New Technology May Alleviate BPL Interference Concerns
December 8, 2003
Users of the high frequency and VHF spectrum have been concerned that proposed broadband over power line (BPL) systems may render the spectrum unusable due to radiation from the power lines. The ARRL has a web page devoted to broadband over power line and its impact on amateur radio. That page has a video showing the impact of BPL interference on HF communications. Broadcasters and government agencies are concerned because BPL radiation, including harmonics, have the potential to cause interference to VHF TV, FM radio and military and public safety communications in the HF and lower VHF spectrum.
It now appears there is a technology that will provide faster data rates (demonstrated up to 216 Mbps) than the high frequency systems operating in the 1.7 to 80 MHz range and also eliminate interference to HF operations. The system, developed by Corridor Systems, uses microwave spectrum instead of HF frequencies. The press release Corridor Systems Announces Breakthrough Technology for Broadband Over Powerlines -- Demonstrates 216 Mbps over PG&E's medium-voltage grid said the technology can leverage existing low-cost 802.11 chipsets, achieving lower cost than competing BPL solutions. Latency, according to the company, is less than 500 microseconds and the 216 Mbps capacity allows simultaneous, bi-directional, and symmetrical end-to-end delivery.
Corridor Systems' technology uses the spectrum between 2 GHz and 20 GHz, avoiding the HF and VHF frequencies entirely. How can microwaves be transmitted over a wire?
Ham radio operators may remember a UHF/microwave transmission line system that used a single wire. This technology, referred to as G-line, used cone shaped launchers at each end of a single wire. DSLReports.com has a Forum on Urban Electrified Broadband that includes some comments on the technology. A comments from N0JCG agrees that the technology behind the Corridor Systems BPL technology is a variation of G-line. Indeed, the photo on the Corridor news page clearly shows the cone shaped launcher!
A description of the technique is available in the oral history interview with Milford Richey at The Cable Center. It shows that it is possible to couple microwaves onto a single wire for distances of 5 to 10 miles without running into interference problems. There are some blurry pictures of a G-line installation at www.emachine.com/tower.htm. G-Line - A 1955 Application of Scalar Technology says the name G-line came from its inventor, Dr. George Goubau. The web page describes articles in the April 1955 and April 1956 issues of "Radio & Television News Magazine" about G-line and includes a drawing showing details of the launcher cone.
Corridor Systems sent an Open letter to the FCC regarding ARRL's opposition to BPL. In the letter, Glenn Elmore, Chief Technology Officer, who also holds Amateur Radio call sign N6GN, said, "Corridor Systems has demonstrated a BPL technology which is completely compatible with the Amateur Radio Service and, indeed, with all users of the HF-VHF spectrum, one which can be operated completely within Part 15 guidelines, is compatible with other services and is an excellent cohabitant of the public spectrum." During the 216 Mbps BPL test using 2.4 GHz and 5.3 GHz ISM/Part 15 spectrum, the letter said "Corridor Systems demonstrated normal system operation, maintaining high dynamic range, in spite of the presence of a high-level, 2.4 GHz fixed wireless transmitter located approximately 1/2 mile and within line-of-sight of the BPL system and sharing common spectrum."
Measurements at ground level directly under the power line used during the test showed a maximum radiated power level of less than -10 dBm, 20 dB less than the typical power level from 802.11 based wireless cards.
As expected with the frequencies used by this system, no signals were detected below 2 GHz. The letter describes two tests:
"Corridor Systems has tested and demonstrated simultaneous operation of its BPL technology and amateur radio HF communications. Utilizing a 100 watt, 7 MHz, 21 MHz and 28 MHz amateur SSB/CW transmitter connected to a dipole antenna located within 20 feet of an operating BPL system, there was not any evidence whatsoever of the operation of one system in the other. Amateur UHF communications at 446 MHz and at a 25-watt power level were similarly unaffected and in turn were not detected by the BPL system. Examination of the .1-30 MHz HF spectrum with a quality communications receiver also revealed no evidence of the BPL system."
"Spectrum analysis has been utilized to examine the entire .01 MHz to 2000 MHz spectrum present at the terminals of several types of antennas immediately adjacent to a BPL system which was operating normally. No BPL related signals at all were found within a variety of resolution bandwidths, from 10 Hz up to 5 MHz, and down to a lower measurement limit which was set either by normal incoming signals or the noise figure of the analyzer; approximately -155 dBm in a 10 Hz bandwidth. This measurement was also performed just after a solar coronal mass ejection when HF propagation changes had caused normal signals within the HF spectrum to be particularly weak or absent and thereby created an even more sensitive environment for measurements."
These results indicate there is no reason for the FCC to allow high frequency based broadband over power line systems when an alternative exists that offers greater bandwidth and no threat to existing HF, VHF and UHF spectrum users. Users of the 2 to 20 GHz spectrum may have some concerns and these should be addressed, but if G-line technology or a later refinement of it is being used, as appears likely, there should be little radiation from the power lines.