This is something I've always wondered about. We've probably all noticed that when driving beneath big high voltage transmission lines, AM radio audio often sounds "phasey", like a SSB signal. It doesn't seem to happen beneath lower voltage "local" power lines.

I would guess that the long lines create a sort of tuned waveguide that emphasizes one sideband over the other, thus causing this effect, but I don't really know. Can anyone offer a good explanation of what causes this?

HV transmission lines have a huge, dense corona. It sounds 'phasey' because it is.

If you lived by a support tower, you could take a fluorescent tube out to it and it would flicker and glow while you were standing on the ground.

Power lines, especially if they are relatively close to the transmitting antenna, have a tendency pick up and re-radiate an AM signal. Depending on the circumstances, the re-radiation can be as much as 180 degrees out of phase at the carrier frequency but maybe not so much at the sideband frequencies. Therefore, if the main and re-radiated carriers enter your radio antenna out-of-phase, the result is that the radio sees a reduced carrier relative to the sidebands producing what sounds like overmodulation or even SSB. What you have are the two signals essentially fighting with each other.

Dave

Interesting about the AM. As far as I know, the radiation from the lines tends to simply diminish or cut off other radio signals.

Some power companies may still be using power line carrier communications between substations
for supervisory control. It operates in the LW band and the last one I worked with used a 50 watt
transceiver coupled to the line with capacitors that looked like lightning stacks. It was a 66kV line
13 miles long, and it got into AM radio to a certain extent.

Its cheaper than microwave or fiber especially if its been there forever and works.

Yes, that is correct...they can do that, too - either from radiating their own interference or simply from the shielding effect that the lines have. But David asked about what causes the "phasey", SSB sound under certain conditions when driving under a power line while listening to a station.

Dave

And the lines are ususally three phase...

Ah, that makes PERFECT sense. Then the I think "driving under power lines sound" is similar to the phasey (or what I might call "comb filter") sound that is heard when you listen to a semi-distant station and pick up a mixture of ground wave and skip signals, which can also be out of phase and fight each other.

Thanks for the reply, Dave. I had been wondering about this for ages.