The propagation of waves

A little must be said here of the passage of radio waves through space after they leave the aerial and of the echoing properties of tar­gets. If the similarity between radio waves and light waves is borne in mind there will be little difficulty in appreciating their behaviour.

REFRACTION IN THE ATMOSPHERE

It is well known that, to an extent depending on its angle of in cidence, a ray of light in passing from one medium to another of different density may be bent or refracted from its path. This occurs when the speed of the wave is different in the two media. The dif­ference between the speed in a particular medium and that in free space is usually expressed as a ratio, known as the refractive index of the medium. The atmosphere has a refractive index which changes with height above the Earth, so that light rays seldom travel on straight paths but are slightly though continuously bent. Radio rays are similarly bent, to an extent which depends on their wave­length.

The radar horizon

If a straight line were drawn from a point at a given height above the sea, tangential to the Earth's surface, its point of contact with the Earth would be the geometrical horizon from the point of origin. Since light rays do not travel on straight lines but are slightly bent towards the Earth, the eye at the same height above the surface would see the optical horizon at a somewhat greater distance; it would see slightly "over the curve of the Earth". With radio rays, the bending

being greater, the horizon distance will be greater than in the opti­cal case. With a standard atmosphere, which will be defined later, the radar horizon for 3 cm. waves exceeds the optical horizon by about 6 per cent and the geometric horizon by about 15 per cent.

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1. Radar pulse being radiated Echoes from both buoys returning

2. Echo from Bi has reached scanner just before transmission has ceased

3. Transmission has ceased. Echo from b2 reaches scanner. Pulse-length 0.25 p-sec: 82 yards minimum range 41 yards Fig. 23. Minimum range

This is important because the sum of the radar horizon distanc­es of the aerial and of a target defines the maximum possible dis­tance from which that target could return an echo.

Diffraction

When a^wave is partly obstructed by some object it is diffracted. The effect of diffraction is to bend the wave round behind the ob­struction. The shadow cast by the obstruction will not be complete in the geometrical sense; the width of total shadow will be less, but the width of partial shadowing may be greater.

THE RADAR SET

This description of wave character and behaviour has brought out the fact that the radar transmitter must produce short, high-power pulses of radio-frequency energy on a wavelength of a few centimetres, separated in time by certain minimum intervals; that the aerial or antenna must direct the radio wave in a narrow beam and rotate so as to scan the surface continuously; and that the re­ceiver will have to make possible the use of echo pulses of extremely small power to operate a display or indicator, on which the ranges and bearings of the targets which returned them can be observed simultaneously.