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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

Pulse duty factor. This is also known as the duty cycle. It is the ratio of the pulse duration TD to the pulse spacing TR. Since TR = 1/PRF the pulse duty factor is also the product of the

pulse duration and the p.r.f. If the pulse duration is 1 us and the p.r.f. 500 p.p.s. the pulse duty factor is 10-6 x 500 or 2,000 us which is usually read as 1 in 2,000. It means that there is one 1 us pulse to every 2,000 us. If we drew each pulse with a width of 1 inch to represent the pulse duration of 1 us the next pulse would occur 2,000 inches further along, i.e. approximately 55 yards away!

Peak and average values of power. We have seen that the power output of the transmitted pulses should be as great as possible; but we also know that when power is developed in a circuit heat is produced in the components. However, because the transmitter works in pulses we are able to develop a very high power in the circuit for the short duration of the pulse and, in the comparatively long resting time between pulses, the heat produced can be removed. If the transmitter uses a pulse duration of 1 us and a p.r.f. of 500 p.p.s. in one hour of operation the trans-mitter is switched on for a total time of only l.8 seconds. Although the peak power developed during each pulse may be very high the average or mean power over a long period is quite low. The power rating of the components used in the transmitter circuit can therefore be much lower than might at first seem necessary.

Relationship between the various factors. There is a very simple relationship between the peak power during the pulses, the pulse duration, the p.r.f. and the mean power. From Fig 3 we see that the mean power is equal to the peak power multiplied by the product of pulse duration and p.r.f. Since pulse duration x p.r.f. equals the pulse duty factor the mean power PA may be written as:

PA = Peak power Pm x duty factor.

Example. If a radar transmitter radiates a peak power of 1MW with a pulse duration of 1 us and a p.r.f. of 1,000 p.p.s. the mean power is:

PA = Pm x TD x fr

= 106 x 10-6 x 1,000

therefore PA = l kW.


 

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