To what extend do I need to worry about nearby power lines causing interference problems?

Actually, we have had very few problems associated with power lines or transmission towers. In general, it is the other sources of EMI/RFI that are of most concern, such as CRT computer monitors that are in the VLF range. Most of these devices have a horizontal sweep of 24.8 KHz and NLK, one of our transmitter stations, transmits on... 24.8 KHz.

Our initial testing was done at a school with power lines (transmission towers), and we never saw any trouble from them. However the computer lab in the classroom next door did. The solution, we found, was to move the antenna about 10 feet (3 meters) or more from the troubling source(s) and the SID monitor will work fine -- if the source is more intense then further away is better... It doesn't have to be outside or on a roof, but these seem to work best as they will not be disturbed.