 During the course of the war we have developed many different radar sets, each built to do one certain job better than anything else. However, regardless of size, design or job, all pulsed radar sets operate on the same basic principles, and the component parts of all these sets are essentially the same. And whichever set you work on, your job, the repairman's job, is the same, to keep your set working and working right all the time. That goes for every set, from the latest search light set, the ANTPL, to this early settler, the SCR 268, on alert in the theater of operations. This one's in Italy. But what's about to happen to it can happen anywhere, on any set, and often does. The operator is on target. Whoa there, wait a minute, what goes on here? That's better. Hey, that does it. Well, what is it, set trouble, jamming, mice in the local oscillator? Whatever it is, it's got to be fixed in nothing flat. Bombers won't wait. And it's your job. It's you radar repairman who will be handed the problem and the nothing flat to do it in. So you have to know your stuff. You have to know if it's set trouble or not. And if it is, locate it and fix it. In the first film of this series, we reduced all radar sets into a simplified block diagram. You saw how the timer originates the pulses, the transmitter converts them into RF energy, and the antenna flings them into space. At the same time, remember, the timer sends part of each energy pulse to the indicator, where it starts the sweep of the baseline. The main pip at the beginning of the baseline is formed when part of the energy leaving the transmitter is picked up by the sensitive receiver. The baseline is synchronized with the outgoing pulse and provides us with an accurate electronic yardstick for measuring the distance traveled by each pulse. When a target comes within range, echo signals are reflected. These signals are picked up by the receiving antenna, amplified by the receiver, and finally register on the baseline of the indicator as a target pip. The position of the target pip on the baseline is calibrated by the set. And in that way, you find the range of the target. Each set transmits high-frequency radio energy in the form of short pulses, followed by a listening period, one long enough to permit a target echo to return from maximum range. In that way, echo signals can be seen clearly by themselves. To make sure that the proper time intervals are maintained and that at these intervals each unit is triggered off to a time accuracy of one millionth of a second, every set is equipped with a master timepiece called a timer, or sometimes synchronizer or keer. Here's what the timer of a typical long-range early warning set, the SCR271D, looks like. And here's its heart, the vacuum tube in the timing oscillator. All the tubes won't look like this, but they all do the same job. Here's what it is. First, it generates a sine wave, where each cycle is identical with all the others. If the sine wave's frequency is a thousand cycles a second, we have actually divided one second into a thousand equal parts. But in this sine wave, it's impossible to tell where each thousandth of a second begins and ends. That's why it's changed into this, a peaked wave, which can be measured from positive peak to positive peak. This wave is the basic controlling wave of the set. All the component actions of our radar are synchronized with these peaks. For instance, it's part of this peaked wave voltage that passes from the timer to the indicator to trigger off the baseline. This synchronizes the baseline with the outgoing pulse because at the same time, the same peaked wave triggers a pulse-forming circuit, which creates a new wave form, pulses with flat tops. Notice that throughout these changes, the exact subdivisions of our second and time have not been lost. We still have precisely the same number of positive peaks per second that our original sine wave did. But here is the important difference. In the original sine wave, the voltage rises and falls gradually. The action is easy. Never sudden. When we use the rectangular trigger pulse, the voltage rises from zero to its peak value instantly, remains at that point for the length of the pulse duration, then immediately returns to zero. Pulse duration is also called pulse width. This action of the trigger pulse causes sudden powerful bursts of energy to be generated and transmitted. Then with equal suddenness, the energy is turned off while the outbound pulse travels through space. This transmission of radio energy may last from half a microsecond to several microseconds, then goes off for several thousand microseconds. On again, off again. So many pulses per second. Some sets transmit wider pulses than others. These are frequently the early warning sets, which pick up targets as far away as a hundred and fifty miles. The echoes from the long-range target register on the baseline as a distinct pip. But as the target comes closer to the set, the echo pip on the baseline approaches and may even merge with the main bang and be lost. The long-range sets don't worry about this. They turn the target over to short-range sets. But to track short-range targets effectively, we must keep the target pip from merging with the main bang. That's why short-range sets must transmit narrow pulses or pulses of short duration. With this type of pulse, the set will get a distinct and separate echo pip on the baseline, even at extremely short ranges. We can conclude therefore that the width of the pulse largely determines the effective minimum range of the set. Now let's examine the time interval between these pulses. This time interval, or listening period, must cover the time it takes for an outgoing pulse to reach a target at maximum range and return. For a set designed to operate at a maximum range of twenty miles, the timer might generate as many as forty-five hundred pulses per second. But if the desired maximum range is a hundred miles, the listening period has to be longer. And the timer might have time to generate no more than nine hundred pulses per second. You can see then that one of the basic factors in determining the maximum range of the set is this fixed rate of pulse repetition. It's known as the pulse recurrence frequency or P R F.