2. Fundamentals of the use of radar

A marine radar equipment consists of a means of generating ra­dio-frequency oscillations in a particular form; sending them out into space as a narrow beam of radio waves, which is rotated con­tinuously in azimuth; receiving the echoes returned from any target in the area surrounding the ship; and displaying the returned echoes visually on a screen in such a way that the bearing and range of each target from the ship are immediately evident. From this brief de­scription it will be seen that the main parts of a radar are a trans­mitter, an aerial, a receiver, and a display or indicator unit.

The radio wave

Radio waves have characteristics common to other forms of wave motion, so that it is permissible to explain them by comparison with, for example, ocean waves. In those it is obvious that, to an approximation, wave motion consists of a succession of crests and troughs which follow one another at equal intervals and move along at a constant speed. A piece of driftwood will be seen to rise and fall as the waves pass, but, in the absence of wind or current, not to prog­ress in any direction. This shows that the wave, caused originally by some remote disturbance, progresses through the medium (in this case the ocean) at a constant speed but that the medium itself is not permanently displaced. It will not be necessary to quote evi­dence to show that the wave contains energy.

The distance between successive crests is the wavelength (sym­bol X, the Greek lambda). A complete alternation or oscillation from one crest through a trough to the next crest is called a cycle. The number of complete waves which pass a fixed point in a given time is the frequency (symbol f) and may be expressed in cycles per second (c/s). Obviously the number of cycles per second depends upon the wavelength and the speed at which the wave is moving (symbol c) (Fig. 20). A wave of 2-metre wavelength moving at a speed of 10 me­tres per second must oscillate at a frequency of 5 cycles per second. So the expression connecting these factors is:

Speed=FrequencyxWavelength; or c=fxl.

Radio waves belong to the same family as light waves though their wavelengths are much longer, which factor greatly affects their characteristic behaviour. When speaking of them electrical rather than magnetic terms are used, their strength being measured in volts and their power in watts. The energy in them is made apparent by the fact that they induce electrical currents in materials with which they come into contact.

The wavelengths of radio waves which are at present usable lie between about 20,000 metres and 1 centimetre (Fig. 20). Their speed in free space is constant at about 300 million metres or 161, 800 nau­

tical miles per second, and this value is commonly used when consid­ering their passage through the atmosphere. As the speed is con­stant, when the frequency is increased, the wavelength will decrease.