05.17.2005 12:00 AM
OFCOM Field Trial Shows Amperion BPL Exceeds FCC Part 15 Limits
Most broadcasters are aware of the potential for Access Broadband over Power Line (BPL) to interfere with communications and broadcast reception on frequencies from 2 to 50 MHz and possibly higher. The main threat is to shortwave broadcast reception and, if higher frequencies are used or harmonics are present, to low VHF TV broadcast reception.

A report released last week by Ofcom, the United Kingdom's telecommunications regulatory agency, found that the Amperion BPL equipment "as tested is not and cannot be FCC Part 15 compliant above 30 MHz." Ofcom also found that the notching facility present in the equipment "is compromised," "because notches cannot be placed in the 'upstream' spectrum, and because FCC Part 15 limits are too relaxed to permit the notched spectrum to afford any significant protection to weak signal reception."

The Ofcom report Amperion PLT Measurements in Crieff raises concerns about the effectiveness of the FCC Part 15 limits when protecting high frequency reception, noting that even with the 20 dB notch the PLT (the European name for BPL) leakage emissions at 10 meters from the overhead line would be approximately 40 dB above the naturally occurring spectrum noise floor of -10 dBµV/m. At 100 meters they would still be 20 dB above natural noise floor. Ofcom concluded, "Accordingly, FCC Part 15 compliant leakage emissions from a notched PLT line would contribute noise to the HF spectrum at distances as far as 1 km from the line." Ofcom notes, "Various radiated emission limits have been proposed, either for establishing network compliance or less rigidly, for the purposes of adjudication in cases of reported interference. It appears, however, that none of the proposed emission limits can currently satisfy the dual objective of protecting radio reception whilst, at the same time, allowing PLT to operate in a commercially viable manner."

For the magnetic field regression measurements below 30 MHz, Ofcom used a Rohde and Schwarz HM525 active loop antenna. The report notes that "This antenna has a noise floor between 17 and 20 dB below that of the standard Rohde & Schwarz HFH2-Z2 60cm active loop antenna that is normally used for compliance measurements." This would indicate that measurements with the "normally used" antenna would not show the full impact of BPL emissions on HF reception.

In a discussion on the effectiveness of notching, the Ofcom report states, "It is interesting to note that the noise level at the bottom of this notch is close to the noise floor of a typical CISPR measuring system using the standard 60cm active loop antenna such as a Rohde & Schwarz HFH2-Z2. Had such an antenna been used for this measurement, it may have led to the misleading impression that part of the Amperion spectrum had been 'notched out' completely, whereas this is clearly not the case."

While BPL is obviously a concern for shortwave radio listeners and ham radio operators, why should broadcasters be concerned that the FCC appears to be ignoring the damage BPL has been shown to cause to the high frequency spectrum? I'm concerned that precedents set here may find there way into the interference analysis of unlicensed devices on TV channels and other sharing of broadcast spectrum by devices that offer wireless Internet access, with one excuse for allowing the increased interference being that few people watch over-the-air TV and, unlike shortwave listeners and ham radio operators, TV viewers have the option of receiving the same programming by satellite or cable TV.

The Ofcom report is 39 pages long and filed with graphs and photos. If you have any interest in the impact of BPL on radio communications, I encourage you to read it. Also see the ARRL story U.K. Regulator's Study Points up Limitations of Amperion-Equipped BPL Trial, which has links to other reports and articles.


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