Understanding Antenna Gain
Antenna Gain is 'the relative increase in radiation at
the maximum point expressed as a value in decibels (dB) above a reference
antenna'
Phew, let's try and decipher that.
The basic reference antenna is a ½-wavelength dipole by which all other
antennas are measured. The reference figure is known as 0dBd (zero decibel
referenced to a dipole).
Sometimes the quarter wave ground plane is used as a reference, especially
for mobile antennas, as it is equivalent to the quarter wave dipole.
Vertical antennas achieve gain by varying the
wavelength and can have omni-directional radiation gains of up to 9dB.
Figures beyond 9dB are impractical owing to the fact that the focusing is
directly related to the length (in wavelengths) of the antenna. A quarter
wave ground plane antenna has a radiation pattern that produces maximum gain
at about 25 degrees and a half wave antenna drops that angle to 20 degrees,
and the 5/8 wave antenna further drops that angle to 16 degrees.
Increasing beyond 5/8 wave gives very high angles of radiation so
we've reached the maximum gain at this point.
Some manufacturer’s state gain figures without giving the reference source
from which their figures were calculated. (i.e. 3dB gain, etc.)
This can be deceiving as the figure could be obtained by referencing an
isotropic antenna. The isotropic antenna is a mathematical representation of
an imaginary antenna that radiates equally in all directions in a spherical
pattern. This gain figure is expressed as dBi
Unfortunately dBi figures are approximately 2dB more than dBd figures for an
identical antenna. This is because a ½-wavelength dipole antenna has a gain
of 2.14 dBi
Therefore an antenna referenced as having a 3dBd gain by one manufacturer
could be referenced as as having a 5dBi gain by another. Yet overall gain is
the same.
So how does an antenna have gain?
Firstly, gain is a relative term. No power increase is actually produced by
the antenna. Assuming no losses, power in equals power out.
To start with let us take a standard ½-wavelength dipole and "suspend" it in
free space (i.e. ignore all possible surroundings that could affect the
antenna). The radiation pattern of this antenna is typically referred to as
the "doughnut".
In order to achieve gain, the radiation pattern is focus into a more useful
direction, typically achieved by flattening out the "doughnut" in a
horizontal plane rather than towards the sky and into the ground.
Gain, or focusing, can be intensified further by focusing the radiation in
only one direction.
If a reflector is placed next to a dipole, most of the energy that would
have radiated in the direction of the reflector is now reflected back in the
direction of the dipole. This makes all the energy appear in only one
hemisphere and thus results in a doubling of radiated energy in this
direction or a 3dB gain.
This radiated energy can be increased further with the use of "directors".
Introducing more directors makes the angle of radiation smaller and narrower
by packing all the radiation into one direction. By this method, gains as
high as 20dB can be achieved.
Because you can never reflect 100% of the signal, some radiation will
escape, this gives rise to the term Front-Back Ratio.
This is the ratio of the maximum directivity of an antenna to its
directivity in the opposite direction.
To summarise.
Antennas do not create power. Assuming no losses, 50w in equals 50w out. The
radiated RF power is simply focused into narrower patterns such that there
appears to be more power coming from the antenna in the required direction.
An antenna with the effective radiated power of twice the input power would
therefore have a gain of 10*log(2/1) = 3dBD.
Using 50 watts as an example:
0dB gain = 50w radiated power
3dB gain = 100w effective radiated power or 2 times
6dB gain = 200w effective radiated power or 4 times
9dB gain = 400w effective radiated power or 8 times
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