des a measure of how the array behaves towards Balanced unbalanced interfering signals arriving from different directions. antenna transformer It is particularly useful in confined areas such as cities Unbalanced where interfering signals 'bounce' off high buildings and interfere with a strong desired signal. In such Balanced cases, it is often better to select an antenna with transmission line a large front to back ratio to provide rejection to the interfering signal than trying to get maximum an- tenna gain. Fig. 2.1-21 Balanced antenna system and (a) balanced distribu- 'Acceptance angle' is the beamwidth angle in degrees tion system; (b) unbalanced distribution system. where antenna gain remains within 3 dB of its stated maximum gain. An acceptance angle of 20° and should refer to these figures while you are reading the a maximum array gain of 10 dB means that for any descriptions given below. signal arriving within +10° of the maximum gain A balanced antenna (Figures 2.1-21(a) and (b)) is an direction the antenna will provide at least (10-3) dB, aerial which has neither conductor connected directly to i.e. 7 dB of gain. However, you should be aware that earth; it is balanced because the impedance between the acceptance angle itself is not accurate and that it earth and each conductor is the same. A folded dipole is can vary by 3° as well. a typical example of a balanced antenna because the impedance from each end of the antenna to earth is equal The values given in the table are representative of the and balanced. An unbalanced antenna (Figures 2.1-22(a) middle range of commercially available Yagi arrays. The and (b)) is an aerial which has one of its conductors figures quoted above have been measured by manufacturers connected directly to earth. The impedance between under ideal laboratory conditions and proper installation is earth and each conductor is not the same. A monopole essential if the specification is to be achieved in practice. aerial is a typical example of an unbalanced aerial because its other