Wednesday, September 18, 2013

Wi-Fi Potatos

Broadcasting a radio signal inside any structure is a challenge. I've covered one corner of this topic previously in a 2008 post about radio in tunnels. There is a similar situation encountered today by radio engineers trying to offer WiFi inside airplanes. It's a long tunnel shaped space, filled with human beings who would like to use the Internet. Unfortunately each one of those people acts as a couple hundred dielectric pounds of joy. Physics is a bummer.

Generally speaking, a human being is a big bag of saltwater. I've explained this before as well in a 2010 post on hand capacitance "A dielectric is just an insulator... Chemically the human body is about 75% water, that's H20, Oxygen and Hydrogen. Water is not a good conductor of electricity. That's why we are a good dielectric."  In an enclosed space like your apartment, a human body has the effect of attenuating radio signals, like WiFi . More here.

The problem is even worse on an airplane. An airplane stuffs in human mass like sardines in a tin making for less airspace than there is dielectric mass. Both the segments of the 700MHz-3.5GHz range used for mobile telephony and both the 2.4 GHz and 5.0 GHz ranges used for WiFi are particularly sensitive to this effect. This is a serious engineering obstacle. Nonetheless Delta, American Airlines, United, Virgin and even Jet Blue tout their WiFi service on flights.

It was Boeing that first solved the problem of modeling a long metal tube filled with manimals. It turns out that for the purposes of science we are very similar to a sack of potatoes. We and the humble spud have a couple things in common in terms of water content. Boeing described the experiment  on their website.
"Using a de-commissioned airplane, the team from Boeing Test & Evaluation laboratories conducted a series of tests. The team determined that sacks of potatoes were ideal stand-ins for passengers, given their similar physical interactions with electronic signal properties. Much of the testing was conducted on the grounded airplane with the seats filled with 20,000 pounds of potatoes in sacks. The test data was then validated with human stand-in "passengers."
This is why science is awesome.