mally made to be a quarter wavelength or longer at the lowest frequency of operation, but to give a good match it is necessary to use the antenna with a matching unit. It also needs to be operated with a good earth system. A poor earth will reduce the efficiency of the system. The directional pattern of the system will depend upon the length of the antenna. A quarter wavelength system will have its maximum sensitivity at right angles to the axis of the wire. Longer ones will have lobes which move progressively towards the axis of the wire itself. If the antenna is made several wavelengths long it becomes what is known as an 'end fire' antenna where the main lobes are almost in line with the axis of the wire. This is what a true long wire is. Phased arrays It has already been mentioned that phasing techniques can be used to create a directive antenna. By changing the phase between two driven 100 Newnes Guide to Radio and Communications Technology Direction Direction of minimum of maximum radiation radiation Feeder Direction Direction of maximum of maximum radiation radiation Feeder Figure 4.21 Phased antennas using two dipoles elements the signal can be cancelled out or reinforced in various directions. There is a wide variety of possibilities that can be created, but to demonstrate the principle take the simple example of two dipoles spaced a quarter wavelength apart. If they are fed 90 degrees out of phase then the result is that the signals from the two antennas are in phase in one direction and they cancel because they are out of phase in the other. A bi-directional pattern can be obtained by feeding the antennas in phase with one another and spacing them one wavelength apart. In this way the signals from both antennas reinforce one another as shown in the diagram. Antenna systems 101 Directions of maximum radiation Feeder Figure 4.22 A broadside array Another type of phased antenna is known as a broadside array. Here the antenna is arranged in such a way that the maximum radiation occurs in a plane perpendicu