NGO information exchange
|The next threat to international radio|
|Monday, 16 June 2003|
The latest communications fad is a way of transmitting high data rate internet traffic over your house electrical wiring and the wires that connect your house to a nearby pole. There is a standard, developed by an industry consortium called the HomePlug Alliance, that plans to use the HF spectrum between 2 and 30 MHz for this connection. You plug your modem into a convenient power outlet. A similar modem near your service transformer picks the signals off the power line and converts them to a series of light pulses which travel via fiber optic cable to the internet interface. Because the power wiring in residential settings is unshielded, these signals will radiate and may cause interference to SW reception. Powerline communications systems also must design around interference. We all know about power-line noise at HF. Electrically noisy motors, fluorescent lights, halogen lamps, switching power supplies, and light dimmers have long been the bane of shortwave listeners. Received data bits tend to have significant error rates due to these interference sources. The HomePlug technology uses a combination of forward error correction (FEC), interleaving, error detection, and automatic repeat request (ARQ) techniques to correct bit errors. This technology is very similar to the methods used for reliable transmission of digital HF radio signals.
The power line distribution architecture also presents unique problems for the transmission engineer. In us neighborhoods, a distribution transformer usually provides power to a small group of homes. The distribution transformer acts as a choke at RF frequencies which effectively blocks the HomePlug signals from crossing into the main power distribution grid.
But signals from your neighbors house are connected in parallel with your own house wiring at the secondary of the distribution transformer. So even if you do not subscribe to broadband power-line services, your neighbors signals can radiate from your own house wiring into your antenna.
HomePlug uses orthogonal frequency division multiplexing (OFDM) as the basic transmission technique. ofdm divides the high-speed data stream to be transmitted into multiple parallel bit streams, each of which has a relatively low bit rate. Each bit stream then modulates one of a series of closely spaced carriers. In this respect the HomePlug standard is similar to the DRM standard for digital SW transmission.
The HomePlug standard uses multiple carriers between 4.5 and 21 MHz. The raw bit rate with all carriers active is 20 Mbps. The actual bit rate excluding the overhead is about 14 Mbps.
Amateur radio operators are also concerned that their use of the HF spectrum will be impacted by these signals. Luckily for the hams, the presence of a 1.5 kW transmitter near the power lines will also tend to disrupt the HomePlug power line communications.
The American Radio Relay League (ARRL), representing the interests of US hams, has been working with the HomePlug Alliance for several years to quantify and mitigate the threat. Because the HomePlug Alliance recognized the threat hams posed to their technology, they worked with the ARRL to help. The compromise solution for hams is reduced HomePlug modem transmitter power in each of the currently allocated ham bands. (I am concerned that this solution will forever constrain ham band allocations at hf to what exists today if this technology is widely deployed but thats not pertinent to SWLs or this article.)
ARRL has conducted real world tests using typical residential ham radio stations. The results of those tests confirmed the threat from this technology to SWLs. These tests were conducted in December 2000. The following excerpts from the ARRL test report are used here with permission of the ARRL. W1AW is the station at ARRL headquarters in Newington, CT. The complete report is available at the ARRL web site.
"A series of measurements were made using two antennas at W1AW. These results are contained in Table 1. The table shows the measurement frequency, a reading in dBµv with the HomePlug signal off, reading in dBµv with the HomePlug signal on, comment on audibility, and the antenna used for the measurement. All measurements were made using a quasi-peak (QP) detector and 10 kHz bandwidth."
Note that in the two SW broadcast bands measured, the interference was 3 dB above the ambient noise level at 6.54 MHz and 11 dB above ambient noise at 9.65 MHz. On 13.55 MHz the interference was less than 1 dB above the ambient level. Note that in all cases the notch suppression reduced interference inside the ham bands to less than 1 dB and barely audible subjective levels at worst.
"The second day, testing was performed at a single family dwelling, hosted by Paul Shafer, KB1BE. A quick measurement was made with the ESH2 connected to a PB5EM antenna pointed toward the house. Ambient signal level was measured at 6 dBµv QP (frequency not recorded). An increase of 2 dBµv was noted with the HomePlug signal on. (Anecdotally, a signal level of 20 dBµv was recorded when a fluorescent light in the next room was turned on.) An on/off comparison was made at 21.04 MHz. Both showed a reading of 8 dBµv, and the HomePlug signal was barely audible.
"A second set of measurements was made with a Moregain folded dipole antenna. The antenna was located on a mast approximately 40 feet high with one end approximately 10 feet from the house. The signal was injected in a basement outlet near the radio rig. Results are shown in Table 2."
Note that at 9.68 MHz the interference was 14 dB above the ambient noise level at 10.13 MHz.
At 6.7 MHz the interference was 5 dB above the ambient noise level at 7.02 MHz. In all cases the 30 dB notch suppressed interference inside the ham bands sufficiently to make it inaudible.
"A third set of measurements was made with a sloper antenna. The antenna was mounted on the same mast as the others at a height of 45 feet. One end of the antenna was anchored to the house. It was noted after the tests that the antenna terminated less than 10 feet from the service entrance to the house. Various outlet locations were measured as noted in the results found in Table 3."
Note here that the close proximity to the house of the bottom end of the sloper permitted some interference to be registered even in the protected ham bands. You can guess how much worse it would have been in the SW broadcast bands if measurements had been made at those frequencies.
"The afternoon of the second day, more testing was conducted at ARRL headquarters at station W1AW. The HomePlug signal was injected upstairs in the main radio equipment building with antennas located approximately 60 feet away. In a quick scan of the Ham bands, the HomePlug signal could not be detected at W1AW.
"Tests showed in general that with moderate separation of the antenna from the structure containing the HomePlug signal that interference was barely perceptible. The cases of objectionable interference were noted for the sloper antenna at KB1BE in which the antenna was physically close to the power lines. Although this location was chosen to mimic as much as possible a situation in which the HomePlug equipment is in one house and the amateur radio in another, the very close location of the antenna termination to the service entrance made this comparison impossible. It was agreed that further testing was desirable at other stations, preferably in which either the HomePlug signal could be injected in the neighbors house or in which the antenna was mounted 30 feet or more from the house."
Of course keeping carriers out of the ham bands only makes the problem worse for SW listeners because those carriers have to go someplace else on the dial. Unfortunately, NASWA does not have the resources available to ARRL. The FCC is currently running field tests of this technology in the Maryland and Virginia suburbs of Washington, dc and ARRL is participating in these tests.
FCC Chairman, Michael Powell, was shown visiting one of these sites on ABC-TVs World News Tonight. Mr. Powell commented on how impressed he was with the potential of this system as a way to provide an additional competitive broadband link to the internet. The competition with cable tv broadband and dsl via telephone lines will hopefully hold down rates and eliminate any need for the FCC to regulate rates.
Shortly after that program, the FCC issued a formal Notice Of Inquiry (NOI) to solicit inputs from interested parties. This is the first step in a process of formal rule making which will eventually authorize the deployment of this technology. Now is the time for us to influence how this system will protect the interests of shortwave listeners.
I urge every NASWA member and every foreign broadcaster targeting the us audience to let the FCC know that shortwave listeners deserve at least the level of protection afforded to amateur radio operators. An argument will be made that even greater protection is required because shortwave listeners operate their radios on small whip antennas inside the house. These listeners have a greater susceptibility to interference because of the close proximity of their receive antennas to their dwelling wiring.
I believe that we have a right in a free society to listen to newscasts and cultural programs from other countries without the intrusion of a gatekeeper or somebody wanting money for the privilege of listening to something we get today for free.
I am going to ask the FCC to afford SWLs protection at least as good as the HomePlug folks are affording hams and hopefully an additional 20 dB of protection to help mitigate radiation to small portable radios inside houses and apartment buildings.
You should come up with your own arguments and reasons. Copy mine if you so desire but original thoughts are what the FCC is looking for. Quantity also counts because a wide response will serve to impress upon the FCC just how many people rely on shortwave radio to keep current with world events as seen through the eyes of reporters from other nations.
At a time where us media are becoming ever more concentrated and controlled by fewer and fewer large corporations, the diversity of thought and opinion available via shortwave radio serves as an important input to an informed society. A free society must be accurately informed if it is to remain free. The best way to do that is to ensure maximum interference-free access to a diverse universe of news and opinion via free shortwave broadcasts.
The complete FCC Notice of Inquiry can be accessed at:
You can submit your comments electronically via the web at:
Under ECFS Main Links, click on "Submit a Filing." In the "Proceeding" field, enter "03-104" and complete the required field. Comments may be typed into a form or you may attach a file containing your comments. Comments also may be submitted via e-mail, per instructions on the ecfs page. The comment deadline is 45 days after publication of the NOI in the Federal Register so you have until at least the middle of June to get your comments in.
Written replies may be delayed by new security procedures and are therefore discouraged. If you have to file comments in writing, send an original and four copies to:
Office of the Secretary
Federal Communications Commission
445 12th Street, SW
Washington, D.C. 20554
Make sure the subject clearly states it is a response to et Docket No. 03-104. If possible, responses should be received by the FCC by June 13, 2003. Special security issues apply to mail sent to government facilities. Be sure to read and follow the instructions in the noi that apply to your selected paper delivery method.
If you want to do more research, I recommend this ARRL web page. It has links to tests conducted on similar technology all over the world including tests by the Japan Amateur Radio League which convinced the Japanese government to prohibit similar technology "at this time."
The FCC was formed originally to protect licensed radio services from interference. Remember that as you formulate your comments.
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