Black holes of radio reception in San Ysidro and Mount Soledad

Dear Matthew Alice: What causes the “black holes" of radio reception at the international border (Tijuana-San Ysidro) and on top of Mt. Soledad? I travel frequently to both places, and my radio goes berserk every time, alternating between stations without any discernible reason. Whit's creating this transmission turmoil? — Radio Daze, La Jolla/Baja

I’m assuming you’re listening to the FM band, since nobody but reactionary talk-show soreheads and news and sports junkies listens to AM anymore. An FM radio signal (a “line-of-sight” signal) basically travels in a straight line from the transmitter until it bumps into something like a mountain or a building or a car, and then it stops. When you’re tooling along I-5, grooving on the tunes, your radio is pulling in the strongest signal at a particular frequency. Once you get into the border area you’ve come out from behind hills and into an area that’s on the line of sight for a whole bunch of Mexican transmitters, some of which are powerful enough and close enough to the frequency set on your car radio that your tuner starts jumping around, trying to pull in the strongest one. It’s designed to do that, and usually it works to your advantage. (Some of the interference may come from “harmonics” from military or police transmissions in the border area; that’s a sort of ghost interference that occurs at double the broadcast frequency. The full explanation is mind-bending. I’ll leave it at that.) On Mt. Soledad, your radio tuner is being overwhelmed by the signals radiated from the many radio and television broadcast transmitters installed up there. So it’s just chaos in waveland, and your relatively wimpy car tuner has trouble making sense of it all.