FM Radio station break through when close to ground

[bob865]

I have this issue with the radios in my plane. This is actually my Mooney, but I figured the community here could probably help. My local airport, KGMU, has a tower frequency of 119.9. When close to the airport, within 3-5 miles, and close to the ground, below 3000'msl, I have what sounds like an FM radio station breaking through the squelch and making my radios incredibly noisy. This happens on both my GTN-650 and KX-155. I don't remember having the issue once I climb out of the area or at other airports. I've asked other pilots around and they don't seem to have any issues on their radios. I asked the avi shop and they don't seem to have any ideas either. I saw the post recently about the notch filter for GPS and it got me wondering if there was something similar I could use to help reduce the out of band interference from the ground. I thought someone might know of something else that I could look into.

Thanks!

 

[OKAV8r]

Sounds like a phenomenon known as "multiplexing". It is location specific, as the 'participating' transmitters need to be in a certain proximal arrangement to your airplane for the effect to be strong enough. It typically involves FM broadcast transmitters. A filter might work, trick being figuring out what to filter. Also, check the VSWR of your antennas.

 

[Ruready]

Let the FAA kno, you'are just a hoppskipjump from the RYY flight check base, they are flying by gmu all the time, they might send a flight check king air with signal hunting equipment to find the station. Or maybe just asine a different freq to gmu tower

 

[N184DA]

I have this issue with the radios in my plane. This is actually my Mooney, but I figured the community here could probably help. My local airport, KGMU, has a tower frequency of 119.9. When close to the airport, within 3-5 miles, and close to the ground, below 3000'msl, I have what sounds like an FM radio station breaking through the squelch and making my radios incredibly noisy. This happens on both my GTN-650 and KX-155. I don't remember having the issue once I climb out of the area or at other airports. I've asked other pilots around and they don't seem to have any issues on their radios. I asked the avi shop and they don't seem to have any ideas either. I saw the post recently about the notch filter for GPS and it got me wondering if there was something similar I could use to help reduce the out of band interference from the ground. I thought someone might know of something else that I could look into.

Thanks!

Happened to me when I first completed my RV4.
Culprit was the ELT, and proximity of the ELT and com antennae.
ELT was an ACK and apparently the FM radio station turned my ELT into a repeater of sorts.
I called ACK tech support and their suggested remedy: trim the ELT antenna.

I did and it corrected the problem.

Strange stuff

 

[pilot2512]

ELT antenna

Way back when I was an avionics tech I saw this a few times. Always the ELT. Best way to test is to fly it with the elt antenna disconnected to confirm.

 

[BillL]

This is an excellent thread - - I have had the same issue at KBRL, RV7. Super noisy - - I still have not gotten it addressed but have been lots of other airports around the US and not had this issue.

OK - if it is the ELT, how does the "trimming" of the antenna work - dimensionally. And . . should it be done 1/8" at a time and fly cycle? A little guidance please.


Edit - thanks for the comment below - -I will go and recheck this again, then call ACK to see what they say. I have the regional FCC guys name and have talked to him. He, and my airport manager, said this happens and he (FCC) does measurements and they apply enforcement to fix validated issues. My bent whip Comant COM antenna are nearly the width of the RV apart. My 406 antenna is curved along my roll bar inside the cabin.

 

[N184DA]

This is an excellent thread - - I have had the same issue at KBRL, RV7. Super noisy - - I still have not gotten it addressed but have been lots of other airports around the US and not had this issue.

OK - if it is the ELT, how does the "trimming" of the antenna work - dimensionally. And . . should it be done 1/8" at a time and fly cycle? A little guidance please.

I don't recall, as this was sometime ago (circa 2000-2001??)
And I will defer to greater theoretical RF engineering minds on this forum as to the "why"

Having said that, I "think" I started with 1/2", and it only took one trim.

BTW: Whatever advice I had came directly from ACK

 

[sabswbc]

Don,t just disconnect the elt, take the antenna off and fly. I had to go to an internal antenna to make my radios work. I flew near some tv antennas that made my kn170B ‘s go crazy

 

[Sam Buchanan]

Just a datapoint that is is a little off topic for this discussion. In one of my light aircraft I used a handheld COM with an external antenna. When I flew downwind over the VOR that was on the field a strong broadcast station (FM?) would briefly break the squelch. I never knew what caused it (no ELT on the single-place aircraft) but it must have been the VOR transmitter interfacing somehow with a local broadcast station.

 

[Webb]

Sounds like the transmitting stations are transmitting harmonics. Since the harmonics are on or close to aviation frequencies, I think a call to the FAA may be warranted due to interference with communications.

 

[Mel]

Sounds like the transmitting stations are transmitting harmonics. Since the harmonics are on or close to aviation frequencies, I think a call to the FAA may be warranted due to interference with communications.

Or maybe the FCC, or both!

 

[GalinHdz]

Sounds like the transmitting stations are transmitting harmonics.

I highly doubt this is the cause. If it were caused by a local radio station then most airplanes flying in the area would have the same issue which, even he reports, is not happening.

I've asked other pilots around and they don't seem to have any issues on their radios.

 

[bob865]

I don't think it's harmonics or any of the "Call the FAA/FCC" solutions because talking to other pilots I think I'm the only one with the issue. If it were that, the FAA/FCC would already be involved considering GMU is the busiest GA airport in the state. I think it has to be isolated to my aircraft.

For those that said ELT, it's interesting to hear that. I did talk to an AVI shop and they mentioned ELT briefly. I dismissed it at the time as non-sense since this was on the receive side and not the transmit side. I think I'll try disconnecting the antenna from the ELT when I fly again and see what happens. Should at least be able to give another data point.