Antenna scams.

Antenna gain:
Antenna gain figures have long been used as a way to impress the customer.
Although dB gain should be a reliable way to tell the performance of an antenna, manufacturers know how to make the figures look better than they really are. Some of the tricks used are covered in detail in the antenna section of the technical side of this site.
As a general guide:
  * Ignore dB figures, only dBi and dBd mean anything.
  * dBi and dBd are not the same - do not directly compare them.
  * If a small antenna claims a large gain figure, don't believe it.

Antenna length:
Another method used to impress the potential customer is to state the electrical length of an antenna. The antenna may be only a few feet, but they may boast an impressive sounding 18 feet or more.
It is true that, as a rule of thumb, a larger antenna is likely to outperform a smaller one - but this applies to physical length, not electrical. It does not matter how much wire they pack inside a 5 foot antenna, it is still only a 5 foot antenna.

Twin antennas:
A common misconception, possibly started by those with money to make, is that two antennas are better than one. This is not quite true.
The idea is that two antennas will work together to provide more power in some directions than in others. However, for this to work as desired, everything must be correct: distance between antennas, phase,coaxial cable used, matching etc.
The typical vehicle is not wide enough to give the correct spacing and, even if your vehicle is, the person fitting must get the cable and phasing spot on.
Ignore the twin antenna hype, just get one decent antenna.

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