Are you one of the many who had to upgrade their transponder to be ADS-B compliant? Do you hear a crackling/popping sound in your headset now? It just might be the location of your transponder antenna.
I was flying around with an old Narco AT-150 transponder. It had served me well for almost three years after buying N78878. I was one of the holdouts, hoping for something less expensive that would get me compliant. As time was winding down, I was leaning towards the SkyBeacon, to work in conjunction with my dinosaur transponder.
Then it happened, as I was approaching my home field at KFUL, I called up the tower, and after tower responded “November 78878, make straight in runway two-four” he continued with “I’m not getting your Mode-C, recycle your transponder.” I turned it off and then back on, but still no joy.
After landing, and before the next flight, my AP and I checked everything we could, yet nothing seemed amiss. A number of successful flights were made after that, and I was beginning to think I was in the clear. That is, until I was taking a couple friends on a breakfast run. We took off and as we were turning downwind I heard “Mooney 878, recycle your transponder, I’m not getting anything from you.” I asked if it was just my Mode-C and he said there was nothing.
Oh boy…
I looked over and noticed the breaker had popped. I pushed it back in and it popped again. I informed the tower the situation and said we would be coming around to land.
Facing a dying/dead Narco, I decided to just pull the trigger on a GTX 335. Under direction of my AP/IA I installed it and it passed the transponder check with flying colors (no pun intended).
Everything pulled apart. If you are wondering, the thought does cross your mind that you have to put it all back together. 
All the trays mounted in the rack. 
Panel back together waiting on my AP/IA to install the new control cables and inspect my installation. 
New Panel.
Sometime afterwards I started to realize there was a crackling or popping sound in my headset. I wasn’t really sure when it had started, and after taking off and flying I didn’t really notice it much, it was just there every now and then in the background, just enough to be annoying. I knew it wasn’t engine related because the noise was there even if the engine wasn’t running.
After checking multiple things I stumbled across the possible culprit. I read that the new transponder puts out more power than the old dinosaur, and if your transponder antenna was too close to the radios the result could be causing the noise I was experiencing. My transponder antenna was in the same location as many other Mooneys, a few feet behind the exhaust, almost under the co-pilots feet. The way to check was simple, power up your avionics and have your transponder on and transmitting, listening for the sound. Switch your transponder over to standby and see if the sound stopped. I thought, “That’s easy enough to check.”
I headed to the hangar, got in the plane, put my headset on, turned on the avionics, and listened. Sure enough, there was the crackling/popping sound. I reached up, pushed the standby button, and… silence. Could it be? I pressed the ALT button so that it would start transmitting again and there it was, that annoying crackling/popping sound. I pushed standby and again, glorious silence.
A few months later during annual with all the inspection panels off and the interior pulled I ran new RG-400 cable and moved the transponder antenna back on the fuselage to the belly a little aft of the avionics bay. With everything hooked up I put on my headset, powered up the avionics, listened, and heard… nothing. I couldn’t believe it was so simple, but it was. The location of the antenna made all the difference.
Typical Location Up Front 
New location
If you recently upgraded your transponder and are hearing a crackling/popping in your headset, take a minute to put your transponder in standby mode and see if it goes away. If it does, and your antenna is located where most of them are (up front), the next time you have your interior pulled, run some new wire and move the antenna back. That is unless you like the sound of bacon frying in your ears while you fly.
