Jeff Duntemann's Wardriving FAQ

Part III: Your Wardriving Rig

Please give me an overview of a wardriving rig.

This is a fairly typical setup, and very close to the one I use. The GPS receiver may be inside the vehicle. Some GPS receivers connect to the computer via a USB port, and take their power from the USB port, thus making the "Y" connector from the GPS receiver unnecessary.

What sort of antenna works best for wardriving?

You want some sort of omnidirectional vertical antenna, ideally one that can be mounted on the roof of your vehicle. Again, an external antenna is optional: You can wardrive with no better antenna than the one on the end of your Wi-Fi PC card. Such antennas are poor to begin with, and when they're inside the vehicle the metal structure of the vehicle shields them from AP signals to some extent. You will gather a lot more stations using an external antenna. In my own tests, a roof mounted omni antenna increases the numbers of stations sensed by at least 50%, and in some cases 100%, over a PC card's built-in antenna.

The antenna I use is quite popular: FAB Corp's 5 dBi omnidirectional mag-mount, shown at left. The assembly rests on a magnetic disk that sticks to the (metallic) roof of your vehicle, with a length of low-loss coax to take signal to your WI-Fi adapter inside the car. FAB has the same antenna on an NMO connector, in case you're willing to drill the roof of your vehicle for an NMO mount and run low-loss coax down into the vehicle.

I've also had reasonable success wardriving from the back seat of a taxi by literally hanging a blade antenna from a suction cup stuck to the taxi's rear window. The cabbies look at me funny sometimes, but have never objected.

Why not use a directional antenna?

You can, and some do, in certain special circumstances. If you're standing somewhere (on top of a building or hill) and want to sense stations at a distance, you can use a directional gain antenna to "warscan" by rotating it on a camera tripod or just aiming it by hand.

Is a Pringle's potato chip can good for warscanning?

In a word, no. That is, it may work, but it won't work as well as a "cantenna" made of a metal can like a coffee can or (my favourite) a spaghetti sauce can. If you're going to go the distance to make an antenna yourself, make one that works well, not one that works so-so. Pringle's cans are nonconductive cardboard, and thus do not operate in waveguide mode, as a metal can does. Rob Flickenger's shotgun yagi adaptation of the "classic" Pringle's antenna works, but it's a lot more work, and in my experiments doesn't do as well as a single tin can without the yagi elements.

I don't do a lot of warscanning, but I've tried several different things. At right is a handheld waveguide antenna made from a tin can that once contained a bottle of spiced rum. (And yes, it's propped up on an old Tom Swift book!) It works, but it's extremely directional. A shorter can would make it less directional. I've had better results in warscanning by mounting a directional antenna on a camera tripod, and steering with the tripod pan levers. Holding a highly directional antenna steady long enough for NetStumbler to get a good fix is a bit of a challenge, especially for distant stations.

I provide detailed instructions for building cantennas like this in my book, including all the math and plenty of photos and technical diagrams.

What is a pigtail?

A pigtail is a short (emphasis: short!) length of microwave-friendly coaxial cable with connectors on both ends, used to connect a piece of Wi-Fi gear to an antenna. It's a microwave jumper cable, basically. It must be short because coaxial cable is very lossy at microwave frequencies, and the loss is proportional to the length of the cable.
Pigtails typically have an industry standard N female connector on one end, and one of the several species of Wi-Fi specific connectors on the other end. You select a pigtail based on the length and on the type of Wi-Fi connector. (The N connector on one end is usually a given.) For wardriving you'll probably need one of only two kinds of pigtails: One for the RMC connector used with the Orinoco PC cards, or one with the MMCX connector used for several of the lesser known cards. Such pigtails are typically 19" long and are quite flexible. Longer pigtails are less flexible, because lower-loss coax is inherently stiffer.

Ready-made pigtails are available from several firms online, including FAB Corp. They cost from $20-$30 typically, depending on the length and the kind of connectors.

That's a lot of money for 20" of wire! Wouldn't it be cheaper to just make pigtails myself?

Yes, a little. However, it's very easy to attach coaxial connectors to cable badly, especially the very tiny connectors that are most often used in wardriving. At microwave frequencies (where Wi-Fi operates) losses in coaxial cable are high, and those losses skyrocket when a connection is badly trimmed and soldered. Think of it this way: You might save $5-$8 on the cost of a pigtail by doing it yourself, but unless you have considerable experience soldering small connectors to coax, you could end up losing 25% of your station count (maybe more) on a typical wardrive. Let the professionals do it.

What GPS receiver works best?

Pick one; they're all good. Any GPS receiver that can get pick up enough GPS satellites to get a fix on your position will work fine. Most people choose a GPS receiver in terms of what other uses they can make of it beyond wardriving. Many GPS receivers are designed for independent use (in other words, without connecting to a laptop or PDA) and have small graphic screens for displaying your current position. Products like these are a good choice if you're a hiker or go offroading out in the wilderness. The downside to using such receivers is that they're typically kept inside the vehicle, and sometimes that makes it difficult for the receiver to pick up enough satellites for a fix.

The GPS receiver I use is an entirely different concept: It's a little magnetic puck that sticks to the roof of your car, with a single USB cable leading in through a window, to plug into your computer. There's no screen or any other kind of readout on the receiver itself; it's designed to work with a computer and not independently. The one I use is called the Holux GM-210, shown at left. The Holux line is popular in Europe but is little known in the US. I found it at Amazon Store. It costs $150 at this writing (early 2003).

Note well that NetStumbler does not directly accept GPS coordinate data from USB ports. The Holux is a USB device, but it comes with an installable "port bridge" driver that grabs one of the PC COM ports (I use COM5) and places data arriving from the receiver on the USB port onto the COM port, where NetStumbler can find it. This works very well, and because power is provided to the receiver over the same USB cable as the serial data itself, this makes a separate power cable and connector unnecessary. I also use the Holux to display a "you are here" marker while we're traveling by car, in conjunction with Microsoft MapPoint, so its use is not limited to wardriving. (My wife does the navigating and reads the laptop screen, by the way. Watching a laptop while you're driving is dangerous, and illegal in some areas!)