Transponder antenna in wing?

Anybody mount a transponder antenna in one of the wing access panels (the wing NOT containing the autopilot roll servo)? See any problems with this?

Also, is the "minimum of 3' from the transponder" rule a "line of sight" rule, or a "cable length" rule?

Thx.

It's an antenna-to-antenna line of sight issue.
You should find cable length restrictions in your transponder installation instructions, and it will depend on what kind of cable you use. That restriction will be likely be more than 3 feet. It was 7 feet for my choice of RG-400 cable and Mode S transponder.
As we all add ADS-B antennas, the 3 foot antenna placement restriction is going to be a problem. My transponder antenna is nearly in the center of the usable real estate on the belly, and a 3 foot radius from there doesn't leave many choices that I like. Out on a wing might make sense.

I answered a different question. I didn't have to worry about the minimum cable length issue, but I remember Stein saying I had to have the cable be at least 3'.

chipf said:

I answered a different question. I didn't have to worry about the minimum cable length issue, but I remember Stein saying I had to have the cable be at least 3'.

Click to expand...

Interestingly, Dynon has specifically said their transponder coax will not need a min/max length.

Here's a Table

This is from MicroAir depicting MAX lengths of various coax for transponder installs.

http://www.microair.com.au/admin/uploads/TransponderAntennaInstallation01R1.pdf

Depending upon your routing you will likely have a longer cable than the recommended maximums. I've seen several aircraft with coax exceeding these lengths with no reported problems.

macrafic said:

Anybody mount a transponder antenna in one of the wing access panels (the wing NOT containing the autopilot roll servo)? See any problems with this?

Also, is the "minimum of 3' from the transponder" rule a "line of sight" rule, or a "cable length" rule?

Thx.

Click to expand...

I've seen in on an access panel you describe. Might have a picture of it will look tomorrow.

Since transponder is "line of sight" and these aircraft have some dihedral, mounting it too far out on the wing will result in poor coverage off the opposite side of the aircraft as the fuselage will effectively be placed between the antenna and ground station and lower operating altitudes.

As I watched my hangar mate working on tires and tubes I realized that one also wants to place the antenna where working on a flat tire isn't going to result in the antenna lacerating one's noggin...

If you have a minimum distance between the transponder and antenna, the distance would be "line of sight".
I have had an experience with a Pitts where the transponder continued to "reply" to itself because it was too close to the antenna.

Minimum cable length:

You may be thinking of a 430W/530W. When the waas versions came out Garmin just bought an OEM antenna to go with the radios. The antenna turned out to be too sensitive. Rather than fix the real problem, Garmin required that you use a minimum cable length (they actually specified a minimum loss, which many installers met by using extra long cables!).