Lu and all,
Slight confusion here - the original post refered to high power RF antenna, whereas high voltage power lines are something completely different. Although, there will be a reasonable magnetic field around power lines and some low power RF interference, generally power lines are not much of a problem (except if you run into them!!!). RF transmitting antenna on the other hand, do emit substantial amounts of radio energy that can easily overwhelm sensitive electronics. A couple of years ago in an R22 I flew within sight of but not near a load of aerials on the way to Carlisle from Leeds in the UK and some strange broadcasts were picked up very clearly by my radio (over powering anything else). Nothing else was affected. I have also routed past Emley Moor UHF TV transmitter (which is at least a 1MegaWatt) in R22 and R44 but nothing affected - it very much depends on what frequencies are being transmitted, how and what power output, but the general advice to stay well clear is very good advice.
PS I find the clutch motor on the R22 interfers (whines) like mad on the radio