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Hack 84 Taking Advantage of Antenna Polarization

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, or it will only receive 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 [Hack #77] 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, 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, and provide twice the bandwidth using the same
channel.