Why do my radio stations sometimes change when I'm near a semi

It isn’t that the tuner changes, but if I am listening to a station like 89.1 and it is playing a program sometimes when a semi approaches or passes the program will change to something else, then change back when the semi passes. It doesn’t happen with all semis, and the stations can vary, but it does happen about once a week.

Normally in a situation like this I assume the most convoluted, persecutory, superstitious explanation is probaby the correct one (Costanza’s razor, the concept is called). So a la the movie Frequency I assume the disembodied victims of a cabal of homicidal truckers are trying to communicate to me via song and music. However that theory doesn’t explain why I don’t get that from all semis, or why the other murderers I tend to hang out with don’t give me that weird radio vibe.

Either way, is it CB radios or something? It is like my radio station is blocked out and another one comes up. It isn’t other people talking that I hear on my radio it is a different station, and once I am a few hundred feet from the semi my old stations come back.

FM radio is propagated by a ground wave, so the truck may be getting between you and the transmitter.

If it is FM radio stations, I think you may be experiencing the “capture effect”.
http://en.wikipedia.org/wiki/Capture_effect
There is probably some minor reflection or blocking of a signal that allows another otherwise weaker signal to capture your receiver.

Once occurrence a week would suggest that the station you are listening to is low power and/or a long distance away.

Sometimes satellite radios, iPods and other audio devices use an FM transmitter to get the audio to the car (truck) radio. An FM transmitter is basically a mini radio station. Normally they are very weak and will not overpower a radio signal, you need an empty station. But I have heard that some of the satellite radios came with transmitters that where overpowered and could overpower a radio station even in a nearby car.

The lower numbers on the dial are most popular for FM transmitters, those are the stations that are empty more often. When I used to use an FM transmitter I would often get briefly interrupted when I passed another truck, most of the time it would be Howard Stern :stuck_out_tongue:

I thought that ground waves were limited to lower frequencies, and
this page agrees. But of course the truck can still get between you and the transmitter.

I think the flat metal side of the truck is acting as a mirror and an obstacle. If you were locked onto a strong signal from your right, and pass the truck on its left, the truck is between you and your original signal. But it is also reflecting anything to your left back at you, maybe doubling the signal strength too (if the distances are right).

Just a guess, though.

This is almost certainly what’s going on. To speculate a little further, the FCC passed some regulations that vastly reduced the power of devices with FM transmitters in them sometime around 2006 (IIRC). But truckers were early adopters of satellite radio in particular, so they might be more likely to have the older more powerful radios. I have a 2005-vintage Audiovox Sirius receiver that I used to leave on in my car and I could tune in an FM radio on my desk about two dozen yards and a couple of cement walls away. I’m sure it probably causes the blank out to any mere mortals in the next lane using a newer transmitter on the same frequency.