This is very interesting. I had always considered power lines to be the enemy, especially to vertically polarized antennas. Did you happen to notice this effect on several different frequencies? I don't recall seeing, so I will have to infer, that you were primarily working SSB? I have a set of transmission lines, very similar to the picture shown, just a couple of miles from the QTH. I have always tried to get away from them. Perhaps this summer I will give it a try. Thanks!