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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1

CHAPTER 2

Basic Requirements Of A Pulse-Modulated Radar System

Aerial System

Aerial. To locate an object in space we need an aerial capable of producing a narrow beam of energy so that the high peak power output from the transmitter during each pulse may be con-centrated to cover a small region of space and so that accurate bearings in both azimuth and elevation may be obtained. To produce such a beam some form of. aerial array is required and, as we have seen in Part 1 of these notes, the higher the frequency the smaller are the aerial elements in the array. At v.h.f. and above, the physical size of the aerial elements are such that we are able to construct small aerial arrays capable of rapid movement.

Centimetric radar uses frequencies in the band 3,000 to 30,000 Mc/s. At these super-high frequencies the aerial assemblies used for radiating a narrow beam are very similar to the structures used for focusing light rays. Typical of these is the parabolic reflector which may be com-pared with the headlight of a car (Fig 5). We shall learn more about the construction and operation of such aerials in later chapters of this book. It is sufficient to know at this stage that centimetric radar aerials are small, that they produce a narrow beam of r.f. energy and that they can be rotated and tilted fairly easily.

Scanning. Locating an object in space with a narrow beam of r.f. energy is rather like looking for a needle in a haystack. The beam must be capable of being swung in any required direction and the search for an object in a given volume of space must be carried out systematically to ensure that the whole volume is covered. To do this the aerial beam is made to scan the whole region which is to be investigated. Fig 6 shows one very simple form of scanning. A small overlap of the beam between adjacent horizontal scans is usual to ensure complete coverage. When the aerial is pointing maximum right at the minimum angle of elevation to be used the movement is reversed and the aerial returus to its original position in a similar manner. The scanning is repeated continuously to give the operator a complete indication of all targets within the region which is being scanned. Other methods of scanning are used and these are considered in more detail later.

T-R switch. Most pulse-modulated primary radars use an aerial that is common to both trans-mitter and receiver. This may be done because while the transmitter is working we do not need to use the receiver, and while the receiver is working the transmitter is switched off. We can therefore connect the transmitter to the aerial for the duration of each pulse and connect the receiver to the aerial for the interval between pulses.


 

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