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228 ANTENNA BASICS

and many other effects like signal degradations in real receivers and transmitters (i.e. distortion,
intermodulation, etc.). These topics are far beyond the scope of this course, and the reader can
consult specialized literature for further information [37]. Note that the inequity (5.64) can
serve as a starting point for antenna gain estimation in communication links. The schematic
diagram of signal power explaining (5.64) is shown in Figure 5.3.1.

5.3.3 Monostatic Radar Equation
The core of radar performance is
the detection of a signal scattered
back by a target or group of the
targets. A simplified diagram
illustrating a monostatic radar is
displayed in Figure 5.3.2. By
definition, a monostatic radar
antenna is responsible for two
tasks: to send a powerful pulse
signal in the direction to the target
and later with some delay to
receive the backscattered signal.
A switch keeps the radar
transmitter connected to the
antenna while a powerful Figure 5.3.2 Schematic of monostatic radar link budget
searching pulse is emitted to
search for targets. The resulting echo signals are then processed in the antenna receiver. This
transmit-receive switch or a branch duplexer is a fascinating device and can be mechanical,
electrical (i.e. with gas discharge tubes or pin diodes), or ferrite (i.e. passive like circulators to
be considered in Chapter 6). The reader can find more information about radar switches in the
specialized literature [35].

It is evident looking at the diagram of Figure 5.3.2 that we can get the monostatic radar equation
as a simple update of the Friis transmission formula by an overall path loss doubling. Nothing
changes in the Friis formula until the emitted wave hits the target that scatters the portion of
incident power back to the radar receiver. Assuming that the equivalent scattering aperture of
2
the target is = [m ] we can estimate the power intercepted by the target and reflected

back in the direction of the radar receiver as
[W]

= 2 (5.66)
4
The parameter is called the Radar Cross Section (RCS) and is mainly a function over the

coordinates (, ) of incident wave, polarization, frequency as well the geometry, material, and
space orientation of the target. Finally, the expression (5.66) for Monostatic Radar Equation
(MRE) can be written in the form allowing the estimation of the antenna gain