tenna that radiates equally in all directions. It can be calculated that a dipole has a gain of 2.1 dB over an isotropic source. When any antennas are quoted against an isotropic source the gain is measured against a dipole and then 2.1 dB is added. In view of the different gain figures obtained against the two standard antennas it is necessary to state what the gain of the antenna is being compared against in any specification. To achieve this the gain of an antenna over a dipole is quoted as a certain number of dBd (dB gain over a dipole). Similarly figures against an isotropic source are quoted as dBi (dB gain over an isotropic source). The other factor that is important for antennas exhibiting gain is the width of the beam or beamwidth. This is the angle between the two Antenna systems 81 Beamwidth Front to back ratio = 1/1/ Figure 4.4 The polar diagram of a typical directional beam antenna showing the beamwidth and the front to back ratio points on the polar diagram where the intensity falls by 3 dB from the maximum. In view of the fact that the measurement points are the -3 dB points, i.e. the half power points, it is sometimes called the half power beamwidth. As would be expected the higher the gain the narrower the beamwidth. Gain is very important in antennas for two reasons. One is the fact that the antenna gain enables weaker signals to be received, or the transmitted power to be used more effectively giving a stronger signal where it is required. The second is the fact that the limited beamwidth means that when receiving, sources of interference can be reduced if they are coming from a different direction to the wanted signal. Similarly when transmit- ting it can be used to reduce the level of signal travelling in unwanted directions thereby reducing interference caused to other stations. Another factor that is important in some applications is the front to back ratio of an antenna. This is the ratio of the maximum signal in the forward direction to the signal in the opposite direction. This figure is