sed bandwidth over a basic dipole. This is of great advantage when the antenna needs to be used over a band of frequencies as in the case of VHF FM or UHF television broadcasts. Half wavelength Figure 4.18 A folded dipole 98 Newnes Guide to Radio and Communications Technology 20 000 10 000 40 000 a:d ratio Figure 4.19 Length factor of an antenna plotted against the wavelength, element diameter ratio The length of the dipole is of prime importance because it determines the resonant frequency. This can be determined quite easily although it is not quite the same as a half wavelength in free space. Instead it is slightly shorter. This is due to a number of effects including one referred to as the end effect. This is largely dependent upon the length to diameter ratio of the wire or conductor that is used. In most cases the reduction in length is around 5 per cent, although it is possible to determine a more exact value from the graph in Figure 4.19. From a knowledge of the length reduction factor it is possible to calculate the antenna length in either metres or inches: A x 150 length (metres) = A X 5905 length (inches) = where f is the frequency of operation in MHz. When making a first version of an antenna it is always wise to cut the elements slightly longer than the calculated length. This enables the length to be reduced when making alterations to reach the optimum performance. It is always more difficult to add wire or metal that has been previously removed. Antenna systems 99 End fed wire One of the easiest types of antenna to install for short wave applications is an end fed wire. Often misnamed a long wire, this type of antenna can be erected very easily and can be used on a variety of frequencies. As such these antennas are used in a number of applications including ships, aircraft as well as being very convenient for short wave listeners and radio amateurs. Insulators Securing nylon Pulley Antenna Receiver Nylon Antenna tuning unit Earth system Figure 4.20 An end fed wire antenna system The antenna is nor