Peter Cochrane's Blog: Inside a giant wi-fi antenna

Who would have thought an aircraft fuselage would boost wi-fi signals?

Written on a flight from London Gatwick to Charlotte, North Carolina, and dispatched to the same day via my hotel's free wi-fi service.

I never thought of an aircraft fuselage as a resonant cavity or a phased-array antenna before, but a recent wi-fi experience has changed all that.

Tarmac view

Because of the onset of an early winter and the chaos wrought in the UK, I was recently stranded in Munich for two days. After many false alarms and six cancelled flights, I managed to return to London Heathrow's Terminal Five after a flight lasting less than 90 minutes.

This was one of those rare occasions when the flight was ahead of schedule and we spent no time at all in the stack above the airport. Unfortunately, this advantage was confounded by a lack of free gates, mobile steps and buses.

So we spent 95 minutes sitting on the runway, apparently disabled by enough snow to fill a teacup, as the picture through my aircraft window (right) shows.

What can you do? Out came my laptop, and unexpectedly up popped various wi-fi signals at five bars. The best bit rate I could get was 9.5Mbps down and 3.5Mbps up. Strange or what?

We were parked way out on the apron with the nearest buildings some 300m or more in the distance. I logged on, did my email, Skyped a few folks, and checked a few websites.

Videoconferencing with Mrs Cochrane

Then I couldn't resist the temptation, I videoconferenced with my wife at home in her kitchen for 20 minutes. What fun. The videoconferencing quality (shown right) was pretty good.

But how was all this connectivity working? A quick assessment of the windows and their spacing revealed the magic formula that was producing the five bars on wi-fi and mobile phones. I was sitting in an RF cavity that, while not perfect, was sufficient to gather signal energy along the length of the cabin and cumulatively deliver it to my devices.

A poor physical analogy here for lay purposes might be the experience - if you could shrink small enough - of sitting inside a penny whistle with air blowing over the finger holes.

A more accurate technical analogy would be to liken the aircraft body to a slotted waveguide antenna or cavity.

A slotted waveguide antenna

The similarity between the fuselage of a passenger aircraft with regularly spaced windows and a slotted waveguide antenna is apparent in the image, right.

Anyway, the next time you get stranded in an aircraft on an apron with the engines powered down, give it a try. It's far better than wasting 95 minutes or whatever and it also provides a bit of entertainment for those around you.

Photo credits: Peter Cochrane