AP 3302 Pt. 3
Section 1
CHAPTER 3
Factors affecting the performance of Pulse-Modulated Radars
There is an area surrounding any pulsed radar within which no targets can be detected. For long range search radars this 'blind' area is relatively unimportant. However if the radar is an airborne interception (Al) equipment carried in a fighter aircraft it is important that target echoes are displayed on the radar right up to the interception point. This may be only a few hundred feet.
A pulse duration of 10.75 us produces a blind area of one mile (5,280 feet). If we require interception down to 100 feet the pulse duration must not exceed (10.75 x 100)/5280 = 0.2 us. This is a typical value of pulse duration for the low ranges of an airborne interception radar.
b.
Target discrimination in range. Since em waves travel 300 x 106
metres in one second, a pulse of 1 us will extend 300 metres along the direction
of propagation. If there are two targets 150 metres apart, the leading edge
of the pulse will travel to the second target and arrive back at the first target
whilst energy is still arriving at the first. Thus, the two pockets of reflected
energy will overlap and a single echo will result (Fig 20). Short pulse durations
are needed for good target discrimination in range. To distinguish between two
targets on the same bearing and elevation, but separated from each other in
range by 100 ft, a pulse duration of less than 0.2 us is needed (see sub-para
a above).
c. Frequency used. The lower the frequency the longer must be the pulse duration to ensure an adequate number of r.f. cycles in each pulse (only important below 400 MHz).
d. Mean power available. Given a certain available mean power and a selected p.r.f. the pulse duration may have to be adjusted to produce an acceptable peak power output. This may be seen from the relationship:
Peak power = Mean power/(PRF x Pulse Duration).
If the mean power available is 1 kW and the p.r.f. is 5,000 p.p.s. a pulse duration of 2 us will produce a peak power output of 103/(5 x 103 x 2 x 10-6) = 100 kW. By reducing the pulse duration to 1 us (the other factors remaining constant) the peak power output increases to:
103/(5 x 103 x 10-6) = 200 kW.
Again, what we are really doing is adjusting the pulse duty factor.
e. Receiver bandwidth. We have already noted that the shorter the pulse duration the greater is the band of frequencies associated with the pulse and the greater must be the bandwidth of the receiver accepting such pulses. If the pulse duration is too short for the available receiver bandwidth, distortion and attenuation of the pulse result (Fig 21). This leads to inaccuracies in range measurement. Thus where the receiver bandwidth has been limited for other reasons (e.g. reduction of noise) the pulse duration must not be made too short
