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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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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.


 

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