Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 477
In an ideal antenna system, 100 percent of the radiated power is in the main lobe,
and there are no other lobes. But in real antennas, certain design and installation anomalies
cause additional minor lobes, such as the sidelobes and backlobe shown in Fig.
20.22A. Several problems derive from the existence of minor lobes:
• Loss of useable power for the (desired) main lobe. For a given power density required
at a distant receiver site, the transmitter must supply additional power to make
up for losses in the minor lobe(s).
• Intersystem interference. A major benefit of directional antennas is the reduction
in mutual interference between neighboring co-channel stations. In radar
systems, large sidelobes translate to errors in detecting target bearings. If, for
example, a sidelobe is strong enough to detect a target, then the radar display
will show this off-axis target as though it were in the main lobe of the antenna.
The result is an azimuth error that could be important in terms of marine and
aeronautical navigation.
Gain
Antenna gain G is a measure of the apparent power radiated in the peak of the main
lobe relative to the power delivered to the antenna feedpoint from the transmitter. Thus,
G incorporates the effect of ohmic and other losses in the antenna. Specifically:
G = ξ D
(20.26)
Antenna/transmitter systems are often rated in terms of effective radiated power
(ERP). ERP is the product of the transmitter power and the antenna gain:
ERP = G P
DLVD
(20.27)
where P DLVD is the actual transmitter RF power delivered to antenna terminals. If an
antenna has a gain of +3 dB, the ERP is twice the transmitter output power. In other
words, a 100W output transmitter connected to a +3-dB antenna will produce a
power density at a distant receiver equal to a 200W transmitter feeding an isotropic
radiator.
There are two interrelated gains to be considered: directivity gain G d and power
gain G p .
Directivity gain is defined as the quotient of the maximum radiation intensity divided
by the average radiation intensity. (Note the similarity to the directivity definition.)
This measure of gain is based on the shape of the antenna radiation pattern and
can be calculated with respect to an isotropic radiator (D = 1) from
G
D
Pa
= 4π
P
RAD
(20.28)
where G D = directivity gain
P a = maximum power radiated per unit of solid angle
P RAD = total power radiated by antenna