Broadband communication over power lines (BPL) appears to offer the opportunity for increased revenues and profits to owners of power transmission and distribution facilities by opening another broadband channel to the Internet. But technical and economic factors may make it difficult to achieve these potential economic benefits,as we will point out. The frequency range and power levels of BPL systems are very likely to result in serious interference with existing, licensed radio services. The transmission and distribution system is an effective receiving antenna at high frequencies (1–100 MHz), and consumers of BPL services should anticipate service interruptions due to transmissions from these licensed radio services, seriously impacting the economics of BPL systems.

The radio frequency spectrum is another member of what many economists have called "the common." In a way it is like the atmosphere and the water supply. Its availability is limited, and it is shared by all; it is not constrained by political boundaries. The existing approach to the problems associated with potential misuse and interference is to establish national agencies like the U.S. Federal Communications Commission (FCC) and an international organization (International Telecommunications Union) to coordinate frequency allocations and national regulations. The radio spectrum is allocated to users licensed by the national agencies on the basis of internationally agreed upon allocation tables.

The high-frequency (HF) spectrum is allocated to a variety of functions, including international broadcasting; radio astronomy; standard frequency and time; and maritime mobile, aeronautical, land mobile, fixed, and amateur services. Fixed and land mobile users include emergency management functions and point-to-point communications. Some countries use HF radio for communications with their embassies. Maritime mobile is used both for regular operations and emergencies. The very high frequency (VHF) spectrum (30– 300 MHz) is allocated to FM and television broadcasting, space operation and research, fixed and mobile (public safety, e.g., fire and public works services; railroad communications), aeronautical radio navigation and communications, radio astronomy, and amateur services.

In the United States, unlicensed users like power line carrier (PLC) and BPL may not cause interference to licensed users (called "Part 15" in current FCC rules). A BPL system is not only potentially a source of interference to licensed users of the radio spectrum, but it is also susceptible to interference itself from strong signals from licensed transmitters because power lines are good receiving antennas at the proposed BPL frequencies.

PLC has been around since the early 1920s, first used for voice communication on the transmission system and later for protective relaying, supervisory control, and telemetry. The frequency range is 30–300 kHz in the United States and 10–490 kHz in Canada. Restricted bandwidth and resulting low data rates have caused PLC to be used less frequently. A transmission line carrier communications channel is rather simple, consisting of a transmitter, receiver, coupling capacitor, and wave trap at each end of the line. Reflections at discontinuities, such as taps or overhead/underground transitions, complicate the application because of reflections of the traveling waves. Distribution line PLC has been developed, but because of reflections and attenuation problems, the frequency range is over the range from about 5–50 kHz, lower than for transmission carrier.

Higher data rates require greater bandwidth, and greater bandwidth requires higher carrier frequencies. BPL requires frequencies in the HF to lower VHF range (1–100 MHz) for successful operation. This raises several technical challenges; here we will address the problem of interference with licensed radio services.

PLC has had to contend with interference with licensed radio services from the very beginning because its frequencies were allocated to radio navigation and other services, including broadcasting in some parts of the world. PLC has coexisted successfully with licensed radio services because the small dimensions of power line conductor configurations as compared with an electrical wavelength at PLC frequencies make the power line an acceptable radio transmission line. At frequencies proposed for BPL, this relationship is no longer valid, and the power line becomes a radiating antenna rather than a transmission line.

The subject of electromagnetic radiation from overhead power lines has been extensively studied in relation to corona and spark-generated radio and television interference. When transmission lines are properly designed for conductor, insulator, and hardware corona, virtually all of the television interference cases and 95% of the radio interference complaints result from spark discharges at one location or another. Almost all of the interference complaints from distribution lines are a consequence of spark discharges. The location and correction of these interference sources is an art in itself.

Spark discharge generated radio frequency interference covers 0.5–800 MHz, so understanding the propagation and radiation characteristics of power line noise is instructive for what may happen with BPL. Location of noise sources is difficult because once a radio frequency signal is coupled on a power line:

• Some is radiated from the location of the source.
• Some is propagated down the line to be radiated and received at a distance from the source.
• Some is coupled to other nearby lines and propagated down those lines to be radiated and received at additional distant locations.

The propagation and radiation characteristics of overhead power lines makes location of noise sources and resolution of interference complaints a challenge, one that some power companies approach more diligently and successfully than others. The fact that the FCC occasionally has to be involved to force a resolution of interference issues illustrates both the technical and institutional difficulties involved.

Power lines are designed for power frequencies, not to be transmission lines at radio frequencies. A good radio frequency transmission line has spacing between conductors that is small compared with a wavelength, and it is impedance matched to minimize reflections and standing waves. Neither condition holds for a power line. As a consequence, radiation is expected so the signal can be heard some distance from the line. Power line configurations and electrical parameters are subject to change over time so that their signal radiation characteristics may change in unpredictable ways. Such simple events as storm restoration activities could be the cause.

FCC Rules (Part 15) specify that unlicensed RF emitters are not to cause interference to licensed services. The FCC Notice of Proposed Rule Making (NPRM) on BPL reaffirms the no interference principle of Part 15, and it gives requirements that BPL systems shall incorporate adaptive interference mitigation techniques and a shut-down feature to deactivate units found to cause harmful interference. BPL signals have been detected from half to three-quarters of a mile from the power line during preliminary tests in the United States. One reason for this may be that power line BPL is an electromagnetic line source in contrast to normal Part 15 devices, which are point sources.

The difficulty in obtaining a leakage-free long distance radio frequency transmission line is illustrated by leakage fields from cable television systems. Although not the problem it was in the early years of cable television, it is still possible in some cases to detect the cable TV signals with a television receiver and antenna at some distance from the cable because of the leakage field, even though coaxial cable is designed to be a low loss pathway for RF signals.

The reciprocity theorem of electromagnetic theory means that a transmitting antenna is also a receiving antenna. This is illustrated by the opposite problem with cable TV systems, that of interference caused by leakage into the cable system. Consider the case when a strong local television station on channel 6 is carried on channel 6 by the local cable system. Channel 6 may be unusable because the signal from the cable and the leakage signal arrive with different phase shift (time delay) and interfere with each other sufficiently to make the picture unviewable.

The fact that power lines make good receiving antennas can be demonstrated by the fact that occasionally a resonance will develop so that a local radio station will be exceptionally strong at one location because of reradiation of the signal from a power line. These reradiation effects can be sufficiently pronounced to cause distortion of the pattern of the radio stations antenna. Detuning of the power line structures and opening of the shield wires are two of the techniques used to solve the reradiation problem. Thus, a BPL system is not only potentially a source of interference to licensed users of the radio spectrum, but it is also susceptible to interference itself from strong signals from local licensed transmitters. Unlicensed users of the spectrum in the United States must, by FCC rules (Part 15), accept interference from licensed users. In a future where BPL is widely available on a power system, the mitigation of interference caused by licensed transmitters could potentially become a significant cost.

There is a view in some quarters that the HF (short wave) spectrum is obsolescent, somewhat like traditional PLC. This is not true. The HF spectrum is a unique natural resource. The refractive characteristics of the ionosphere allow radio signals of relatively low power to travel world wide under the proper conditions. Simple equipment and antennas are all that are necessary. This makes the HF spectrum especially valuable for emergency communications, especially during storms or other events that take out the normal communications channels. It also makes worldwide interference possible from BPL installations. During this time of concern for homeland security, now is the time to conserve backup resources rather than to degrade them with increased noise floor. For example, international broadcasters are developing digital broadcasting techniques to take them into the next generation of long-distance radio. Fixed, mobile, and marine users still frequent the HF spectrum.

A characteristic of the uses of the HF spectrum is low received signal levels, made possible by the relatively low background noise level. A general increase in this noise level would render many of the uses difficult or impossible. In a sense, this is an environmental pollution issue, albeit a strange one in that it is the electromagnetic environment under attack and not the normal human environment. It is again the "problem of the common." However, it is an issue with possible serious long-range consequences. Many interfering sources envisioned under FCC Part 15 emit signals for a short time, and most users can usually tolerate short-duration interference. BPL would be a continuous emission at significant signal levels, and it has the potential of dominating the frequencies it uses. Thus, widespread application of BPL is of great concern to the licensed users of the radio spectrum who depend on a low background noise level. This is in addition to the reciprocal problem of interference to BPL systems by ingress of signals from high-power transmitters. The BPL system operators and owners are responsible for solving this problem when the BPL service is affected. These interruptions will surely remove some of the consumer appeal of BPL, and the needed solutions may well result in higher than anticipated BPL system operating costs. We would expect that wireless Internet access would become available in the next few years at a cost to consumers at the same level as today's broadband access channels, further reducing the appeal of BPL. (See the 26 April 2004 issue of Business Week, pp. 94–102, "No Wires No Rules," which reports on developments in wireless networking and cites an installation in Ireland to supply a rural town with broadband Internet service.) As the market becomes saturated with providers, wireless may well have the competitive edge.

The views expressed in the "In My View" column are those of the individual, not of this magazine, its editorial board, or the IEEE Power Engineering Society. If you agree or disagree with the opinions you see here, we want to know. Please send your comments to Melvin Olken at m.olken@ieee.org.