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22 April 2009

Electromagnetic waves and antenna

Radio signals are a form of electromagnetic wave. They are the same type of radiation as light, ultra-violet and infra red rays, differing from them in their wavelength and frequency. Electromagnetic waves have both electric and magnetic components that are inseparable. The planes of these fields are at right angles to one another and to the direction of motion of the wave. The electric field results from the voltage changes occurring in the antenna which is radiating the signal, and the magnetic changes result from the current flow. It is also found that the lines of force in the electric field run along the same axis as the antenna, but spreading out as they move away from it. This electric field is measured in terms of the change of potential over a given distance, e.g. volts per meter, and this is known as the field strength. Similarly when an antenna receives a signal the magnetic changes cause a current flow, and the electric field changes cause the voltage changes on the antenna.

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. 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 meters per second although a more exact value is 299 792 500 meters per second. Although wavelength was used as a measure for signals, frequencies are used exclusively today. It is very easy to relate the frequency and wavelength as they are linked by the speed of light as shown:

lambda = c / f

Where, lambda = the wavelength in meters
f = frequency in Hertz
c = speed of radio waves (light) taken as 300 000 000 meters per second for all practical purposes.

It is also interesting to note that close to the antenna there is also an inductive field the same as that in a transformer. This is not part of the electromagnetic wave, but it can distort measurements close to the antenna. It can also mean that transmitting antennas are more likely to cause interference when they are close to other antennas or wiring that might have the signal induced into it. For receiving antennas they are more susceptible to interference if they are close to house wiring and the like. Fortunately this inductive field falls away fairly rapidly and it is barely detectable at distances beyond about two or three wavelengths from the antenna.