Page 39 - Antennas for Base Stations in Wireless Communications

12 Chapter One

directivity of the antenna, and G the antenna gain. We usually deal with
relative gain, which is defined as the power gain ratio in a specific direc-
tion of the antenna to the power gain ratio of a reference antenna in the
same direction. The input power must be the same for both antennas
while performing this type of measurement. The reference antenna is
usually a dipole, horn, or any other type of antenna whose power gain
is already calculated or known.

P
G = G ref P max max ref (1.22)
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In the case that the direction of radiation is not stated, the power
gain is always calculated in the direction of maximum radiation. The
maximum directivity of an actual antenna can vary from 1.76 dB for a
short dipole to as much as 50 dB for a large dish antenna. The maximum
gain of a real antenna has no lower bound and is often –10 dB or less
for electrically small antennas.
Antenna absolute gain is another definition for antenna gain. However,
absolute gain does include the reflection or mismatch losses:

G = e G = e e D (1.23)
abs eff refl cd
As defined before, e refl is the reflection efficiency, and e includes the
cd
dielectric and conduction efficiency. The term e is the total antenna
eff
efficiency factor.
Taking into account polarization effects in the antenna, we can also
define the partial gain of an antenna for a given polarization as that
part of the radiation intensity corresponding to a given polarization
divided by the total radiation intensity of an isotropic antenna. As a
result of this definition for the partial gain in a given direction, we can
present the total gain of an antenna as the sum of partial gains for any
two orthogonal polarizations:

G total = G q + U f (1.24)

4 π U 4 π U
G = P in θ & G = P in φ (1.25)
θ
φ
The terms U and U represent the radiation intensity in a given direc-
q
f
tion contained in their respective E-field component.
The gain of an antenna is a passive phenomenon; power is not added
by the antenna but simply redistributed to provide more radiated power
in a certain direction than would be transmitted by an isotropic antenna.
An antenna designer must take into account the antenna’s application