The VOR thread has prompted me to ask about xpndr antenna locations in use on our planes. Mine is up front between the gear and it does not work!! In fact, it was there when I bought it and I can never remember having this problem in the past! I can connect the xpndr to the RNAV antenna (yes, still have it installed ) and the darn thing works fine Radio guy on the field says to move the antenna since it is blocked by the lower cowl lip and gear.....funny, I would think the nature of aircraft movement wrt atc facility and height above would offset the small lip and gear.... I have installed a new antenna, cable, etc and continuity checks are good.
Where do you guys have your transponder antennas mounted????
What problem are you having? ATC can't receive? It's failing the FAR 91.413 transponder check?
Did you install the new cable and antenna to try to solve the problem or is that when it started?
I would definitely not start cutting new holes in your airplane to solve this problem (whatever it is). I doubt the gear legs or cowl lip could be responsible.
I definitely do NOT want to cut a new hole in the airplane. The problem arose on a "VFR flight following" request a while back. "primary target only, no alt or code readings from the plane....despite seeing the xpndr light apparently working.
Radio shop pulled the xpndr, bench tested it and found power out, signal, etc all within specs. They then did a continuity check of the antenna and found it good (later, replaced that antenna while troubleshooting with a brand new one), shop replaced cable as it was about 15 ft long and wound up in the belly. That done, it still didn't work. Swapped xpndrs between planes and the xpndr was dead on with atc. flew the 70 with replacement xpndr and it showed the same indications with no output.
Finally, connected the xpndr antenna to the RNAV antenna and went flying. Low and behold, xpndr seen just fine by ATC. This, with a second patch cable to reach the RNAV antenna under the rear seats. Still need to fly it with the RNAV hooked to the old xpndr antenna to see if the RNAV works there. If it does, I'll leave it swapped. However, it does not make sense to me why the "front" antenna wouldn;t work. Shop claims that the intricate signal combined with the freq of the IFF xpndr results in it being blocked by the lip and gear........ being a career military flyer, seems like bs to me but......
Have seen this issue before . Firstly the floor or the inside skin must be clean from oil and contaminates . I mean hospital clean . Next no paint on the inside skin area . The aerial must earth it self to the skin of the aircraft . The aerial edges where it joins the fuse large on the outside skin must be sealed with the appropriate sealant.this stops water and oil getting in to this area. There is no reason why you would have trouble with the system ( aerial) in that position that you indicated. Plus do not torque the screws holding the aerial on to the fuse large too tight. Torque the screws as per manufactures instruction .
I cleaned it wen replacing the antenna but I will check it again, thanks! wrt too tight, do you mean not to tighten the antenna so much that you risk breaking it inside?
What type of antenna are you using a blade type or a pole type ? Which transponder? Which encoder? Did you say when you have an issue ATC doesn't know your there?
I'm using a new pole antenna on the transponder (the expensive one) and the rnav antenna is also apole. My transponder is a Narco at-150, same as the one in my Stearman (they both work fine when swapped back and forth in the stearman). Encoder.....hmmmm, I'll have to check.....
In the 170, with the transponder connected to the front antenna, ATC only see a primary contact. By that, I believe they only see a radar hit. No IFF track or alt info. On the back pole antenna, they have no problems seeing me with IFF.
I positioned my blade antenna about 18 inches aft of the gear.
By no means do I have extensive knowledge here but I've had a few issues with my plane. I had a small (what I call pole type) antenna which appeared to be ok visually but was the cause of a bit of grief. Likely it had been hit/damaged by debris which had struck the mast and effected its performance. I've since switched to a blade type which I think are more robust. The second problem I had was with a new transponder which worked mode A but would not work mode C. I rang out the encoder to transponder wiring several times thinking it was at fault which it wasn't, gambled on a new transponder and solved the problem.
Its possible its an impedance matching problem with one component on the edge of performance.
My transponder antenna is located between the gear legs.
I experienced similar issues.
I tried a different transponder, which worked for a while then went intermittent, then quit.
The avionics shop ramped checked it and , it checked out normal.
Then the transponder failed again.
Then we tried a 3rd transponder. It worked for a while, and then quit.
I changed the antenna, which was a King KA-60, no change,....
I then pulled out all the wiring for the transponder, and rewired it.
We installed a new circuit breaker.
When avionics shop came to check the transponder after the new wiring, they found the coax was intermittent,
but only intermittent, when the cable was being moved around, near the antenna.
A new coax cable was installed, and the transponder has been 100 % for the past 8 months.
( so now I have 2 good, spare serviceable KT-76 transponders )
Had a chance today to check to see where my transponder antenna is located. It has been in its current location for 25 years and has worked at all times as designed. Mine is about 8-10 inches forward of the main gear. Guess whomever did the installation knew not to put it between the gear legs.
The belly location between gear legs is not the problem. Anywhere on the belly is fine (but consider placing it slightly aft of a line directly between the gear legs in order to accomodate a jacking-cradle for maintenance purposes.)
Directly beneath one of the pilot's seats is my personal suggestion (after personally viewing scores of them there that never had a problem.)
It sounds as if your problem might at least partially be your avionics shop. Their opinion of the cowl lip "blocking" the signal is patently ridiculous, and brings their credibility into question.
It is most likely a connector and/or cable issue that you are fighting. A common volt/ohm meter is not adequate to trouble shoot that cable because continuity may appear fine when in fact there is a leak when your AT-150 blinks-out a 100 watt burst.
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) and the darn thing works fine 
Radio guy on the field says to move the antenna since it is blocked by the lower cowl lip and gear.....funny, I would think the nature of aircraft movement wrt atc facility and height above would offset the small lip and gear.... I have installed a new antenna, cable, etc and continuity checks are good.
