How much gain is
Antenna performance is primarily established by its gain.
Gain must always be measured against a know reference. Unfortunately,
there are many "so-called" gain references. Choosing
the wrong reference could cost you up to 2.0 dB in performance.
Most commercial antenna suppliers specify gain in dBd (gain over
a half-wave dipole). The half-wave dipole is one of the simplest
and most efficient antennas. It is relatively easy to construct
and has a very predictable radiation pattern similar to that
of a donut. This is why most commercial antenna suppliers reference
gain to a half-wave dipole.
Another reference, especially at microwave frequencies, is dBi.
This term refers to gain over an isotropic radiator, a theoretical
antenna that radiates equally well in all directions (such as
the Sun). If an antenna is specified in dBi gain, it will appear
to have approximately 2.15 dB higher gain than dBd. This may
be a nifty way to impress the customer but it does not mean that
you get more gain if dBi is the reference.
Beware if the supplier
only quotes antenna gain in just "dB". What is the
reference? For instance, some mobile antennas are specified in
dB gain over a quarter wave whip which is about 0.6 dB less gain
than a dipole.
Furthermore, gain influences the type and size
of an antenna. The higher the gain, the larger the antenna. Table
1 shows some of the common communication antennas and the approximate
gain ranges to expect.
Antenna Gain Table 1.
As a rule of thumb, doubling the gain (a 3.0 dB increase) will
increase the size of an antenna by a factor of 2-3. Properly
combining two identical antennas will also increase the gain
up to about 3.0 dB over that of a single antenna.
the gain of a 6 foot high unity gain omni by 3 dB, the antenna
will height will have to be increased to 12 to 18 feet tall!!
Likewise, a Yagi antenna with a 4 foot long boom will have to
increase to 8 to 12 feet along with additional elements! Table
2 can be used as a guide to assist in the selection of a Yagi
To increase the gain of a 6 foot high unity gain omni
by 3 dB, the antenna will height will have to be increased to
12 to 18 feet tall!! Likewise, a Yagi antenna with a 4 foot long
boom will have to increase to 8 to 12 feet along with additional
elements! Table 2 can be used as a guide to assist you. A reminder,
increasing gain will decrease your beamwidth.
Antenna Gain Table 2.
Technologies, Inc. and the author retain the rights to all intellectual
This information should be used as a guideline
only to help you in the appropriate selection of an antenna.
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