While we were moving out of the old building and we were taking down all of the hardware that was strung up throughout the building I saw a stack of antennas that were taken down.
These were a stack of WiFi antennas that were taken down the from the ceiling. These were all directional antennas in the sense they covered a hemisphere of their surroundings in an omnidirectional way — they were mounted in the ceiling and they covered the cubes all under them.
I figured they might be interesting inside so I took a couple of them apart.
I was right and wrong.
If you look closely you’ll notice that the outer shield of the coax is simply soldered to the “circuit board.” (I put it in quotes since there’s no circuit at all) The board acts as nothing other than a ground plane.
To make things odd the center wire, the signal conductor, is attached to a tab that itself is bolted to the ground plane.
Isn’t that just shorting things out?
Well, at low frequencies it is.
At the design frequency it acts really, really differently.
The antenna you’re looking at up there is for the 5.4GHz band. In the open air that works out to be a 5.5cm wave. In copper, on the other hand, it’s closer to around 3cm. As it turns out the distance between where the center conductor diverges from the shield is around 1.5cm — about 1/2 wavelength in copper. Interesting…
This is a patch antenna.
The design of the antenna is such that it’s resonant on the design frequency and the ground plane acts as a reflector so it covers the side you want.
Such a stupid simple design works so well. That is just freakishly cool I think.
Of course each one of these antennas is very carefully tuned to the band your transmitting. The one on the left is for a 2.4GHz signal, while the one on the right is for a 5.4GHz signal. It’s not a whole lot more than an issue of scale.
This design of antenna works great for high frequency signals, but as things get to longer wavelengths, they become increasingly harder to manage. Imagine one for FM which is centered at around 100Mhz (3m wave). You’d have to have a chunk of copper (or other conductor) that’s around 3 feet square and a massive ground plane behind it!
All in all though it’s interesting to look at something that looks so high tech and realize that it’s so incredibly simple inside!
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