Transponder antenna fairing

[krw5927]

Don, as always very helpful information. Thank you. Makes me even happier that I have four of your antennas on my 9A.

[912ry]

Originally Posted by az_gila

Is there a TED antenna under the fairing?

yes, those are the antennas. I used pink insulation foam to make the shape, and incorporated a nylon block at the base. I put a set screw in the block that tightens to the nylon insulator on the antenna, just enough to hold it steady. Although the purists will disagree, there was no change in the performance of the antenna; At least not in my empirical testing.

[912ry]

Originally Posted by krw5927

Very nice! Can you please provide some detail on how these are attached? Do they attach to the aircraft or to the antenna, or both?

just a bit of foam, shape as you wish, a nylon block at the base with an allen set screw, one layer of fiberglass. I left the foam in there, it was really light already.

[Walt]

If the fairing is removable, I'd be willing to do some ground tests to check the effect.

[912ry]

What? nobody noticed the beautiful sunrise photo my wife took?

[DanH]

Originally Posted by dpansier

With that said what does this mean to the average builder? By covering the ball and rod transponder antenna with a fiberglass / micro cover (Dielectric material) the electrical resonate frequency is changed from its design frequency of 1030 to 1090 to a new frequency of 830 to 890 MHZ. The VSWR also increases to level that may exceed the transponder spec?s.

Hi Don...we talked about this before, which is why I have not mixed up some micro and cast a airfoil shape around my rod'nball antenna.

Curiosity question; it sounds like the dielectric described above is in physical contact with the antenna mast. Does theory predict a different result if the airfoil fairing is a thin glass/epoxy shell, not in contact with any part of the antenna?

[912ry]

Originally Posted by Walt

If the fairing is removable, I'd be willing to do some ground tests to check the effect.

Sounds fun! Ill try to swing by next week and we'll play with it.

[dpansier]

To answer questions I fabricated a foam shape similar to what I believe Robert displayed in the first post to test the effects on antenna performance.

The test consisted of a TED monopole transponder antenna mounted on a 8? dia ground plane and 1? thick blue "Home Depot" type foam. The foam shape did not touch the antenna element.

Data collected with Anritsu antenna measurement equipment and calibrated with traceable standards.


Results:

The antenna resonate frequency was pulled down but the VSWR was still within reasonable limits. Based on what I tested I would not be hesitant to attach this antenna system to my transponder.

Suggestions:

Minimize material on the exterior of the foam shape, construct a thin FG layup to form the hard shell.

Do not use the antenna element to attach the foam shape, the results improved when the foam did not touch the element.

A base plate could be constructed from a material that will adhere well with epoxy. A wood paint mixing stick may work well in this application.
Studs or nut plates could be bonded on the base plate to attach the foam shape on the airframe.
Position the mounting studs or screws with the hole layout shown below as it will allow mounting a Delta Pop or Comant CI-105 transponder antenna in the future without drilling any additional holes.


As always, use caution when modifying antennas, the proper test equipment should be used to insure the modified antenna meets the system requirements before attaching to your expensive avionics.




Depiction of test antenna

[912ry]

I hadn't looked at this thread for a while, thanks for the testing. My fairings sound like just as you describe. I hollowed out the foam after laying up the shell. Minimum weight and not touching the antenna. Unfortunately this makes the fairing cross section a little thicker than if it touched the antenna, but not by too much. I have noticed no reduction in reception with my ADSB antenna. Quite honestly I did this for the fun of it, but hopefully it makes the plane go faster too!

[flightlogic]

I am pretty darned impressed....
A guy who has a commercial enterprise delving into the experimental efforts of a do it yourselfer....
Taking time to do research.... sit at the computer.... and SHARE.
Look around at other forums. That set of ethics may be hard to find.
I think I might order a DP just on principle. And paint it candy apple red just for aesthetics... NOW where did I leave my lead paint can???? Gotta be here somewhere.....