Wednesday, January 9, 2008

Wireless Hacking

I've very much been on the wireless hacking kick lately. I've been amazed at how a little ingenuity in constructing antennas pays huge dividends in decibels.

This is a "parabolic dish" made from a USB wireless dongle and a $5 colander from Target (plus a couple of wood scraps from Lowe's). It provided a measured 11db gain over the USB dongle plugged into the laptop sitting inside the Alternate HQ.

It wasn't terribly scientific, and I didn't have great hopes for its effectiveness, but it blew away my expectations. I got the idea from a clever site in New Zealand, who reported a 12db gain on their version.
So what do you do with this setup? I'm using it to connect to public, but distant, wireless hotspots. Experimentation has shown that it is possible to detect and connect (at somewhat low data rates) to wireless access points several miles away under the right conditions.

The dish, however, is pretty unwieldy and requires that you zip-tie a colander to the outside of your house (or apartment, trailer, RV, tent, etc.). I wanted something that was a little less obvious.

These are corner reflector antennas made by hacking the simple 2db wires that come with the ubiquitous WRT-54G wireless access point. They were constructed by taking the plastic covers off of the stock WRT-54G antennas and soldering in new elements onto the stubby little stock antennas. This setup gives an estimated 8db gain over the stock antennas.

I then made corner reflectors out of disposible baking pans from the dollar store, and mounted them to the cut-off bottom of a plastic storage jar using 3M double-sided outdoor mounting tape (I LOVE that stuff).

The modified antenna elements were covered with lengths of 1/2 inch black PVC tubing, and the whole assembly was attached directly to a WRT-54G re-flashed with the latest DD-WRT firmware (v24). I normally use OpenWRT, which is a lot more adaptable and extensible, but DD-WRT has as a unique feature: the ability to repeat a wireless signal on a separate virtual subnet.

For example, if the ESSID of the distant network is "linksys" on channel 6, with the v24 version of DD-WRT, you can receive the signal from "linksys" and repeat it, also on channel 6, with a different ESSID, for example "ddwrt". So wireless clients could connect to "ddwrt", which might only be a few feet away, and be automatically routed to "linksys", which might be miles away, depending on the conditions and the antennas connected to the WRT-54.

For my next trick, I'm working on various power sources for the WRT routers, so that the repeater doesn't have to be connected to anything - it can be completely self-contained.


John the Scientist said...

Seriously cool homemade geekery.

I have a big parabolic dish, solid metal, somewhere. Do you think mesh gives better S/N or does solid?

Jim Wright said...

CW (hey that's not short for Carrier Wave is it?), great idea. I might rig up something like this to connect my laptop to the house WAP while I'm out in detached woodshop.

You could build a cosmetic/protective cover over your colander dish using half of a kid's plastic ball (the kind you find for cheap in those big bins at Wal-Mart.) They come in a variety of sized, shouldn't be hard to find one with the same diameter as the colander/dish. Cut the ball in half, following the seem, punch 3-4 holes around the diameter and attach it edge on to the colander with cable ties. The thin plastic or stiff rubber they make those things from should be RF transparent and I doubt you'd get any significant signal loss.

Just a thought.

CW said...

The guy in New Zealand who is the guru of wifi-dishes-made-out- of-cookware says that radomes made out of plastic balls, etc, are a very good idea.

Because of the wavelength (several centimeters), the mesh is as good as solid in most cases (unless it was VERY large mesh, like large chicken wire).

I've been on a kick to see how much range I can get out of off-the-shelf routers not plugged into anything - powered with 12V UPS batteries, and using the colander dishes. So far the answer is several miles.

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