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direction of greatest gain. In most cases, you will see Yagis in the microwave fre-
quency ranges (above 1 GHz) encased in a plastic cylindrical covering made of the
same material as a radar's radome, transparent to the radio waves.
It is interesting to note that all practical antennas exhibit a phenomenon called
effective area . Effective area is sort of a “capture” area of the antenna. Thus, an
antenna with half the effective area would capture half the EM signal. For a dipole
antenna tuned to the frequency of operation, the effective area is inversely proportional
to the frequency of operation. For wireless networking, this means that the antennas
used for 5 GHz will have about half the effective area of those we use at 2.4 GHz. This
accounts for a received signal that is about half that of the 2.4 GHz case, so we can
expect that there will be about a 3 dB lower signal. 1 The net effect of all this is that we
can expect 5 GHz networks to be more limited in range for the same transmitted power
and that we may have to use higher gain antennas to compensate.
So-called parabolic antennas (really paraboloid-reflector antennas) are a spe-
cial case. The actual antenna is a small dipole or a feed horn at the focus of the par-
abolic dish. The dish essentially captures extra signal, thus artificially increasing the
effective area, and then reflects/concentrates it onto the feed point. Dish antennas
are capable of truly awesome gain, and the increase in effective area is proportional
to their size. Practical dish antennas for 2.4 GHz are usually 12 to 18 inches in
diameter, and can produce about 15 to 25 dB of gain, which is a little better than a
multi-element Yagi.
Dish antennas are also more obtrusive, harder to mount, and more affected by
wind. Still, if you could use a 30-m dish…wow!
Interference Sources
All radio transmission is subject to interference from a variety of causes. With the tiny
signals (20-100 milliwatts) involved in the unlicensed low-power transmission of 2.4
GHz and 5 GHz wireless networking, the most significant issues are the strength of
the available receive signal, relative to other unwanted signals. Unfortunately, many
other types of RF equipment operate at nearby frequencies and all digital equipment
produces unwanted or spurious signals. For example, a microwave transmitter,
microwave oven, radar, or certain industrial equipment may produce interfering sig-
nals. For that matter, other wireless devices operating in the same (or mathematically
related) frequency ranges may interfere with a wireless network.
1 A decibel (dB) is simply 10 times the log of the power input/output ratio, or 10 log ( Pi/Po ). Thus, a
power ratio of 2:1 equals approximately 3 dB. Likewise, 4:1 corresponds to about 6 dB, and 10:1 cor-
responds to 10 dB, exactly. The use of the logarithmic dB unit transforms all the gain/loss calculations
into simple addition and subtraction.
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