Radiocommunications Agency  EMC Awareness

Power systems

Description

The FCC’s Seattle Office reportedly responded to a request from the Langley Air Force Base Rescue Co-ordination Centre in September 1994. It reported that its Search and Rescue Satellite was receiving interference on its 121.5 and 243 MHz distress frequencies.

The area over which interference was a problem was around 8 square miles, which was significant because normal emergency transmitters on these frequencies can only be detected at ground level for about one mile.

Mobile direction finding equipment only found power line noise, which included some emissions on the affected frequencies. When personnel started shaking the pole guy wires at one location, the interference dropped significantly. The local power company was asked to check the pole and found loose connections. When they were repaired, the problem disappeared.

Commentary

Electrical arcs and sparks, even if they are little more than quiet ‘fizzing’, create electromagnetic interference right throughout the radio spectrum. The emissions tend to be at harmonics of the supply frequency (60Hz in the above case) with the amplitude modulated randomly by the very same noise that one can hear them making (e.g. a fizzing sound).

The natural resonant frequencies of the power lines, acting as ‘accidental antennas’ will cause there to be higher levels of emissions at some frequencies than at others.

Power system engineers often use radio receivers with very directional antennas, sometimes when leaning out of the doors of helicopters, to help them locate bad connections.

The only practical solution to this problem is to make sure that arcs and sparks don’t occur at electrical power connections. This is also an aim of the power system engineers, but in their case it is because poor connections can cause significant damage to power system equipment (and to network reliability) if allowed to continue.

References and links

“Power lines interfere with rescue satellite” by Isidor Strauss, ‘Newswatch’ Editor, Compliance Engineering European Edition, January/February 1995, page 6.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Suppressing arcs and sparks
Bonding