Interference to PLC systems from Amateur Radio Operation

From a post to the amwindow.org discussion forum by Ed Hare, W1RFI

Interference is a two-way street, and PLC systems are at significant risk from amateur HF operation. In the US, amateurs are limited to 1500 watts PEP RF output, but there is no limit to the antenna gain. As a practical matter, few amateur antennas exceed 13 dBi on HF. This means, however, that the EIRP from amateur stations can exceed 20,000 watts.  These stations can have antennas that are as close as about 10 meters or so the the electrical distribution systems. 

Here is an estimate of the interference potential of a more modest HF station on 7.15 MHz:

Those unshielded overhead power lines are not great antennas, but they can and will pick up our signals. Here is a quick calculation:

Transmit power: +26 dBW (400 watts)
Transmit frequency: 7.15 MHz
Distance between antenna and power line: 20 meters
Path loss: -15.6 dB
Transmit antenna gain (with ground reflection): 6 dBi
Power-line antenna gain: -10 dBi (estimate)
Power picked up on power line:
          +26 dBW
          -15.6 dB
          +6 dBi
          -10 dBi
          -------
          +6.4 dBW (4.4 watts)

The total power of their signal inside the line is going to be about 10 milliwatts, and when we transmit, PLC wiring may pick up 4 watts of our power right inside the frequencies PLC is using. It is unlikely that PLC systems will continue to function in the presence of these signal levels