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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1


Factors affecting the performance of Pulse-Modulated Radars

cone is in line with the target (Fig 14a) the aerial is correctly 'aimed'. If the aerial is incorrectly aimed (Fig 14b) the signals received from the A and B positions are proportional to OA and OB respectively. By comparing the two signals the magnitude and sense of the error can be calcula-ted and a correction applied to give automatic 'following' or 'tracking' of an object.


Spiral scanning is a development of the conical scan. A pencil beam is produced by the aerial and the beam is then made to rotate about a horizontal axis in ever-increasing circles to produce a spiral as shown in Fig 15 Angles up to about 60o in both azimuth and elevation may be scanned in this way.


In all the systems so far considered the scan is the result of physical movement of the aerial. In large ground radars, where the aerial assembly is very wide, it is inconvenient to swing the aerial because of the large forces involved and because of the space taken up by a large rotating aerial. In such cases the beam is caused to scan, without physical movement of the aerial, by varying the phase of the input currents to the aerial elements in a cyclic manner. These electronic scanning methods are efficient and economical and will be considered in more detail later.

Pulse Repetition Frequency

The p.r.f. selected for a particular radar depends upon several factors, the most important being summarised below:

a. Maximum required range. Each pulse must be given time to travel to the most distant required target and return before the next pulse is transmitted otherwise there will be a risk of confusion on the display. If the maximum required range of a radar is 100 miles, the time taken for a pulse to travel to a target at 100 miles range and return is 100 x l0.75 = l075us. The transmitter must be quiescent for at least this time, i.e. the pulse spacing TR must have a minimum value of 1075 us. Since p.r.f. =1/TR the maximum value of p.r.f. is 106/1075 = 930 p.p.s.

If the p.r.f. is adjusted to this maximum value then any signals received from targets at a range greater than 100 miles would appear in the next pulse spacing period. Thus in Fig 16 target A, within the required range, is indicated on the c.r.t. during every pulse spacing period. Target B, outside the required range, misses the first pulse spacing period. The persistence


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