AP 3302 Pt. 3
Section 1
CHAPTER 3
Factors affecting the performance of Pulse-Modulated Radars
that
at the output is known as the 'noise factor' of a receiver. It is a measure
of the noise introduced by the receiver itself (Fig 8). The design problem is
to produce a receiver with as low a noise factor as possible. This is complicated
by the fact that the receiver has to accept very narrow pulses and hence a wide
bandof frequencies. Wide bandwidth tends to increase the noise factor of a receiver.
Thus the design of a receiver is a compromise between high sensitivity, wide
bandwidth and low noise factor.
Frequency
of Operation
The frequencies used in radar are high for three main reasons (see Fig 9):
a. To obtain a good echo the radar wavelength must
be less than four times the size of the target.
b. For good angular discrimination
between adjacent targets, for accurate indication of bearing and for adequate
concentration of the radiated energy we use aerials which can provide a very
narrow beam. This can be achieved much more easily at high frequencies.
c. High frequencies are needed to ensure an adequate number of r.f. cycles in
each pulse.
The frequency chosen for a particular radar depends upon the job it has to do. A high-resolution radar which is required to discriminate between targets very close together in bearing will use super-high fre-quencies in the microwave region~in the band 3,000 to 30,000 Mc/s. For long range radars, where early warning is the criterion and accuracy of range and bearing of less importance, the v.h.f. band around 200 Mc/s may be used. The lower frequencies have the advantage of smaller atmospheric absorption and longer ranges.
In
radar the wavelength at which the equipment is operating is quoted as often
as the frequency. The relationship be-tween frequency f in cycles per second,
velocity c of e.m. waves in metres per second and wavelength l
(lambda) in metres is illus-trated in Fig 10.
