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Installing Transponder Antenna

John Slade

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The transponder antenna needs a 5.7 inch diameter circle of aluminum or copper as a ground plane. The antenna itself is 2.85 inches long and sticks out below the ground plane.


I'm told that interference from nearby metal objects isnt much of a problem - one guy installed his in the wheel pant next to the brake caliper and axle with no problem. I found a spot for mine in the forward nose above the gear. One way to install this item would be to make an elevated pad on the floor about 1 inch high just forward of the pax rudder pedals for the ground plane to sit on. The tip of the antenna would then be just short of the outside skin.


I put mine on top of the AC blower box, but not everyone will be installng one of these. :)


I can be reached on the "other" forum http://canardaviationforum.dmt.net

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  • 8 months later...

I put mine in the center of the wing. The wing thickness is sufficient there for the antenna - I am building an SQ2000 canard, don't know if Cozy wing thickness is similar. I was told that the transponder antenna should be sufficiently removed from passenger compartment for radiation safety.

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You've got access panels in the nose. Why not just velcro the ground plane to the inside of the panel (also have a coax quick disconnect). It would be a short antenna lead, wouldn't get in the way of much of anything, and you could change your mind if you wanted. Velcro'ed under the canard might be another place


Just a reflexive brain fart ... Jim S.


PS I was just reading somewhere (AeroElectrics?) about folks using an AOL CD they got in the mail as a ground plane. Makes sense to me ...

...Destiny's Plaything...

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we put two shelves in the nose, one left and one right of the nose gear pivot bolt square, for just this kind of thing.


we figured that a gps antenna might fit nicely on one and they are there ready for use in the future


enjoy the build



maker wood dust and shavings - foam and fiberglass dust and one day a cozy will pop out, enjoying the build


i can be reached at



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I had a number of problems with getting my transponder to work reliably. The 2nd airplane in my hanger (a Mustang II) has also been working out some transponder bugs. Here are things that we found:


Rule of thumb from the Avionics shop- No metal within 3 feet at or below the ground plane. Any metal in the path will create shadows (such as an aluminum landing gear).


Use the good RG400(?) coax. They have replaced alot of the RG58 coax. Some of the Garmin transponders barely work with RG58.


Use good connectors and good crimpers. I have a decent crimper, but my crimp was not successful. Theirs was. For the money, I would have them make the cable. I had them make my cables when they did the transponder certification test. The cables for the Mustang II were about $10 more for the crimped and checked version than for what I could have bought the materials. If anyone needs cables made, and can't get a reasonable price from their avionics shop, I can connect you with the folks that made my cables.


I ended up putting a sheet of aluminum in the bottom of the engine cowl. It fits that extension of the scoop, and is maybe 10 inches x 15 inches? The antenna mounts down through the cowl. To remove the lower cowl, I reach in through the scoop and disconnect the cable.


With the antenna in this location, I have had zero complaints about lost transponder reply, all the way out to their radar termination radius. The initial installation was intermittant, and had shadows. The transponder 'lack of reply' issue was a hassle during the first couple flights. It feels really good to know that everything is working well now.


I could relocate the antenna to the front and see if it works. But it is really tough to justify moving it back when the system works so well right now. It just isn't that big a deal to unhook the antenna when changing the oil.


FYI, the transponder needs to be checked every two years. Even if it is homebuilt, experimental, VFR, etc. The test box plugs into the coax line after the transponder. They verify that the encoder is reading out the right altitude (no crossed wires) within some tolerance. Then they connect the cable and put a box over the transponder antenna. They verify signal strength, frequency, and a couple other things like that. Total cost was something like $130 for the transponder test, and $70 for the materials for cables. Half of that was for the connector that fit the back of the transponder.



Norm Muzzy

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