For what it's worth, in the short time I've been flying (~200 hrs) I've noticed two "holes" in the area where I fly where there are RF funnies that I can't explain. In these cases, I stopped receiving.
1) Doing IFR training near CXO. Was getting vectors to the ILS 14 from the east, and overshot the localizer because I never got instructions to intercept. Tried to call just before crossing, no response. Two calls later, I get a response loud and clear that Approach had been trying to vector me and I wouldn't respond. (There was little traffic on freq and no chance that I just missed the calls.) Got vectored to intercept from the west with no problems, flew the missed, and began another approach from the east. Encountered the same "black hole" and missed the localizer again. Ended up trying a third time with a different radio, and it was fine, but we also got vectored a little differently that time, so not sure if it was the radio or different flight path that solved the issue.
Sounds like maybe an auto-squelch set too tight or a radio with a deficient antenna/coax so signals aren't strong enough to break the auto-squelch. If you have a chance to fly that aircraft again in that area, pull the squelch knob out and put up with the static and see if you hear them weakly in the noise. From reading further I think you know that, though.
If the auto-squelch is too tight, it can be adjusted on most radios on the test bench in about an hour, start to finish. Sadly, these days that'll cost about $100-$150 and it's really only about ten minutes of work once the case is off. Might as well have the avionics shop sweep the antenna on the aircraft while the radio is out of the rack also.
But then again, RF sometimes just doesn't reach everywhere the system designers intended. We have a hole in coverage in one of the UHF repeaters that I maintain that I can't explain fully. There's a ridgeline in between but it's not high enough to block line of sight. There's a phenomenon known as "knife-edging" that might explain it, but it really doesn't. I suspect it's a null in the antenna pattern and twisting the antenna in its mount 10-20 degrees would move it elsewhere, but since the intended coverage is omnidirectional and that would just move the problem, we just live with it.
2) Flying direct between T41 and DTN, there is a region over E. TX where all of a sudden, I stop hearing Center. I can hear other pilots responding to Ctr on my freq, but I can't hear Ctr. Even with squelch defeated I hear _nothing_ on either radio (430W and KX155) except pilots. This has happened on three different trips. First time I didn't clue in to what I (wasn't) hearing for several minutes, and about the time I convinced myself there was a problem, I suddenly started hearing Ctr just fine again. (It's a sharp cutoff, too...not like I hear Ctr fading out, then fading back in--they're loud and clear until they're silent, and silent until they're loud and clear again.) Second time, I recognized it when it first happened, and dug up another freq from A/FD and tuned it on 2nd radio; I could hear them on the alt freq. Third time, I had the alternate freq in the flip-flop and was ready when it happened.
Weird...
Are all the other pilots higher than you? The controller may be keying a far away transmitter instead of the one closest to your location. The alternate freq may also be being keyed simultaneously, but they didn't switch you to it. Basically if they want to find you, they will. They'll either relay the alternate freq through another aircraft or they'll switch transmitters. If they have no traffic in your area and don't expect any, they may just let you fly along and forget to switch ya. Heh.
I've helped with a relay for a lower aircraft before to have him switch frequencies with Abuquerque Center. They forgot to switch the guy, basically. Usually they ask the jet jocks to do the relaying so I must have been the only aircraft near him for a looooong way around.
I laughed when I saw that the avionics shop used "RG-8" (supposedly, I didn't see it) for the re-run of our transponder cable. RG-8 SUCKS for 1 GHz. And I was surprised it wasn't listed as RG-8X mini-foam. It probably was. RG-8 is fat.
Of course, I'm doing weak signal stuff and 13dB of loss in a 100' run is completely unacceptable. A couple of dB for a ten foot run in an aircraft, isn't any huge deal, but sheesh. Try some LMR-240. Maybe that nice fancy "ultra-flex" variety for environments with constant flexing/vibration. LMR-240UF.
On a 50W transponder, RG-8X gives roughly 35W to the antenna. Shameful! And I know that blade antenna doesn't have much gain.
LMR-240... Same size as RG-8X... Gets 41W to the antenna. It'd cost a whopping $0.79 a foot.
Sigh. Avionics shops are apparently stuck in the 1960's.
