Tuesday, September 30, 2008

Simple 802.11a/b/g Antenna Mod

There are scads of 802.11 antennas available commercially, and many home-brew variants floating around as well. What does mine have that the others lack? Nothing really, just another alternative. However, I do recycle some stuff and do it all with no special equipment or calculations which is nice.

So I happen to have a bunch of old torchiere lamps that the HR department discarded. Some work, but they are electricity hogs. The bulbs are getting hard to find, and they really aren't in style any more. What to do? I often need to access a wireless AP that is marginal, and it occurred to me that a simple parabolic dish and a USB wifi stick would do the trick. Mentally the two came together and voila!

So to explain further, I stripped the dish part off and took out the bulb assembly. It is left as an exercise to the reader to find a use for the base and shaft. The center of the dish in this case was partially plastic, but that wasn't a deal-breaker. I used some of the original all-thread and some zip-ties to attach a USB extension cord to the dish. Once it was more or less secured I inserted the wifi stick and it was done. I used some soft 1/4" aluminum wire I happened to have for a hook/stand and wrapped a loop around to fill in the empty spot from the plastic piece. It doesn't have to be solid, just smaller aperture than 1/4 wavelength of 2.4GHz (a little over 2 1/2 cm).

Well, almost. I know that there is an optimal focal length for a parabola, but had no desire to measure and calculate it. I also know that not being close to the optimal focal length can lose you several dB of gain. Well I want to get it fairly close, so I tested.

I used a laser pointer to "spot" the focal length. This worked better than I thought it would, see the pic. Unfortunately I discovered that the dish is not a true parabola at all; the focal length changes at different distances from the center. Oh well. All things considered it should still boost my gain a few dB.

I have tested this design a little and it does improve the reception and more importantly the transmission enough to make the difference between barely having the AP visible and getting a pretty decent connection.

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