. Justuse flat head screws through theskin, connect the antenna elementsto the balun as shown in Jim Weir'sarticle — Figure 11. Cut the ele-ments to length using the formulaon Page 76 in Jim Weir's article. Ifmounting the antenna on wood orplexiglass, shorten the elements toabout 8(Ft of the free space lengthto compensate for the dielectric ma-terial. Anytime an installation of thistype is made the V.S.W.R. shouldbe checked to make sure the an-tenna is operating properly.

So, I believe flush or internallymounted antennas are the best bet.They are out of the way so they arenot likely to get broken or pokesomeone in the eye. They are out ofthe weather so corrosion is not somuch of a problem. Also they causeless drag. A VHR whip antennatakes about IVi horsepower to drag

Een E<*G CPO

PLANE: 6=

FIGURE 4

Swept BackDipole on Top

of Fin— No Sweep

it through the air at 150 miles perhour. An airplane that doesn't haveantennas poking out all over it, atleast to my eyes, is a much betterlooking machine.

A properly designed, built and in-stalled flush antenna can work justas well, and possibly better than thenormal outside types. The anten-nas we build for the T-18's are alsocompletely grounded and so tend toreduce radio noise.

This is more than a few words, Iguess, but there is a great deal to say

113 Mhz.Horiz. Pol.Horiz. Plane

Swept DipoleUnder Horiz. Tail

SPORT AVIATION 25

about antennas once I get warmedup. I've neglected completely ADF,Transponder, and Marker Beaconantennas, which are ALL candidatesfor internalizing. If there is interestperhaps I could expound on theselater.

All antenna radiation patternssupplied with this article are repro-ductions of actual patterns, whichwere run on a typical model aircraft,on an antenna pattern test range.

EflO E<2>D

PLANE: 6=

CPD

E6D E*D CPD

PLANE: 0= °, <t>

FIGURE 6

FIGURE 7

113 Mhz.Horiz. Pol.Long. Cut

128 Mhz.Vert. Pol.Horiz. Cut St. Stub

on Fin Tip

Swept Dipoleon top ofFuselage

ABOUT THE AUTHOR: Bob Archeris an Antenna Engineer for T. R. W.Systems at Redondo Beach, Califor-nia. He has restored a 1946 BellancaCruisair and a 1943 Laister Kauf-man glider. He is presently buildinga Coot amphibian and is convertinga Mazda rotary for use as its power-plant.

26 DECEMBER 1977