Redundant Links
All antennas show a characteristic known as
polarity,
which refers to the direction that the electrical field moves in as
it leaves the antenna. Simultaneously, magnetic waves leave the
antenna at a 90-degree angle to the electrical waves. Most common
antennas show a linear polarity (i.e., vertical or horizontal). Some
antennas, like a wound helical
antenna, actually
demonstrate
circular polarity, in which the waves
move outward in a spiral, always perpendicular to each other.
In order for one antenna to receive the signals of another, the
polarity must match. Omnidirectionals (and most sectors) have
vertical polarity. Dishes and yagis can be mounted vertically,
horizontally, or somewhere in between. The Pringles can is just a
yagi, and its polarity is determined by the position of the antenna
connector. A circularly polarized antenna (like the helical) has its
polarity determined by the direction of the outer winding: either
clockwise or counterclockwise.
You can use polarity to your advantage to
try to eliminate some
noise on a long-distance link. First
try each end in vertical polarization and measure the perceived
noise. Then rotate each end 90 degrees and measure the noise again.
Use the position that shows the least amount of noise, and you should
have a more stable link. Figure 7-5 shows typical
polarization.
Since an antenna can receive signals only from antennas whose
polarity matches its own, you can also use this property to make more
efficient redundant links. For example, suppose you wanted to use two
radios at either end of a link to provide 22Mbps total bandwidth to a
remote location. Usually, you would need to use two channels
separated by 25MHz (i.e., 1 and 6, or 2 and 7, or 3 and 8). If you
use one antenna with vertical polarization and another with
horizontal polarization, and separate the antennas by a few feet, you
could use the same channel for all of your traffic. This means less
noise in the band for you and your neighbors (and it theoretically
gives you up to six possible simultaneous links without interference,
where normally only three were possible).
As far as IP goes, you can either route the traffic independently or
use the channel bonding features of the Linux 2.4 kernel to weld all
of your connections into one big pipe. This mode of operations is
new, experimental, and left as an exercise to the reader (hey, if
you're anything like me, you could stand more
exercise in your life).
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