Dear Editor,

As a radio frequency design engineer I have read and listened to much discussion regarding the proposed Two-Way Automatic Communication System (TWACS). This is also known as low frequency power line communications (PLC). TWACS is a low-speed data over power-line metering system that San-Miguel Power Association (SMPA) is considering for advanced metering.  There appears to be much misinformation out there regarding this particular metering system.  Perhaps the greats myth about TWACS is that it operates at radio frequencies (RF).  TWACS, in fact operates with frequency content in the 60-650 Hz, audio frequency range, theses same frequencies pass through the wires that go to the speakers in your home audio system and through your telephone lines.  These frequencies are not RF by any stretch of the imagination.

TWACS signals are designed to pass through power line transformers without significant impairment and travel long distances (tens of km) over power lines without repeaters.  Radio frequencies over power lines do not satisfy either of these criteria; hence lower audio frequencies are used for the TWACS system.  The TWACS system is specifically geared to rural power utilities where the long distances between homes and substations would make an RF system with its requisite repeaters completely impractical.

The wavelength of these low frequencies is extremely large, and, therefore, even the longest unshielded power lines are still extremely inefficient radiators. Furthermore the TWACS signal is operated at power levels far below that of the 60 Hz power that is also delivered on the power line.

TWACS should not be confused with Broadband over Power Line (BPL) systems. BPL is another power -line data technology that you may have read about.  BPL does often operate at RF frequencies with additional circuitry to bypass transformers.  However, BPL is not a system being considered by SMPA.



Eric Funk, Ph. D.

RF Design Engineer

Red Mountain Radio LLC

Ouray, CO