RF Hacker Improvement Kit Antennas and Gain

Often, a attacker is not able to get a signal strong enough to decode the embedded communications protocol successfully. This is where the attacker applies his or her understanding of the concept of gain and that corresponding application of the proper antenna type to enhance signal reception and transmission.

For a signal to be received or transmitted, it has to go through an antenna. An antenna serves to radiate or collect RF energy and can be omnidirectional or directed. An omnidirectional antenna is a device that radiates a signal in all directions. Theoretically speaking, it does not favor any direction. However, most so-called omnidirectional antennae on store shelves today are of the monopolar/half-wave dipolar variety and thus have a toroidal or doughnut-shaped emission pattern and a semi-blind/blind spot, depending on distance directly above the vertical axis of the antenna.

Directed antennae are devices with high-gain in a particular direction. Gain is the amount of RF you are favoring in a particular direction. It is a measure of directionality and is measured in dBi or decibels over isotropic. Isotropic refers to the theoretical perfectly spherical emission pattern of a purely theoretical antenna that radiates equally in all directions. Isotropic antenna cannot physically exist. Instead, they are used as theoretical comparators with real-world antenna types. Thus, an antenna with a 10 dBi gain would have a longer, more torpedo-like shape toward a particular direction compared with an antenna with a relatively more doughnut-like 3 dBi omnidirectional.

Among the different types of directed antennae are the Yagi-Uda and the parabolic antennae. Developed by Hidetsugu Yagi and Shintaro Uda, the Yagi-Uda design is best exemplified by the TV aerials common on the rooftops of houses and apartment blocks in many countries. The makeup of a Yagi-Uda is reflected in Figure 8-3.


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Figure 8-3 Yagi-Uda antenna design

The Yagi-Uda design seeks to direct the RF signal along the boom axis out to the front of the antenna. With directionality of this nature, the RF footprint looks something like that shown in Figure 8-4.

Figure 8-4 Yagi-Uda antenna RF footprint

Obviously, the Yagi-Uda can shoot a RF signal much farther in a specific direction than an omnidirectional antenna. However, this is clearly at the expense of all-around coverage. From the Linux enthusiast's point of view, he or she can use a Yagi-Uda to provide network connectivity by reliably bridging two Linux boxes, configured as routers, via their master-mode-enabled WNICs, sited at separate locations up to 1.5 km from each other. Alternatively, a long-range audit of a wireless network can be effected using tools that are mentioned in the next section.

Like the Yagi-Uda, a parabolic antenna is a directional antenna. However, it has a much narrower beamwidth. Consequently, the narrower beamwidth results in a parabolic design, gain-for-gain, typically outranging the Yagi-Uda. An example of the performance envelope of a parabolic is the fact that parabolics are mounted on spacecraft to communicate with Earth-based controllers and used on naval warships for surveillance and fire-control (see Figure 8-5).

As the beamwidth is much narrower, boresighting the antenna becomes even more critical. Let's take a look at how a hacker would build an external cantenna.

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