RF Amplifiers
Whereas the antennas achieve passive gain by focusing the
energy, amplifiers provide active gain by injecting external DC power into the
RF cable. This power is sometimes referred to as "phantom voltage" and is
carried by the RF cable from a DC injector to an amplifier. There are two types
of amplifiers: unidirectional (which only increase the transmitting power) and
bidirectional (which improve the receiving sensitivity as well). In addition,
both amplifier types come as fixed or variable gain devices. For a network
design purpose, fixed power gain amplifiers are recommended for overall
stability reasons and because all necessary RF power calculations should be done
prior to the network deployment and you should be aware of your network power
needs. Traditionally, amplifiers are deployed to compensate for loss due to
significant cable length between an antenna and the wireless device. It is
unlikely that you will need one in your penetration testing procedure, as it is
cheaper and more convenient to use a highly directional antenna. However, if you
have additional cash to spare, you might want to purchase a bidirectional
amplifier to use in conjunction with the directional antenna for typical
power-demanding security experiments such as long-distance connectivity and
traffic analysis, or jamming and Layer 1 man-in-the-middle attacks. Unlike the
actual network design case, variable gain amplifiers are recommended for testing
purposes, security testing included. For example, you might want to tweak the
amplifier power to find at which EIRP a Layer 1 man-in-the-middle or DoS attack
will succeed.
The main problem with using amplifiers for security
evaluation is providing a mobile power source. For this reason, amplifiers are
rarely used by casual attackers. However, the use of one by a determined
stationary attacker cannot be excluded.
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