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To compare antennas, the radiation pattern is
compared to that of an imaginary “perfect” antenna that
radiates all the available power in all directions. This “perfect”
antenna is called an isotropic radiator. If an antenna is able to
concentrate more of the available power in a particular direction,
it is said to have “gain” compared to the isotropic
antenna. Gain is measured in "decibels" or “dB”;
3 dB is double the power. This “perfect” or isotropic
antenna is said to have 0 dB of gain, so if an antenna is able to
put out double the power in a particular direction, it is said to
have 3 dB of gain.
Antenna gain works for both transmitting and
receiving. That is, a 3 dB antenna is able to transmit out twice
as much power in a particular direction as an isotropic radiator,
but it’s also able to receive a signal twice as weak. So,
if you point two of these 3 dB antennas at each other, the transmit
one has 3 dB of gain, and the receive one also has 3 dB of gain,
for a total gain of 6 dB.
Antennas radiate both in the horizontal plane
and vertically as well. These characteristics are usually both specified
by a manufacturer as horizontal and vertical beamwidth. It’s
important to examine both when choosing an antenna, since manufacturers
make tradeoffs between these characteristics to produce antennas
suited to various uses. For example, an omnidirectional antenna
with a 10 degree vertical beamwidth mounted on a 500 foot tower
might not hit a user on the ground 500’ away, but would shoot
right over his head!
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