Additional RF Equipment: Antennae and Cables

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The steel and aluminum body panels in most auto and light truck bodies block radio signals very effectively. As a result, most WarDrivers will want an external antenna on their vehicle to improve the detection of APs. An external antenna for WarDriving functions in the same way as the AM/FM radio antenna on your car. It allows a weak signal, which otherwise might not penetrate the car body, to be brought in where the receiver is located. Conversely, it also allows the transmitted portion of the signal to extend past the card body. Because of these reasons, usually an antenna is the first accessory purchased for the WarDriving kit.

Generally, an antenna capable of transmitting and receiving the signal equally in all directions is preferred. These antennae are known as omni-directional, or omni for short. Most WarDrivers use an omni antenna in the 3dBi to 8dBi range, as these give the best compromise on signal gain and pattern. Commonly, this type of omni-directional antenna has a magnetic base that sticks readily to a steel car body roof. Such “mag mounts” as they are known, allow the antenna to be transferred between different vehicles without a complicated installation. Figure 3.14 shows a typical 3dBi mag mount antenna.

Figure 3.14: 3dBi Omni-Directional Antenna with Magnetic Mount Base

Most antennae of this type come with an attached 10- to 20-foot cable. This cable allows the signal to be carried easily inside the car or truck. The cable end opposite the antenna usually terminates in a Type N-Male connector. If the attached cable is too short or absent, a cable of the appropriate length will have to be purchased.

In addition to an omni, directional antennae of different types are available from a wide variety of sources. A directional antenna is confined to sending and receiving in a single direction, as the name implies. Directional antennae also come in different gain levels, and usually are described as having a particular “beam width” in degrees. Think of it like a flashlight beam of light. The beam width on a directional is the area the antenna will send and pick up radio signals.

Due to the narrow focus of the “beam,” directional antennae are of limited use for general WarDriving, since the signal is confined to sending and receiving in only one area. Most times while WarDriving, you are attempting to find as many APs as possible, in all directions. However, a directional is a must if you are attempting to locate a specific AP. For example, when attempting to locate an unauthorized or “rogue” AP on a company network, a directional antenna is an indispensable tool.

Pigtails

The frequencies used by 802.11 WLANS fall into the microwave area of the radio spectrum. In order not to lose too much of the radio energy in the connectors, the connectors must be of the proper type and rated for microwave use. The “N” type connector is one of the most commonly used for microwave RF communications, and is seen on many brands of antennae and cables. Unfortunately, an N connector is huge in comparison to the average WLAN cards. The combined weight of the cable and the connector would almost certainly destroy most cards in short order. For this reason, the card manufacturers use tiny connectors on the cards. To convert from the tiny connector used on the card to the N connector, we need what is known as a “pigtail” cable.

The term “pigtail” comes from radio engineering and ham radio. A pigtail is nothing more than a short piece of antenna cable, with different connectors on each end. They are used to convert one connector type to another. Usually, the cable used for a pigtail is a smaller diameter and type than the main cable, but this is not always true. The exact origins of the term seem to be lost, but since most small diameter coaxial cables curl rather tightly after being unwound from a cable spool, it seems reasonable that the name came from the fact that a small length of cable might resemble the curled tail of a pig.

The most common pigtail used for WarDriving is the ORiNOCO proprietary connector (Type MC) to a Type N-Female connector, which is a reflection of the popularity of the ORiNOCO card, as seen in Figure 3.15. However, there are others pigtail types. Another fairly common pigtail is the Type MMCX connector to a Type N-Female connector. When purchasing a pigtail, you must determine the connectors that you need on each end of the cable. This will depend on the brand and model of your card, and the connector on the antenna cable. If you have more than one card that you will be using for WarDriving, most likely you will need more than one pigtail. Most pigtails will be about 12 to 18 inches (30 to 45cm) in length. Longer pigtails may be found, but are generally best avoided. This is because the thin cables have a high signal loss, and the longer the cable, the more signal is lost before it gets to the radio card.

Figure 3.15: Pigtail (Type N to Type MC)

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