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Broadband over power lines (BPL), also known as power-line internet or Powerband, is the use of PLC technology to provide broadband Internet access through ordinary power lines. A computer (or any other device) would need only to plug a BPL “modem” into any outlet in an equipped building to have high-speed Internet access.

BPL seems, at first glance, to offer benefits relative to regular cable or DSL connections: the extensive infrastructure already available would appear to allow people in remote locations to have access to the Internet with relatively little equipment investment by the utility. Also, such ubiquitous availability would make it much easier for other electronics, such as televisions or sound systems, to hook up.

However, variations in the physical characteristics of the electricity network and the current lack of IEEE standards mean that provisioning of the service is far from being a standardized, repeatable process, and the amount of bandwidth a BPL system can provide compared to cable and wireless is in question. Some industry observers believe the prospect of BPL will motivate DSL and cable operators to more quickly serve rural communities.

PLC modems transmit in medium and high frequency (1.6 to 30 MHz electric carrier). The asymmetric speed in the modem is generally from 256 kbit/s to 2.7 Mbit/s. In the repeater situated in the meter room the speed is up to 45 Mbit/s and can be connected to 256 PLC modems. In the medium voltage stations, the speed from the head ends to the Internet is up to 135 Mbit/s. To connect to the Internet, utilities can use optical fiber backbone or wireless link.

Differences in the electrical distribution systems in North America and Europe affect the implementation of BPL. In North America relatively few homes are connected to each distribution transformer, whereas European practice may have hundreds of homes connected to each substation. Since the BPL signals do not propagate through the distribution transformers, extra equipment is needed in the North American case. However, since bandwidth is limited this can increase the speed at which each household can connect, due to fewer people sharing the same line.

The system has a number of complex issues, the primary one being that power lines are inherently a very noisy environment. Every time a device turns on or off, it introduces a pop or click into the line. Energy-saving devices often introduce noisy harmonics into the line. The system must be designed to deal with these natural signaling disruptions and work around them.

Broadband over powerlines has developed faster in Europe than in the US due to a historical difference in power system design philosophies. Nearly all large power grids transmit power at high voltages in order to reduce transmission losses, then near the customer use step-down transformers to reduce the voltage. Since BPL signals cannot readily pass through transformers — their high inductance makes them act as low-pass filters, blocking high-frequency signals — repeaters must be attached to the transformers. In the US, it is common for a small transformer hung from a utility pole to service a single house. In Europe, it is more common for a somewhat larger transformer to service 10 or 100 houses. For delivering power to customers, this difference in design makes little difference with power distribution, but it means delivering BPL over the power grid of a typical US city will require an order of magnitude more repeaters than would be required in a comparable European city. One possible alternative is to use BPL as the backhaul for wireless communications, by for instance hanging Wi-Fi access points or cellphone base stations on utility poles, thus allowing end-users within a certain range to connect with equipment they already have. In the near future, BPL might also be used as a backhaul for WiMAX networks.

The second major issue is signal strength and operating frequency. The system is expected to use frequencies in the 10 to 30 MHz range, which has been used for decades by amateur radio operators, as well as international shortwave broadcasters and a variety of communications systems (military, aeronautical, etc.). Power lines are unshielded and will act as antennas for the signals they carry, and have the potential to completely wipe out the usefulness of the 10 to 30 MHz range for shortwave communications purposes.

Modern BPL systems use OFDM modulation which allows the mitigation of interference with radio services by removing specific frequencies used. A 2001 joint study by the ARRL and HomePlug powerline alliance showed that modems using this technique “in general that with moderate separation of the antenna from the structure containing the HomePlug signal that interference was barely perceptible” and interference only happened when the “antenna was physically close to the power lines”.

Much higher speed transmissions using microwave frequencies transmitted via a newly discovered surface wave propagation mechanism called E-Line have been demonstrated using only a single power line conductor. These systems have shown the potential for symmetric and full duplex communication well in excess of 1 Gbit/s in each direction. Multiple WiFi channels with simultaneous analog television in the 2.4 and 5.3 GHz unlicensed bands have been demonstrated operating over a single medium voltage line. Furthermore, because it can operate anywhere in the 100 MHz – 10 GHz region, this technology can completely avoid the interference issues associated with utilizing shared spectrum while offering the greater flexibility for modulation and protocols found for any other type of microwave system.

And What About Hams (Amateur Radio Operators) opposing BPL? 

(It’s important to understand that Hams oppose BPL interference to wireless spectrum, and do not oppose BPL itself or broadband, despite what several BPL carriers and vendors may have said publicly.)Q: Why are Amateur Radio operators, also known as Hams, in an uproar over Broadband Over Powerline or BPL?
A: BPL is a system that is being tested to provide broadband Internet service via powerlines. The system uses radio frequencies that do radiate into the air. They can cause interference to licensed services including Amateur Radio. This interference has been seen and documented in most of the BPL trial areas. BPL experimental licenses have been issued allowing some systems to operate between 2 and 80 MHz (megahertz). This particular band of frequencies are generally known as HF (which is actually 3 – 30 Mhz) or “shortwave” frequencies. This part of the radio spectrum has very special properties not found elsewhere. With this band, one can communicate around the world with very low power levels, without the need for any equipment in between, such as satellites or repeaters. This is due to the fact that radio waves in this band can bounce off the ionosphere multiple times to get to the destination. Other portions of the radio spectrum are essentially line-of-sight. This means that the signals cannot bend or bounce off the ionosphere, but they can only propagate like light – in a straight line. Many consumers have enjoyed listening to shortwave radio broadcasts from around the world.

Q: What is technically wrong with BPL?
A: The medium of BPL (the powerline cable), unlike any other broadband medium (copper twisted pair, fiber, coaxial cable), is inherently unsuited for carrying the frequencies BPL uses. Power lines, copper twisted pair, and coaxial cable all act like natural low pass filters, meaning higher frequencies are attenuated more than lower frequencies when attempting to transmit them through the medium. The exact slope of the graph of attenuation depends on the specific construction of the material, but in general, twisted pair is suitable up to 100 Mhz and coaxial cable can go up to about 3 Ghz. Again, these are very general figures and determining the suitability for any application depends on other factors. Power lines would be suitable for up to perhaps 20 Khz, maybe 350 kHz at a stretch, with caveats. The exact figure is unimportant for this discussion, but note that this is kilohertz, not megahertz or gigahertz. These are essentially audio frequencies, and equate to a data rate in the neighborhood of ISDN.

Power lines are designed to carry electrical power. They were not designed to carry radio signals. They do this very poorly, loosing much of the signal to losses and, more importantly, radiating them as radio signals that can and do affect nearby receivers using those frequencies. Amateur radio operators, CB operators and shortwave listeners are all found commonly in the residential neighborhoods where BPL will be installed. They will all suffer strong interference if BPL uses their frequencies at the permitted levels. Other uses of HF spectrum include business, government, military and aeronautical. Many of these users and their organizations have expressed strong concern about BPL and its interference potential.

Other wires and cable, such as telephone or cable TV systems do actually radiate to some extent, but in proportion to the amplitude of the signal they are carrying, it is minuscule. (This doesn’t apply to poorly designed or maintained cable or pairs, or intentionally radiating cable like Radiax used for indoor applications). It’s ironic that many antennas used on HF are very close in construction and visual appearance to typical power lines. It has been clearly demonstrated in technical studies prepared by ARRL, the National Organization for Amateur Radio that BPL often causes interference and that power lines act like antennas.

Putting a signal that will operate 24 hours a day over wide range of frequencies at the FCC limits for unlicensed devices onto wiring that radiates well is a very poor engineering choice. When that system is built as large as an entire community – or several states –noise and interference are inevitable. Basic first-year college courses teach one this. This is very, very basic. The electrically dirty and unpredictable nature of power lines can also produce harmonics and intermodulation that can cause unexpected forms of interference.

Q: Does BPL work?
A: From a consumer point of view and what has been heard from test areas, yes. Amateur radio operators and others do not have an issue with this. So far, the BPL systems installed are generally small, and how well they will work when built out to cover a wider area and more customers remains to be seen, though, as there are few if any heavily loaded systems. However, one would expect the behavior of a BPL network to resemble that of a shared medium like cable or wireless. Systems will have to be segmented further as traffic grows and the available bandwidth in a segment decreases.

There are significant concerns about immunity to interference which are discussed later in this FAQ. Tests done to date show that nearby radio transmitters can interfere with BPL, but this too, needs to be looked at in larger systems. The effects of such interference are unknown at this point and could make BPL service in areas unreliable or unfeasible. Under the FCC rules on BPL, as an unlicensed device, BPL is offered no protection from licensed users that may interfere with BPL in the normal course of their licensed operation.

Q: What are the benefits of BPL?
A: The dream of BPL is to have every powerline activated with BPL and have BPL networks within homes. With the advent of inexpensive chipsets, every appliance in the home could easily be networked without additional wiring. Utilities could use BPL to manage network elements within the power grid and perform automatic meter reading or AMR, although narrowband non-interfering PLC based AMR system existed previous to BPL and continue to be in use today. Grid management functions could be performed as well, although PLC and SCADA systems do this today.

One major perceived advantage of BPL is wiring infrastructure. Utility power is nearly everywhere, so costly copper twisted pair or fiber would not have to be run. While this may be seen as a major advantage, the work and cost to light up every power line with BPL is significant. Furthermore, nearly every home in the U.S. has twisted pair copper lines installed for telephone service. These lines are better suited for broadband transmission and are the basis for DSL, so technically the potential coverage and benefits of DSL far outweight that of BPL and the power line infrastructure.

Q: Is BPL the same as wireless broadband?
A: No. BPL affects wireless radio spectrum but it does not actually use the wireless spectrum through the air to transmit data. There are some BPL systems that use unlicensed 802.11 WiFi wireless as part of the BPL system to deliver the data to the end user, however the core BPL network uses the powerline as the base medium for transmission.

Q: Why can’t BPL coexist with wireless technologies?
A: To allow a wired-based network to make large portions of frequencies unusable in the RF world is tantamount to the way industry used to be permitted to cause serious pollution. Cable companies use frequencies in VHF and UHF bands that were they to leak out into the outside world, would wreak havoc with aviation and public safety frequencies. They are subject to strict limits that include a need for them to regularly test the cumulative effect of the leakage from their system and there’s no reason why any other wired network like BPL shouldn’t be subject to strict limits – it’s a wired medium, it is not necessary for it to affect wireless media in any way.

Q: Is BPL new?
A: BPL has been tested and deployed on a limited basis in other countries. BPL vendors may claim “new technology” and advances have now made it possible, but the fact is they can’t change the laws of physics. High speed data must occupy a certain amount of “bandwidth” and power lines which were designed to operate at 60Hz will radiate RF that is applied to them. The BPL system has to operate at a higher level than the noise on the lines, so by definition, BPL will increase the radiated noise level from the power lines that carry it, on any spectrum it used. Only changing power line construction (i.e. coaxial cable) would eliminate this radiation. BPL proponents reject this as being too costly, but that would be the cost to make this a real viable technology.

Q: Hasn’t Power Line Carrier or PLC been in operation for years without problems?
A: The “original” PLC is a very low frequency, narrow bandwidth signal used for control equipment in the power grid. It is in the neighborhood of 100 to 180 kHz (that’s kilohertz, not megahertz). It is not intended for high speed data transfer, but rather simple commands, like “turn relay on”, “turn relay off”. It is also used to send “messages” up the line when a major failure occurs so that other network elements don’t trip off when senses a fault. This keeps the power grid from falling down like a bunch of dominoes (like what happen recently with the blackout in the Northeast :-).

Usually people refer to the 1-80 Mhz “PLC” as BPL, although some use the acronyms interchangeably and in my opinion, wrongly. Comparing “original” PLC and BPL is like comparing apples to oranges. PLC is brief, primitive commands and is very narrow banded. BPL is wide band noise that’s all over the place.

Recently the Amateur Radio community petitioned the FCC for a low frequency band in the same area as the original low frequency telemetry PLC. It was rejected by the FCC because utility companies complained that Amateurs would interfere with this telemetry PLC. This is very interesting for several reasons. First off, the utilities are saying that BPL won’t interfere with Amateurs and Amateurs won’t interfere with BPL. But PLC is arguably more robust than BPL because it’s narrow band. So which way is it ? The second reason this was so interesting (or disturbing) is that telemetry PLC is operating under Part 15 and is afforded no protection from interference from licensed services. For the first time the FCC disallowed licensed operation and essentially protected a Part 15 operation. This sets a dangerous precedent as any unlicensed “spectrum squatter” can later claim rights to a chunk of frequencies. The third and even worse observation is that the utilities have admitted that the telemetry system of the national power grid is vulnerable to attack, and they’ve based a critical part of the infrastructure on something that cannot be legally protected from interference. Any nutcase with a little knowledge could conceivably control network elements within a power grid with enough time and money.

PLC has been in operation for a long time, but as mentioned, it’s very different from BPL. It seems many BPL proponents are confusing the situation by riding on PLC’s history and merits.

Q: Haven’t power lines always radiated radio energy and caused interference?
A: Previous to BPL, power lines have had a history of radiating noise, but properly maintained lines will radiate only a minute amount of radio energy that is low enough that it does not impact radio communications. On the other hand, a properly maintained BPL system will radiate radio energy.

A distinction between power transmission noise and BPL noise needs to made, and is very important to understanding this issue. Power transmission noise is a product of 60 hertz voltage and is a rough, raspy frying type of noise that is stronger at low frequencies and gradually tapers off. Two types of BPL noise exist. One is results from Spread Spectrum modulation which is a Geiger counter type noise. The other type of BPL noise typically observed is resulting from OFDM modulation. This creates carriers, or little “radio blips” every 1 kilohertz or so across the radio spectrum. Each “blip” has a ringing sound or clicking sound.

Power transmission noise can most always be fixed by changing out worn or defective power line components such as insulators or taps. Radio interference from BPL is a side effect of a the system in operation, not an anomaly.

Ironically, BPL proponents and FCC representatives have claimed that BPL deployments have reduced classic power line transmission noise. This is true to an extent as any noise on the line from power transmission will seriously degrade a BPL signal. Unfortunately, the power line transmission noise is being replaced with a much more devastating signal that is stronger and does not decrease in strength as you go up in frequency.

Q: Does replacing bad or dirty insulators on the powerline fix the BPL interference problem?
A: Replacing such components reduces power transmission generated noise, but not the BPL interference. Often new BPL carriers have to clean up their lines and reduce the power transmission generated noise so BPL will work. BPL is a radio signal on the powerline, and basic electronics theory states that any unshielded conductor a ¼ wavelength long will radiate RF energy.

Q: BPL seems to be more prevalent in Europe. Is BPL interference just a problem in the United States? Is the interference issue related to North American 60 hertz systems versus 50 hertz systems found in the rest of the world?
A: The BPL interference problem is a function of the unshielded powerline, not the frequency of the power signals, 50 or 60 hertz. European systems (50 hz) tend to be better suited for BPL than North American systems (60 hz) from a logistical point of view as utilities in Europe tend to place tens, or a hundred or more customers on one transformer. In the US there is usually only two, three, or four customers on one transformer. Because European voltage service to the home is 240V versus North American 120V service, the I2R losses in transformers and drops in Europe are one quarter that of North American systems, thus more homes can be placed on one transformer. More customers on one transformer translates to less transformer bypasses that are needed to pass the BPL signals to household wiring.

Again, the feasibility of BPL service is not related directly to power service frequency.

Q: Won’t adaptive technology in BPL protect others from interference?
A: Adaptive technology was proposed by the FCC in the Notice of Proposed Rulemaking in 2003 as a way to mitigate interference. While sounding high tech, this technology is actually administrative functions that are present in most BPL equipment today. This includes:

• Power control, dynamic or remote
• Frequency notching
• Harmful interference shut-down feature

First off, these techniques do not provide any protection for mobile or portable stations. Dynamic power control is an obvious no-brainer and should be a requirement as this would keep power levels on the lines as low as possible. Frequency notching, while a possible solution for local interference complaints, doesn’t address long range interference that would be created by ionospheric propagation or the cumulative effect on the noise floor in the HF bands. The harmful interference shut-down feature is rather ambiguous in the NPRM, but it seems to be a manual remote control on/off switch. Most network equipment today can be turned off remotely, so this feature isn’t a real stretch, but it does nothing proactively to lessen interference. Also, many uses of shortwave involve more listening than transmitting, and uses like international shortwave broadcast would not have any transmitters nearby for the BPL system to sense.

Q: If this just affects Ham Radio, why should anyone care?
A: BPL will also affect other licensed services such as government agencies, military, aviation, maritime, public safety, and shortwave broadcasts. Ham radio occupies less than 10 percent of the affected radio spectrum. Many of these users have filed comments in the FCC rulemaking, in both the Notice of Inquiry and the Notice of Proposed Rulemaking, that expresses grave concern about BPL interference.

Q: Are Hams qualified to talk about BPL issues?
A: Hams are licensed by the FCC in the United States and various governing bodies in most every country in the world. Most countries have several classes of licenses. In the United States and most countries, one must pass written tests on electronic theory, communications protocol, and regulatory material. While becoming a licensed Ham isn’t equivalent to an Electrical Engineering degree, many hams have formal educations in electrical engineering.

The experience and knowledge of Hams can vary greatly, and like any hobby or profession, there are hams who speak before thinking. Occasionally you will find an ex-ham dismissing Ham Radio as being dead, or an inexperienced Ham who only uses VHF bands who could care less about the HF bands that are threatened by BPL. Most active hams though are rather knowledgeable. Their decades of experience on radio give them an understanding about interference that is unmatched in any arena.

Q: Why are Hams the only people talking about the negative affects of BPL? Why aren’t other services complaining?
A: Ham radio occupies a minority of BPL spectrum, with government, shortwave, public safety, and ship communications occupying the majority. Ham radio operators are the most vocal because government agency employees quite simply don’t post in Internet forums. Many of these other users are concerned about BPL interference. Comments with the FCC have been filed from groups and agencies like the NTIA, Salvation Army, the Missouri State Patrol, Aeronautical Radio, Inc, and NPR, to name just a few.

Most of the other services on HF radio frequencies affected by BPL like government and aviation usually operate at much higher power, use much larger/more directional antenna arrays, are more frequency agile, and often have their transmit/receive stations at sea, in the air, or on remote reservations of land far away from civilian housing areas and their associated power lines. In the latter case, having acres and acres of open land gives much more flexibility about antenna placement than someone living on a 1/4 acre residential lot. Full realization of each of these advantages is not attainable by even the richest radio amateur. So even if government agencies such as the NTIA say that BPL will be of minimal impact to them, it should be emphasized that their capabilities, and therefore their operating environment, may be very different than that of amateurs.

Q: How can HF radio signals travel across the world?
A: Long distance HF propagation occurs by radio signals bouncing between the earth and the ionosphere, often several times.
This has two consequences with regards to BPL. A lot of long distance wireless HF communications occurs just above the noise floor (the common noise you hear on your radio when you are not receiving a station which is noise leftover from the Big Bang and from various man made sources), receiving very weak signals. A station attempting to receive an HF signal could experience interference from a local BPL system, even if it is notched in the amateur radio bands and 40 or 50 dB below the emissions limit. (A good analogy is it’s hard to hear someone yelling a half mile away when someone is wispering in your ears). The second issue has yet to be proven either way, but with thousands or millions of BPL devices in operation, the noise from these devices combine could raise the noise floor in the HF spectrum and propagate thousands of miles. The jury is still out on this and the NTIA was supposed to release a second study that would explore this. There’s not nearly enough BPL systems in operation to test this in real world circumstances.

Q: Isn’t long distance HF communications more an infrequent anomaly rather than a common occurrence?
A: HF propagation in an area varies on the time of day and on solar activity. It’s usually possible to communicate on some HF band to various places in the world 24 hours a day. During the day, the D, E, and F layers form a thicker layer of ionization. This ticker layer absorbs lower frequencies (below 5 Mhz or so), and enables farther propagation of frequencies between 15 and 30 Mhz. At night, the D, E, and F layers combine. This causes the upper frequecy limit (called Maximum Usable Frequency or MUF) to drop, usually to about 14 or 15 Mhz. During this time, frequencies below 5 Mhz will propagate better. This is why you can hear many AM broadcast radio stations at night, and most AM radio stations decrease their power at night to avoid interference.

Q: Why doesn’t my 802.11 WiFi or cellular phone radio signal travel around the world?
A: 802.11 WiFi uses 2.4 Ghz frequencies (2400 Mhz) which is considered microwave frequencies, cellular is 800 Mhz, and PCS is around 1.3 Ghz. These to not bounce off of the ionosphere, but travel right through it. The reflective characteristics of the ionosphere diminish above about 30 Mhz. Also, microwave frequencies are much more susceptible to absorption by precipitation and water vapor.

Q: Does BPL Affect Shortwave Listeners (SWLs) and Shortwave Broadcast Frequencies?
A: Yes, BPL can affect all of the HF shortwave broadcast bands.

Q: Can’t Hams and others using the spectrum simply be relocated?
A: Users of the affected spectrum cannot be relocated, or at least not economically or in a timely manner. It would be cheaper for the government to subsidize cable and DSL deployment. Plus, all of the services that use HF bands require the characteristics that only HF bands exhibit. There would also be huge international treaty implications with any relocation. Changes in international communications treaties are measured in decades, not months or even years. Relocating government and military services alone would take years as the FCC would have to structure a migration plan. Chances are it would be ten years before this could be completed and it’s likely that power companies will have run fiber to the home or DSL and cable will finally be ubiquitous. Perhaps the largest issue to tackle, though, is where to move these services in what is an already overcrowded spectrum.

If it was determined that relocation was the way to go, this would be very irresponsible as HF radio bands are a unique natural resource. No other radio spectrum can provide worldwide communications without any supporting infrastructure (i.e. satellites).

Q: Why doesn’t the FCC allocate dedicated spectrum for BPL?
A: There are two main reasons for this. The first is quite simply there is not enough open space in the HF spectrum to accommodate a dedicated allocation for BPL. Current BPL systems need at least ten MHz of spectrum. Newer, faster systems may need 25 MHz or more to operate. Accommodating such systems would require the entire HF spectrum to be allocated to BPL. The second reason against a dedicated frequency allocation is that BPL doesn’t actually use the wireless spectrum, it emits energy in the frequency range, thus polluting it. An analogy would be building a highway and dedicating it to one industry or group of people so they could dump garbage on it.

Q: Won’t Frequency Notching protect licensed services?
A: Frequency Notching is a new feature that is reported on some of the second generation BPL systems. The idea is that if a BPL signal is interfering, the system can be configured to not use this frequency or a band of frequencies. This apparently can be done on a subscriber, network, or system wide basis. While this is a nice feature, it still doesn’t make BPL acceptable for four reasons. The first of which is that the possibility of intermodulation still exists. (Intermodulation is described in detail below).

The second reason is that BPL signals can propagate for very long distances due to the characteristics of the frequencies they are using. Interference from a BPL could be experienced a thousand miles away. Tracking this interference down so that the BPL operator could be informed of a Part 15 violation in order to get them to notch the frequency would be logistically difficult. Considering BPL acts like a large distributed antenna and not a classic point-source of interference, it would be hard to direction find the signal to track it down.

The third issue is what’s called the noise floor. The noise floor in simple terms is what you hear in between radio stations on an FM or AM radio. It’s the snowy screen you see on your TV when there’s no station on the channel. The noise floor is essentially noise and radio spectrum energy left over from the Big Bang. This noise is fairly constant, but man-made noise sources such as existing 60 Hz power lines, noisy transmitters and other things contribute to raising the noise floor. BPL systems will contribute to the noise floor and raise it higher. This will make it more difficult to receive weak signals. Taken to the extreme, the raised noise floor could make HF communications impossible.

The fourth reason that notching won’t work is that there just isn’t any significant open space in the HF spectrum. Anywhere you look, there’s a service using the frequency. My guess is BPL providers are banking on the fact that there’s not a local user of particular chunks of HF frequencies, so they will configure their systems to operate in these areas. If we were talking about microwave frequencies that are strictly line-of-sight propagation bands this would be a great solution, but that’s simply not the case with HF. HF has worldwide propagation characteristics, so it’s likely users hundreds or thousands of miles away could experience interference. As mentioned elsewhere here, tracking such wide band no identification interference is nearly impossible.

Q: What is intermodulation?
A: Intermodulation is the mixing of radio signals which produces new radio signals. Think of it as radio waves having children. But just how do radio waves have children ?

This mixing is caused by what are called non-linearities. One non-linear electronic component that you find in most any electronic device is a diode. When multiple radio signals are run through the diode, they mix together. Let’s say we have a 4 Mhz signal and a 6 Mhz signal going into the diode. We would then get:
4 + 6 = 10 Mhz
6 – 4 = 2 Mhz
4 and 6 Mhz had two “children”, 2 and 10 Mhz !

Now, non-linearities are usually good. This phenomenon is used in just about every radio device to either create a signal to be transmitted, or receive a signal that you hear or see. But, non-linearities can occur where you don’t want them and then in causes problems. One such place is in power lines. Bad, corroded connections or dissimilar metals touching can create natural diodes that act like mixers and produce this intermodulation.

So, let’s take a BPL signal and for the sake of discussion, say it’s a grossly simplified consisting of radio signals at 1, 5, 8, 9, and 12 Mhz. Some of the intermodulation products that could be created would be:

1 + 5 = 6 Mhz
8 + 9 = 17 Mhz
9 + 12 = 21 Mhz
12 – 9 = 3 Mhz

But you could also have what is known as third order products:

1 + 9 + 12 = 22 Mhz
8 + 9 + 12 = 29 Mhz
8 – 5 + 12 = 15 Mhz

Or even:

2 * 12 = 24 Mhz
(9 – 5) * 12 = 48 Mhz

You can do the math and figure out each permutation, but you get the idea. If we took a real BPL signal that has signals from 1 – 80 Mhz the number of products and where they would fall are mind-boggling. The resulting intermodulation products in a system could extend well above the band BPL proponents want, falling into FM broadcast, VHF TV, Aeronautical, and more public safety bands. This is just another reason why BPL is so problematic.

It’s arguable that such non-linearities in power lines are exhibited as arcing connections, something that most power companies are actively searching for these days as the RFI (radio frequency interference) effects are well understood. These maintenance issues will be addressed quickly by well run utilities. However, non-linear loads are common in homes, light dimmers being the first devices that come to mind. Theoretically, these devices could create intermodulation that would in turn be radiated by the house wiring and outside power cabling.

Q: What are harmonics?
A: Harmonics are in the same “family” as intermodulation. It’s new radio signals that are created by a non-linear device that are a multiple of a radio signal. Unlike intermodulation which requires two or more “parent” or fundamental signals to be created, harmonics are the product of just one signal. A 10 megahertz signal would have a second harmonic of 20 Mhz, a third harmonic of 30 Mhz, a fourth harmonic of 40 Mhz, and so on. Harmonics created in equipment are generally undesirable and can interfere with other communications services if not filtered out. Harmonics can also mix with other signals to form intermodulation.
Harmonics can be created from fundamental BPL signals on powerlines. These have been observed in the field at typically 30 dB lower than primary BPL signals.

(compiled using informations from Wikipedia and http://bplinterference.wikispaces.com/)