Radar System Engineering

CHAPTER 5
C-W RADAR SYSTEMS
BY W. W. HANSEN
6.1. General Considerations. -We shall begin with a discussion that
is fundamentally useful in understanding what follows, but has as its
immediate object a definition of the scope of this chapter more precise
than is given by the chapter title.
The simplest conceivable radar system consists of a source of r-f
power, an antenna, and a scattering object or target. The antenna
emits waves with time factor eh~, which, on striking the target at a
distance r, are reflected and returned to the antenna, where their time
factor will be e’(”’-’kr) with k = 27r/A. The returned waves will be
reduced in amplitude by a factor a which depends on the target and
various geometrical factors, as discussed in Chaps. 2 and 3.
This returned wave will give rise to currents and voltages in the
antenna, which add to those produced by the power source and so give
rise to a voltage-current ratio, or impedance, which is changed from the
value existing ~vhen no target is present.
In principle, this impedance, or some similar quantity, might be
measured on an absolute scale, and the deviations from the normal, or
no-targrt value, ascribed to the presence of a target.
Practically, this procedure is impossible because the returned signals
are often 10–9 times as large, on a voltage basis, as the outgoing signals,
with the result that the variations in impedance might be of the order
of a part in 10’. Such variations can hardly be measured in the laboratory
at low frequencies, let alone in the field and at microwave frequencies.
It is therefore necessary to cause the returned signals to vary in some
manner so that the variation can be measured, rather than to attempt
such an absolute measurement as that described above.
This desired variation is caused by a modulation process of some sort.
The word “modulation” is used herein a general sense to include changes
induced by the target as well as changes in signal introduced at the
transmitter. All possible radar systems can then be classified by describing
the type of modulation and the use made of the resulting information.
As a specific example of these general remarks, let us take a very
simple radar system which consists, as shown in Fig. 5.1, of an r-f power
source, an antenna, a rectifier, a high-pass filter, an indicator, and a
127