Tragedy of the commons, take two

In 17th-century New England villages, townspeople shared a centrally located common pasture for grazing each household's dairy cow and a few sheep. If no one got greedy and pastured more animals than the land could support, the common system worked fine (Ref. 1).

Fast-forward to the 21st century where a different kind of overgrazing endangers another common—the electromagnetic (EM) spectrum. The latest spectrum glutton goes by the acronym BPL, for "Broadband Internet over Power Lines." As yet another attempt to solve the Internet's "last mile" problem on the cheap, this one's a can of worms.

When I first wrote about broadband interference (EMI) 15 years ago (Ref. 2), I described signal leakage from cable television (CTV) systems in the greater Los Angeles area and its effects on aircraft and other public service communications.

Well, not much has changed. At our home in rural New Hampshire, we receive the Weather Channel—nothing unusual about that, but we don't subscribe to the local cable service. Thanks to the CTV company's leaky coaxial cables, we get snowy but watchable reception via an outdoor antenna.

Unlike well-maintained coaxial cables, open-wire lines radiate RF—sometimes they're called "antennas." BPL schemes use unshielded AC power lines to transfer signals via proprietary broadband, frequency-hopping pseudorandom transmission methods at frequencies in the range of 1.7 MHz to 80 MHz. As amateur radio operators attest, at these frequencies a few milliwatts of RF can span continents when propagation conditions are favorable.

If you're involved in open-air test site (OATS) EMI measurements, BPL may affect background RF levels and compromise OATS isolation. You may also experience an increase in requests for resolution of EM interference complaints. Current FCC regulations don't describe an accurate method for measuring BPL emissions, which may open a market for new test instruments

For the rest of us—consumers, radio astronomers, radio amateurs, and anyone who relies on the HF radio spectrum for routine or emergency communications—BPL offers dubious security to its users and overgrazes the spectrum. Does our civilization really need another cow on the EM common?

Brad Thompson, Contributing Technical Editor, Brad@tmworld.com