Use electromagnetic polarization to avoid noise from other antennas in the same spectrum.
One extremely important property of electromagnetic waves to consider is polarization. An electromagnetic wave is actually comprised of two simultaneous and inseparable fields: the electrical field and the magnetic field. These two fields are perpendicular to each other, and both are perpendicular to the direction in which the wave propagates.
An antenna must be oriented to match the polarization of the incoming energy; otherwise, it will receive only a small portion of it. Practically speaking, this means that antennas with matching polarization will see each other well, while antennas with opposite polarization will hardly see each other at all.
Both horizontally and vertically polarized antennas are common, but in some exotic antennas, circular (clockwise or counter-clockwise) polarization is possible. The polarization of the antenna on each end of a link must match, or the radios will have trouble talking to each other. Omnis and sectors are generally vertically polarized, although horizontally polarized variations do exist (see "Build a Slotted Waveguide Antenna" [Hack #90] for an example of a do-it-yourself horizontally polarized omni). Yagis and dishes can be mounted vertically or horizontally, depending on the application.
On a long distance point-to-point link, be sure to try both horizontal and vertical polarization to see which incurs the lowest noise. Simply try the link one way, and then rotate both dishes 90 degrees and try it again. You can tell the polarization of most antennas by the position of their driven element (the part connected to the center conductor of your antenna feed). The polarization of a dish is indicated by the position of the front element, not the rear reflector, so an oval dish that points up and down is probably mounted in horizontal polarization and therefore won't be able to talk very well to a vertically polarized omni. Sectors and other sealed antennas typically indicate their polarization on the back of the antenna.
You can use polarization to your advantage to use multiple radios on a single point-to-point link. For example, you can run two parallel links on the same channel, one with vertical and one with horizontal polarization. If separated by a few feet, two dishes can operate quite happily on the same channel without substantially interfering with each other, while providing twice the bandwidth using the same channel.
When setting up a point to point link, remember to try various positions to see howpolarization interacts with the environment. Sometimes vertical polarization outperforms horizontal, sometimes not. Occasionally, rotating the antennas to somewhere in between horizontal and vertical yields the best performance. The environment between two points on a long distance shot is complicated, so it's impossible to know for certain which orientation will work best. You'll never know until you try it!
Bluetooth, Mobile Phones, and GPS
Network Discovery and Monitoring
Wireless Network Design
Appendix A. Wireless Standards
Appendix B. Wireless Hardware Guide