Why does my radio get possessed at this time of the year?

I have a clock-radio on my nightstand that, 365 days a year, is tuned to the same public radio station. Except that in the middle of the summer, for some reason, the tuning goes all wiggy, and changes without human intervention.

This is a problem, because it’s my alarm clock, and the gentle hiss of static between stations is not as effective at waking me up as the Morning Edition fanfare.

I think it’s actually changing during the day, because I checked last night before bed and I was getting static, and tuned it to a station that was playing something orchestral . . . which turned out, this morning, to be a Christian station. Ugh. What a way to wake up.

It’s the kind where you turn a knob, not a digital tuner or anything.

I have a couple half-assed hypotheses:

  1. This is the time of year when the bedroom experiences the greatest temperature swings. We don’t have air conditioning, so in the day it gets into the 80s, at least, and at night it gets down into the lower 70s or (blessedly) upper 60’s. Perhaps thermal effects are changing the properties of the resistors, capacitors, etc. in the tuner so that the resonant frequency drifts, or thermal expansion and/or contraction are making the knob to move physically.

  2. Another, nearby station (99.7 MHz) does weird stuff in the summer. I listen to it often in the car, and I think they turn their transmitter (or something?) this time of year, because suddenly my reception gets really bad. It comes back around October. The station I tune the clock-radio to is at 100.9, so maybe there’s some kind of interference thing happening with the other station? I dunno.

For completeness’ sake, I should probably mention that this is one of those “atomic” clocks that recieves the signal from the atomic clock in Boulder and automagically sets itself to the right time, but I don’t think that has anything to do with it.

Your half assed guess number 1 is basically correct.

The old dial tune type radios use a variable capacitor for the tuner. This variable capacitor is made up of a bunch of metal plates in parallel with a bunch of other metal plates. When you rotate the dial, you move the one set of plates so that they are closer together or farther apart from the other set of plates, which changes the capacitance (and since the capacitance changes it changes the resonant frequency of the receiving circuit, which selects what station you are tuned to).

Here are some pics that might help you visualize what I’m talking about:


Since these are made out of metal, they expand and contract with changes in temperature, which means as the temperature of the room changes the tuning of the radio changes.

As for why the other radio station gets worse reception, it is possible that they use a directional antenna and beam the signal more in one direction than another during the summer (maybe to try and catch more listeners in a particular summer resort area) or it’s also possible that the atmospheric conditions (particularly moisture) vary during the summer months and this cuts down the reception.

Changes in the tuning circuit are one possibility. Changes in the atmospheric propagation of radio waves is another. Depending on where your station’s transmitter is physically located and the proximity of other stations, you could be getting interference or just poorer reception. I know that this time of year, my favorite nearby station is often drowned out by some Christian station that’s located 300 miles away off the coast of Maine.

Since you agree with me, clearly you are a genius :slight_smile: however, if that’s the whole story, then shouldn’t the frequency be directly relatd to temperature, e.g. if it’s 70 degrees in my room this morning, I wake up to stack, so tune to 100.9, even if frequency drifts through the day, when it’s 70 degrees in my room tomorrow morning, shouldn’t the radio be tuned again to 100.9, instead of off in la-la land between stations? That’s what makes me suspect that part of the problem might be mechanical, so that in the heat of the day, the parts expand, and something gets out of whack somewhere.

Thanks for the pics! I have a strictly Physics-102-level understanding of electronics, and thought capacitors only had two plates, but it makes sense to have a bunch of them. . .