Antenna Gain
The term antenna gain describes how much power is transmitted in the
direction of peak radiation, relative to an isotropic source. Antenna gain is
more commonly quoted on antenna specification sheets. Antenna gain is important
because it takes into account actual losses incurred.
An antenna with 3 dB gain means that the power received from the antenna is
3 dB higher than it would be from a lossless isotropic antenna with the same
input power. 3 dB is equivalent to twice the power.
Antenna gain is sometimes discussed as a function of direction or angle.
However, when a single number specifies gain, that number is the peak gain in
all directions. Antenna gain, "G," can be compared to directivity, "D."
[Equation 3]
The gain of a real antenna can be as high as 50 dB for a very large
satellite dish. Directivity can be as low as 1.76 dB for a real antenna (such as
a short dipole). Directivity can never be less than 0 dB. However, the peak gain
of an antenna can be arbitrarily small. This is because of losses or
inefficiency. An electrically small antenna is an antenna that is relatively
small, about the wavelength of the antenna's operating frequency. Small antennas
can be very inefficient. Antenna gain is often less than -10 dB, even when not
accounting for impedance mismatches.
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