ut spreading out as they move away from it. This electric field is measured in terms of the Magnetic field component Electric field component Direction of travel Figure 2.4 An electromagnetic wave Radio waves and propagation 21 The wavelength is the length from a point on one wave to The best point the identical point on the next to take is usually the peak Figure 2.5 The wavelength of an electromagnetic wave change of potential over a given distance, e.g. volts per metre, and this is known as the field strength. There are a number of properties of a wave. The first is its wavelength. This is the distance between a point on one wave to the identical point on the next as shown in Figure 2.5. One of the most obvious points to choose is the peak as this can be easily identified although any point is acceptable. The second property of the electromagnetic wave is its frequency. This is the number of times a particular point on the wave moves up and down in a given time (normally a second). The unit of frequency is the hertz and it is equal to one cycle per second. This unit is named after the German scientist who discovered radio waves. The frequencies used in radio are usually very high. Accordingly the prefixes kilo, mega, and giga are often seen. 1 kHz is 1000 Hz, 1 MHz is a million hertz, and 1 GHz is a thousand million hertz, i.e. 1000 MHz. Originally the unit of frequency was not given a name and cycles per second (c/s) were used. Some older books may show these units together with their prefixes: kc/s, Mc/s, etc. for higher frequencies. The third major property of the wave is its velocity. Radio waves travel at the same speed as light. For most practical purposes the speed is taken to be 300 000 000 metres per second although a more exact value is 299 792 500 metres per second. Frequency to wavelength conversion Many years ago the position of stations on the radio dial was given in terms of wavelengths. A station might have had a wavelength of 1500 22 Newnes Guide to Radio and Communications Technology metres. 