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Radio frequencies are usually expressed in terms of kilocycles per second (kc/s) and megacycles per second (Mcls), denoting, re­spectively, thousands and millions of cycles per second. For very short time intervals, microseconds (millionths of a second) are used. The wavelengths commonly used for marine radar are about 10 cm. (3,000 Mcls) or 3 cm. (10,000 Mcls).

Radio waves of this order of wavelength are called microwaves.

Directivity of the transmitted wave

Radio waves may be focused and, from the radar point of view, this has two advantages: the direction of transmission can be accu­rately known and the power sent in the required direction (and hence the likelihood of obtaining an echo) is enormously increased. So far as strength and directivity are concerned, the lighthouse is an excellent analogy.

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The size of the reflector needed to concentrate electromagnetic waves into a beam of given width depends upon the wavelength used; the longer the wave the larger must be the reflector. Hence, when employing a reflector of dimensions suitable for ships, a very short wavelength must be used to obtain a narrow beam. This is one of the reasons for using microwaves for marine radar. On a wavelength of 3 cm. a beam-width of about IV2⁰ can be obtained from a reflector 5 feet wide.

Accuracy in measuring direction is needed only in the horizontal plane that is, in azimuth. In the vertical plane the beam is made wider', so that rolling of the ship will not cause it to miss targets. It will be realized that the wider the beam in any direction the less will be its intensity. This is therefore a matter for compromise and it will be found that some radars use as much as 30° vertical beam-width and others as little as 15°.

As the beam is narrow it must be rotated in azimuth so that ra­dar information may be obtained from all directions round the ship. In common with other forms of radio, the device used for sending the waves off into space is known as the aerial or antenna. Since in this case it rotates and scans the surrounding area it is frequently called the scanner.

A reflector which sends out, in a divergent beam of a certain angle, the energy which is directed at it from its focal point will also concentrate at the same point any energy from external sources which reaches it from within the same angle. That is to say the aer­ial is as directive for reception as it is for transmission. This not only favours accuracy of bearing measurement but also gives a gain in the intensity of the received wave.

RADAR PULSE-LENGTH AND REPETITION FREQUENCY

There are two factors here which affect the design of radar equip­ment: the duration of the pulses, or pulse-length, and the interval between them. The latter is usually defined by the number of pulses per second, called the pulse repetition frequency (p.r.f.) {Fig. 22 at

(с). If it is desired to receive echoes from targets as close as 50yards from the transmitter, the pulse will have to be cut off before the be­ginning of the wave has had time to travel to the target and back; a total distance of 100 yards. A radio wave travels 328 yards in 1 microsecond, so that the pulse, in this case, must not be longer than 0.3 microseconds. If it is also desired to receive echoes from targets up to, say, 30 miles* range, the interval between pulses must be long enough to enable the wave to travel twice this distance, i.e. 370 microseconds. This gives a maximum p.r.f. of 2700 pulses per second.

It will be noted that in the case mentioned the transmitter is required to oscillate for 0.3 microseconds and then to rest for 2699.7 microseconds. Because of this a small valve may be used to generate very high power since it has relatively long intervals in which to cool. In practice the p.r.f. is usually between 500 and 2000 pulses per second. In these circumstances a valve no bigger than a 25-watt lamp will give a peak power of 60 kW.