Determining Antenna Gain

Figure out the approximate gain of your home brew antenna—without a spectrum analyzer.

After building one of the many antennas in this chapter, or perhaps designing one of your own, you will inevitably wonder just how much gain your antenna provides. While an ideal testing rig would include a spectrum analyzer and lab conditions, most people can’t afford to bring such resources to bear on their little antenna project. Fortunately, informal gain tests are simple to perform, given some simple tools and a little patience.

Here is one method for estimating gain. While your results might not be as accurate as those provided by a “real” radio lab, it can give you a fair estimate of how well your equipment performs, for very little cost.

What you’ll need:

  • Two radio cards of the same manufacturer and firmware revision, as well as external antenna connectors (Lucent/Orinoco/Proxim cards, or Prism II cards like the Senao/EnGenius work well)

  • Two laptops

  • The antenna to be tested

  • Two antennas of known gain (preferably low gain and somewhat directional, like sector antennas)

  • Two tripods, mounts, and pigtails for the above antennas

  • A large, flat outdoor space free of obstacles

  • A notebook

  • A friend, and a means of communicating with that friend (such as cell phones or FRS radios)

Connect an antenna to one of the cards, and using a program like NetStumbler [Hack #21], run a simple site survey. Walk around the area a bit, and look for an unused (or lightly used) channel. Once you decide ...

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