[RFI] Powerline Bandwidth Characteristics

[Lee K7TJR]

Greg, one only needs to drive down the street while listening to a low frequency power line noise to find the answer to your question on bandwidth.
  The power lines are the unmatched antennas for radiating a noise source. Being unmatched they have large standing waves.
At any given point along a power line you may be in a null or a peak in these standing waves depending on the frequency you are listening to. One can easily hear the peaks and nulls along an offending line.
 An arcing source is indeed a broadband source but its radiated bandwidth can also affected  by whatever antenna like conducting objects it can inject RF power into. If the energy was injected into a broadband antenna the noise would be radiated strongest across the efficient wide bandwidth of the antenna. When the noise is injected into an essentially unmatched power line, the noise will likely be strongest at resonant frequencies on the line. Being unterminated there are likely many distinct resonant frequencies. 
    All that being said, it is also possible for noise generators like switching supplies to be quite broadband in nature. Most often they have many harmonics and are modulated by sharp line frequency splikes. This produces centers of noise with very wide sidebands at every 60 Hz. Often these sound very much like an arcing power line. Other noise sources like digital circuits are often heard at their clock frequencies and their harmonics. These are also often modulated at low frequencies like AC line or the frame rate of your computer monitor producing a noise that also sounds like a power line arc.
    The only things that I have seen and I have spent quite a few hours in the power companies interference finding truck for identifying different noises is lots of Experience and a small oscilloscope to look at the actual audio waveform of the noise. Sometimes you can identify different kinds of sources by their waveform signature. 
    My current power line guy must be one of the best. He has over 30 years experience in radio. The cab of his truck is full of receivers from DC to 1 GHz. He has 5 different outside snap on antennas and some he can rotate from the cab. He carries several ultrasonic units and several hand held vhf-uhf receivers/antennas. He also has a small oscilloscope for monitoring audio waveforms. Some of his receivers have internal spectrum display as well.
    The bottom line of all this from my perspective is that each offender has to be characterized and compared to known signatures. There is no easy way. You have to turn over all the stones in this game and keep notes. Even with all this equipment we often fail and have to wait until another time. My perserverence and his experience characterizing the interference has produced great results however. One arc took us 2 years to find and many easy ones take less than a day. The last offender we found used nearly all the equipment in his truck and 2 days. Turned out to be an arc in a junction cabinet for underground lines. 

    Good luck    Lee  K7TJR   Oregon