I’ve noticed that whenever I drive under high voltage power transmission lines (i.e. the kind carried on large towers, not just standard commercial/residential service lines) that the radio is drowned out with noise. It’s not exactly a hum, and it isn’t at 60Hz, either. This only happens when listening to AM, not to FM.
I could see how power lines might induce 60Hz AC on the radio circuits, perhaps at the pre-amp or post-amp stage, or in the speakers themselves. This would probably indicate that the amplifiers and/or speakers are not sufficiently shielded. Two problems: 1) I’ve seen this in multiple vehicles and 2) if this were the case, I would pick up this noise no matter what I’m listening to - AM, FM, or CD. Because it only occurs with AM radio, it must be picking up interference with the carrier signal itself.
How does this happen? AM radio band in the U.S. is 520 - 1610 kHz. This well above the 60Hz ELF (Extremely Low Frequency) radiation that you would expect power lines to emit. The fact that I only hear it with major transmission lines and not with service lines is probably just a question of magnitude. Service lines probably throw off the same interference, but in much smaller amounts since they carry less current. But how do any of them emit radiation at 0.5-1.5 MHz at all?
You don’t hear interference on FM because FM receivers follow the signal as it varies in frequency. It’s not the signal itself that the FM receiver decodes. It’s the frequency of that signal (hence, frequency modulation). An FM receiver is going to track the loudest signal on any given frequency band, which means any smaller noise signals just end up being ignored.
With AM, the noise adds into the signal and therefore can’t be separated from the signal.
In other words, the noise is there on both the AM and FM bands, but because of the modulation methods, you only hear it on the AM band.
As for what causes the noise in the first place, it is due to the fact that the field strength near the surface of the power lines is greater than the breakdown voltage of the air, so you get a small discharge of energy around the line. You can sometimes also get a visible glow of light near the line for the same reason. Noise from these types of sources tends to splatter across a very wide range of frequencies.
Power companies will also send communication signals down their power lines (it’s cheaper than running separate communication lines from one place to another). Sometimes some of the noise you hear is due to these higher frequency communication signals that are coupled onto the line.
Mostly, though, it’s the air breaking down due to the high voltage.
I understand the mechanisms of AM and FM transmission, but I hadn’t really thought about the implication for interference. That makes sense. For FM, as long as the power level of the noise is kept beneath the power level of the carrier signal, the receiver should still be able to track the modulation accurately.
I hadn’t considered the possibility of corona. I didn’t think there was an appreciable corona affect for power lines, given that I’ve never seen a visible corona on one. After doing a little research on it, I’ve learned that apparently some power lines do have a faintly visible corona. I’m guessing you don’t see that 110kV lines, though, because I’ve never noticed it. I guess I’m just a little surprised, what with air having breakdown voltage on the order of 1-3kV/mm, depending on humidity.
I also found what might be another contributing factor. It seems kind of obvious, now that I think about it. Basically, the current wave on a power line is not a perfect 60Hz sine wave. There are enough non-resistive loads on the grid that you get some amount of higher-frequency perturbation of the current wave, a.k.a. “harmonics”. Things like variable frequency drives are apparently especially bad about this. The higher frequency fluctuations will obviously produce a certain amount of higher frequency EM radiation from the lines.
All this raises another question, though:
Recently I’ve started seeing cell phone antenna arrays mounted on top of high voltage power line towers. These things will be situated just a few meters above the conductors. How do they handle the interference? As I understand it, cell phones use a flavor of phase-shift keying rather than FSK or ASK. I don’t know if PSK is more or less prone to interference than other modulation schemes, but the signals are very low power. The broadcast power of a typical cell phone is only about 250mW, and I’m sure it attenuates quite a bit before it reaches the tower. I don’t see how an antenna sitting that close to a high-voltage power line could hear such a weak signal over all the noise.
A cell phone (digital or analog, it matters not) does not use Amplitude Modulation so it will not be affected by power lines.
Nextel likes to use high voltage power towers, FWIW. They are often several feet higher than anything surrounding them, and they don’t have to erect anything- just mount the equipment at the appropriate height.