MARK SCHAIBLE

FOR MOST A I R C R A F T TODAY,avionics are a fact of lifebecause they give pilotsfull use of the nation's air- ,space. Selecting and in-stal l ing the properantennas is the first stepin getting the best recep-tion for these devices, andthe best antennas are theones recommended by themanufacturers. Proper selection andinstallation of radio antennas foryour airplane is critical to good re-ception. The best antenna for eachof your radios will be the one thatthe avionics manufacturer recom-mends. Purchasing the least expen-sive antennas often comes with anadditional price—poor reception.

Each avionics device has its ownantenna, which is matched to the fre-quencies on which it operates, andwe'll discuss the most common types,communication, VOR, transponder,marker beacon, GPS, ADF, and Loran,and where to locate them on your air-craft to get the best reception. Ran-domly placing antennas on yourairplane, locating certain types too

AntennaInstallation

Connecting your avionics to the outside worldRON Al.KXANDF.R

close to each other, and locatingthem too closely to such things asstrobe lights can cause problems.

To avoid these problems, map an-tenna locations before ins ta l l ingthem, and position them in areas notsubject to interference. Many avion-ics companies skin map airplanes tofind the spots free of radio-frequencynoise and interference, and thesecompanies can assist you in th i sprocess. Where you route avionics ca-bles is also important, and you shouldavoid locating them near wires or de-vices that use high currents.

Communication AntennaCommunication antennas are usu-

ally a stainless steel or fiberglass whip,

or they may be a strip or glassed-intype. Typically, each communicationradio has its own antenna, and onairplanes with two transceivers, oneantenna is on top of the airplane andthe other is on the bottom. Commu-nication antennas should not bemounted closer than 24 inches to anavigation antenna (36 inches is bet-ter). This reduces any frequency"bleed through" from one radio tothe other, and it removes any recep-tion interference.

The most important thing to re-member about any t ransmi t t ing an-tenna is tha t it has two parts , theantenna itself and the ground plane, ametal plate around the area that sup-ports the antenna. Bonding the an-tenna's ground plane to the aircraftskin is important because this bondcomprises the other h a l f of the an-tenna. Without a good ground plane—a bare mctal-to-metal connection—the

communication radio willhave poor reception and alimited range.

Mounting an antenna..' depends on its type. A

f iberglass whip antennamay have a three- or four-hole mount pattern, and

. , a stainless steel antennaw i l l have a feed-throughmount. Both types have

one thing in common, a backing plateor doubler that keeps the antenna fromvibrating or deforming the aircraft skin.Usually, doubler plates are riveted—not

Whether it's a stainlesssteel or fiberglasswhip, communicationantennas need agood ground planeand a good baremetal-to-metalconnection.

AIRCRAFT SKINBACKING PLATE

92 NOVEMBER 2000

bolted—to the aircraft's skin. Typicalwhip antennas are 22 inches long, andwithout a doubler plate the constantin-flight drag pull on the antenna willdamage the airplane's skin.

Composite materials—except forgraphite—are transparent to radiowaves, which enables you to mountantennas inside the aircraft structure.But this type of installation requiresmore planning because usually youmust incorporate the antenna whenyou're building the airplane.

Achieving the proper groundplane is another challenge on com-posite airplanes. It requires a piece ofa luminum w i t h at least 10 to 12inches of area around the antennafoo tpr in t , and this ground planemust be connected to the a i rc ra f tground. In some instal lat ions thebacking plate may act as the groundplane. There is a communication an-tenna for composite aircraft that cre-ates its own ground, a 36-inch lengthof copper foil that's split in the mid-dle by a coaxial cable connection.

Navigation AntennaNav antennas don't need a ground

plane, and you can mount them in anumber of d i f fe ren t ways. Whatmakes the nav an t enna d i f f e r e n tfrom the rest is that its most impor-tant part is the cable that connects itto its receiver. At the end of this cableis a balun, a radio frequency "trans-former" that connects a "balanced"two-conductor antenna to an "unbal-anced" cable, and it must not be al-tered in any way.

There are two types of nav anten-nas, the dipole, often called "catwhiskers" or a V antenna, and thebalanced loop antenna.

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94 NOVEMBER 2000

Aircraft Building

There are two types of nav anten-nas, the dipole, of ten called "catwhiskers" or a V antenna, and the bal-anced loop an tenna . The balancedloop looks like a U-shaped towel bar ora thin, flat blade, if the loop is encasedin fiberglass, and they are usual lymounted high on the vertical stabi-lizer. Because of its design, cat whiskers(dipole) antennas may cause reducedreception from the rear of the aircraft.Regardless of the type, nav antennasshould not be located within four feetof a communication antenna.

Transponder/DME AntennaTransponders and distance meas-

uring equipment (DME) use "pulsetype" antennas that come in twostyles, the familiar rod-and-ball andthe blade type, but you can use thesame antenna for both devices. Youshould mount the antenna on thebottom of the aircraft, on the cen-terline and at the lowest point whenthe a i rp lane is in level f l i g h t . I tshould be at least three feet awayfrom nosewheel wells, mast andwire antennas, and any other ob-structions for maximum receptionand minimum shadowing.

METALSUPERSTRUCTURE

Transpondersand DME use"pulse type"

antennas thatcome In two

styles, the fa-miliar rod-and-

ball and theblade type,

and you canuse the same

antenna forboth devices.

As with communication anten-nas, a good ground plane is impor-tant to pulse an tennas . Mountrod-and-ball antenna ground planeson bare metal. With the blade type,

OUTSIDE

MOISTURESEAL

it's best to have the sk in bare be-tween the antenna and the aircraftskin itself. A manufactured groundplane should have at least 6 inchesof area surrounding the antenna,and you should use RG8 coaxial ca-ble for the antenna wir ing. UsingRG.S8 cable limits you to a 10-footlength of cable.

Marker Beacon AntennaMarker beacon antennas vary in

shape and size, but generally theylook like a boat hul l (and they areof ten called "boat antennas") .Mount this antenna on the air-plane's belly as far away from thetransponder/DME antenna as possi-ble (three feet is the minimum). Theantenna requi res no s t r u c t u r a lchanges except a backing plate anda hole for the cable.ANTENNA,

LOW PROFILE

XBACKINGPLATE

Because GPS signals are receivedvia line of sight, the location of theantenna is very Important.

GPS AntennaGPS manufacturers should pro-

vide an antenna with the receiver.Mount the small, simple antennaon top of the airplane as far forwardas possible, such as on top of thecabin. Usually a GPS antenna does-n't require a backing plate because

its low profile creates little drag, andmost of them don't require a groundplane. Because GPS signals are re-ceived via line of sight, the locationof the antenna is very important.

ADF AntennaModern au toma t i c direction

finder (ADF) antennas combine theloop and sense antennas in a singleunit, doing away with the need torun a wire antenna between the ver-tical stabilizer and the measuredpoint on the wing or fuselage. AnADF antenna should be as far aft aspossible on the bottom of the fuse-lage, but you can mount it on top ofthe fuselage if it's not too close tothe vertical stabilizer.

Loran AntennaA Lorann antenna looks like a

communication antenna, but the

two are not interchangeable becausethe antenna has a pre-amp circuitmanufactured into its base. You in-stall this antenna like you would acommunication antenna, and theground plane is important. For thebest receptions, don't mount it closeto the vertical stabilizer.

Mounting ProcedureRegardless of the type of antenna,

you must follow certain basics wheninstalling an antenna:

Locate all antennas before instal-lation.

Drill the necessary holes usingthe template, if provided.

Determine the size of the mount-ing area. Mask off this area, removeall paint and primer, and treat thebare metal with Alodine.

Install backing plates and nut plates.Instal l the an tenna and secure

the fasteners.Seal the area as necessary around

the base of the antenna.

Routing CablesKeep an tenna cables away from

each other as much as possible. Whentying them off, do not compress thecable itself because it can damage thebraided shielding beneath the exte-rior insulation and cause a radio fre-quency "leak" tha t can af fec t thereception of other avionics. Don'tpull the tie-wrap too tight.

Mounting antennas is a relativelysimple process if you remember thatproper placement is paramount andthat most antennas need an ade-quate ground plane. Choose yourantennas with care and follow theinstallation instructions to the letter,and your avionics' reception shouldbe clear and strong.

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