Phorgy Phynance

WSJ: “Culprit in Wi-Fi Failures: Chicken Wire”

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On Thursday, the Wall Street Journal published an article

Culprit in Wi-Fi Failures: Chicken Wire

that is consistent with the theme I’ve been talking about lately. This is a perfect example of what I outlined in

The Ultimate Wireless Broadband Speed Limit

where I wrote:

Now consider two people trying to communicate via radio waves, but they are separated by a wall of metal. No dice. The radio waves cannot penetrate. Now, puncture a small hole in the wall. To be concrete, lets say we are communicating at a frequency of 3 GHz with a corresponding wavelength of 10 cm. If the hole is 1 cm in diameter, it is difficult for the radio signal to “ooze” through that tiny hole. Remember, the “size” of anything as far as a radio wave is concerned is only as a ratio of its wavelength. In this case, the hole is .1\lambda.

The ability of a wave to ooze through the hole depends on the size of the hole relative to its wavelength. Roughly speaking, when the size of the hole is greater than .5\lambda it has a much easier time oozing through it.

This example is actually pretty close to the situation described in the Wall Street Journal article. Wi-Fi operates at a frequency of roughly 2.5 GHz with a wavelength of roughly 13 cm. I’m guessing the holes in chicken wire are probably close to 3 cm in diameter or roughly .25\lambda. Since the hole is smaller than .5\lambda, the Wi-Fi signal cannot easily penetrate the chicken wire.

The photo in the Wall Street Journal illustrates another point I was trying to make. Although the 2.5 GHz Wi-Fi signal cannot penetrate the chicken wire, the MUCH higher frequency visible light does easily penetrate it, i.e. you can see through the chicken wire. The physics here is similar to why AM radio signals in underground parking lots are much worse than FM radio signals. The radio can see the FM radio signals, but the lower frequency AM radio signals get blocked.


Written by Eric

January 3, 2010 at 12:26 pm

Posted in Broadband

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