ANTENNALETS . . . O R . . .

HOW TO KEEP YOUR PRETTYPLASTIC AIRPLANE FROMRESEMBLING AN AGITATED

LBOUT THE TIME that some-one asked me where he could putthe second DEM antenna on hisVariEze, I started to get the ideathat perhaps some enthusiastic plas-tic airplane builders were instru-menting their ships a little more ex-travagantly than either Rutan orI expected. In this vein, where thecosts of the radios meet or exceedthe cost of the airframe, allow me toshow you how to do a dual COM, dualNAV, glideslope, marker beacon,plus one OME and one transponderantennas inside a plastic (conductivestructure) airplane.

Since my original article on an-tennas for plastic airplanes cameout,1 there have been a number ofnew foam-and-glass designs intro-duced on the market. One of thosenew designs is a refinement of theship I used as my example in theoriginal article. Since this refine-ment, the Long-EZ, is intended forserious cross-country work . . . i tis my intention to show how all theantennas described above may befastened to the airframe of this lat-est of the Rutan creations. This isnot to say that all these antennasrr'jst be mounted on every ship. No,each of the antennas are separateunto themselves, and some, all, oneor more of them may be installed onyour bird. I will, however, point outthat it is much cheaper and a wholebunch easier to install a winglet an-tenna (antennalet) during wingletmanufacture than after the aircrafthas been flying for 6 months or so.I also hasten to point out that theconcepts presented here are notunique to the EZ series of aircraft.Providing that you observe theground rules about metal structuresnear the antennas, these antennasshould work in any nonconductiveship.

58 JANUARY 1981

COM AntennasThe original COM antenna as a

dipole structure with one dipole earrun down the gear leg and the otherdipole ear strung inside the bottomof the fuselage. At the time, I con-sidered this a best compromise be-tween ease of installation and anten-na pattern. Although most buildersof the straight EZ found this an ex-cellent antenna, the great spirit ofantennas decreed that the prototypeLong-EZ should have problems withthis simple design. Several attemptsto locate this elusive problem failed,so I decided that a new approach wasin order. Not a totally new design,mind you, but an adaptation of thedesign that proved so efficient on theoriginal EZ.

Basically, the reason I chose notto place the antenna on the wingletin the original EZ design was that

PORCUPINEBy Jim Weir (EAA 86698)

VP EngineeringRadio Systems Technology

10985 Grass Valley Ave.Grass Valley, CA 95945

the dipole arms were significantlylonger than the winglet, which meantthat there would be a sharp bend inthe approximate middle of the bottomdipole leg as the foil bent to con-form to the winglet-wing interface.Having had poor luck in the pastwith sharp bends in the middle ofdipole legs, I chose the gear-leg loca-tion, where the bends would be muchmore gentle.

However, Rutan decided to add areaand length to the winglet of the Long-

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EZ, so that now a full-length verticaldipole could be placed on the wingletwith only one minor bend. Placingthe COM antennalet in the wingletoffers several advantages, not theleast of which is getting that criticaldipole tip area a ways away frommetal engines, control cables and saltwater sacks (passengers). One furtheradvantage of antennalets is that adual-COM setup may be easily in-stalled by putting one COM anten-nalet in each of the two winglets.

Construction techniques for theCOM antennalet shown in Figure 2are basically the same as for the oldgear-leg antenna. Once again, themost important detail is to keep theantenna tips away from the bigpieces of metal. Also, it makes theantennalet work a whole bunch bet-ter if you locate the foil dipoles onthe trailing edge of the winglet andrun the coax and balun at right an-gles to the dipole as far forward aspossible before bending the coaxdown the leading edge of the winglet,into the wing, and thence into thecabin. The coax may be run down thesame path as the NAV light wire.Also, it is permissible to bend thebottom dipole ear slightly in orderto miss the rudderlet. Please notefurther that improved measurementfacilities here at RST have shownthat the dipole elements should beslightly shorter (see Figure 1) forthe dipole arms than presented inmy earlier referenced article.

NAV Antennas and GlideslopeThe only change that I am going

to make on the NAV antenna locatedin the canard (antennard? cantenna?)is to shorten both dipole arms slightly

as listed in Figure 1. The reason forthis foreshortening (and likewise forthe COM length reduction) is thatwe have obtained a 12 foot measuringmast at RST which reduces "eartheffect" to a negligible quantity. Theoriginal work was done using 6'masts, and we had some ground-bounce effects that we could not ac-count for. At any rate, install theNAV antenna into the canard in ac-cordance with the drawings in theoriginal article, but of a length aslisted in Figure 1 of this report.

Getting a glideslope output, in ad-dition to getting a second NAV an-tenna output, is done with what iscalled a hybrid splitter. The func-tion of the splitter is to take the onesignal from the canard NAV antenna,

split it into two isolated NAV out-puts, plus provide a glideslope out-put without interfering with the NAVoutputs.

This splitter has been describedseveral years ago in this and otherrnagazines,2 3 but the glideslope cir-cuit has not been covered, so I willgive a description of both circuitshere.

The new splitter consists of twomeasured lengths of 75 ohM coaxialcable, plus one 100 ohM carbon re-sistor. If the 75 ohM cable has a poly-ethelyne dielectric (as opposed to tef-lon), the coax lengths should be about17" long (each). The drawing, Figure3, shows these cables as beingstraight. In fact, the 17" cables maybe coiled up as tight as you pleaseand stuffed into a very small box.Also in the drawing you can see theglideslope-coupling capacitor, a 10picofarad NPO (or mica) capacitorwith a total of one inch lead length.In this particular unit, you will noticethat one lead of the capacitor is three-quarters of an inch long and the otherlead is one quarter of an inch. So longas the total lead length is one inch(±0.2 inch), the glideslope circuitshould work just fine.

Marker BeaconSince I'm just as sure as shootin"

that somewhere, someday, someonewill get his plastic homebuilt blessedfor IFR, we might just as well do themarker beacon design now. In es-sence, the marker beacon antennathat is placed in one wing instead ofthe canard (wing of your choice). Thenice thing about marker beacon an-tennas is that you are dealing withhigh signal strengths and relativelynon-critical receiver sensitivities. The

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SPORT AVIATION 59

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net effect is to make the antenna rela-tively immune to minor design com-promises. If you have to put one an-tenna tip near the NAV light wiring,it won't really affect the antenna per-formance that much. If you need tobend the antenna sharply around acurved surface, again, no great prob-lem. Even the cardinal sin of puttingone tip near, say, an aluminum fueltank will not completely destroy theusefulness of this antenna. The onlyeffect you may see by compromisingthe marker antenna may be a smallamount of range reduction or an in-strument approach that picks up themarker further out when left ofcourse than when right of course.Big deal.

Auxiliary AntennasEvery now and again, someone

comes up with a total off-the-wall re-quirement for an antenna for theirown "special" radio system. Some-one will want to put in a 6-meter hamrig, a VHF business band radio or anantenna for the FM music radio. Us-ing the basic design for NAV/COM/marker antenna, you should be ableto cut an antenna to be placed in theremaining wing by means of the sim-ple formula L=2578/F, where L=length of each arm of the dipole(inches) and F= the frequency ofoperation in Magahertz. Where youneed to cover a whole band of fre-quencies, the F value to use is thecenter for that frequency band. Forexample, if you wanted an FM an-tenna for the 88-108 MHz music band,band center would be 98 MHz, andeach foil dipole arm would be 26inches long.

Antennas from an upper frequencyof about 400 MHz down to about 45

60 JANUARY 1981

MHz may be handled in this manner.If your intention is to make a CBantenna for the 27MHz GarbageBand, a simple calculation will showthat the dipole will be much longerthan a single wing can accommodate.The only out you may have, if youwish to try it is to calculate the dipolelengths and serpentine the copperfoil across the wing. Just think of itas a dipole doing S-turns along yourwing and you've got the picture. Ifreely admit that I haven't tried it,I don't know if it will work or not,so you are pretty much on your own.One ploy that may be worth tryingis a "helical" antenna, where thetape (or, actually wire, if you wish)is wound on a tubular form and in-setted into a slot in the wing. Once

again, I emphasize, you are on yourown for any antenna below 45 MHz.

DME/TransponderFor those among us affluent enough

to be able to afford pulse equipment(transponder and DME) we offer theultimate in antenna sophisticationand expense — one AN4 bolt and aflat sheet of aluminum for each an-tenna! The basis of the antenna de-sign is one quarter-wave radiator(AN4 bolt) above a quarter-wavegroundplane (aluminum sheet). Thewhole thing takes up about as muchroom as a softball and is very light-weight (especially if you substitutean aluminum bolt for the steel ANbolt).

Construction details of the antennaare shown in Figures 4 and 5. Theground plane may be rectangular orcircular, your choice, although therectangular version is preferred forthe DME because of bandwidth con-siderations. Bending the corners ofthe rectangle up for either mount-ing or clearance are O.K., as are trim-ming the sharp corners of the rec-tangle.

The mounting locations for the an-tenna on the fuselage are many. Be-neath the pilot or co-pilot's seat as-sembly are excellent choices, as isthe cavity adjacent to the landinggear legs. Once again, the only caveatis to keep any and all metal awayfrom the radiating rod tip. However,anything behind the ground planewill not affect the rod. Just be surethat you mount the antenna with therod pointing down. And, for those ofyou with more money than goodsense, if you install both a DME anda transponder antenna, separatethem horizontally by at least 24inches.

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Sources shouldn't have much trouble with,Always the question comes up — as they are standard aircraft hard-

where can I buy all the bits and pieces ware. And, although you can certain-to make these fine antennas? The ly purchase copper foil tape at al-AN bolts and aluminum sheet you most any well-stocked office supply

house or stained glass window shop,the foil tape, ferrite beads, cable con-nector, insulating washer and coaxcable are all available from RadioSystems Technology, 10985 GrassValley Ave., Grass Valley, CA 95945.Phone 916/272-2203. A completeNAV or COM foil antenna "kit" con-sisting of copper foil and ferrites isstill only $5, with quick disconnectsand longer foil pieces available atadditional charge. Write or call RSTand ask for a "Plastic-Plane AntennaPrice List".

I would like to thank my researchassistant Brian Plath (KA6CWO) forhis invaluable aid in making theVSWR measurement and the Rutanclan for both installing and testingthe prototype antennalets.

References1) The Plastic Plane $5 Antenna Sys-tem, SPOflT AVIA T/ON, Jim Weir,May 1979, PP 45-48.2) Economy Antennas, SPOflT AVI-ATION, Jim Weir, October 1976, PP71-78.3) An N-Way Hybrid Power Divider,IRE Trans MTT, Ernest Wilkinson,January 1960, PP 116-118.

FM, MKR BCN,HAM ANTENNAS COM

ALL ANTENNAS HAVE UG88/UG89QUICK DISCONNECT FITTINGSAT THE WING ROOT.

TRANSPONDER/DME

NAV ANTENNADETAIL

NAV (VOR,LOC,GS)

QUICK DISCONNECTFITTING

SPORT AVIATION 61