ing directional pattern and beamwidth. It has already been mentioned that the impedance of the antenna changes as the frequency changes. In turn this causes the match between a signal source and the antenna to change. When there is a poor match between the feeder used to supply the power to the antenna and the antenna, power is reflected back along the feeder. Alternatively when used for receiving, the transfer of power to the feeder will be inefficient. In both instances the efficiency is reduced although when transmitting high levels of reflected power can give rise to the possibility of damage. As a result a bandwidth dependent upon the impedance is often used for transmitting. This is usually expressed in terms of the standing wave ratio (SWR) that is explained later. For example, an antenna may operate between 55 and 60 MHz for an SWR of less than 2:1. Changes in frequency mean that the operating point moves away from or towards the resonant frequency. With the changing values of inductive or capacitive reactance phases or current in the antenna can change and this can result in changes to the radiation pattern. Aspects like the beam width and particularly the front to back ratio are affected. For many antennas the forward gain is the main requirement and the radiation pattern bandwidth is defined as the frequency range over which the gain of the main lobe is within 1 dB of the maximum. Angle of radiation In many applications, another aspect associated with directivity called the angle of radiation is important. Essentially this is the radiation pattern in the vertical plane with respect to the ground. The actual angle measured by taking the angle between the ground and the centre of the main lobe of the signal being radiated from the antenna. It is important because for many applications especially at very high frequencies and above where near line of sight communications are used. Here the power Antenna Direction of the maximum radiation Angle of radiation Earth's surface Figure 4.6 Angle of radiation 84 N