Green Windows v. Clear Signals

The apparent impasse between energy-efficient windows and the signals of the “Clear” mobile broadband service seemed cause to look into the basics of radio propagation, and the differing spectral approaches to mobile broadband.

Refresher: Some subscribers to Clearwire’s service in the Dallas/Ft. Worth area complained on of signal strength problems. When they opened the window, it worked great; with the window closed, no bars.

That quickly narrowed the focus to the energy-efficient coatings on “green” or “Low-E” windows. The “E” stands for “emissivity,” or “the relative power of a surface to emit heat by radiation.”

That means that in winter, low-E windows direct indoor heat back into the room; in summer, the sun’s rays do the same thing off the exterior of the glass.

The secret ingredient: A thin layer of tin, silver, or zinc, applied to the window during manufacturing.

Aside to Low-E window manufacturers: This could be a swell marketing feature. Get energy-efficient windows in your house; get your own personal Faraday cage! (Translation: Great for the privacy depraved, or for those who wish their spouse/parent would spend less time on the signal. A Faraday cage is a metal mesh container that exists to deliberately shield against electromagnetic energy. Those I’ve seen could crate a Great Dane, or a human hunched over. Not a whole house.)

Radio frequency propagation basics: Low frequencies (longer wavelengths) tend to penetrate structures better than higher frequencies (shorter wavelengths.) Clearwire transmits in the 2.5 GHz band; AT&T and Verizon aren’t up yet, but paid a premium to run in the 700 MHz range.

Visually, the best way to picture this is the letter “S,” tipped on its side. Tracing your finger from one end to the other of the sideways “S” is one Hertz. In that sense, 700 MHz, if you could see it, would look more smooth and spread out than 2.5 GHz, which would look more bunched up and spazzy.

What’s too high vs. passable for “green” windows? That is the question. Suggestion: Put the big chips on “it depends.” Clearwire officials, when asked about the spectrum/window issue, noted that all cellular carriers could be similarly impacted.

Problems like this can be solved, of course, in one of three ways: More power, more base stations, or larger antennas.

Or, as a wonderfully wry wireless guru noted: “If you were prepared to walk around with a four-sector, active-steered microwave dish on your head, with a car battery on a trolley behind you, and if you had the cash to build a network on every lamppost in North America, you could probably build a network at 40 GHz. The only problem would be that your head would glow.”