o acheive a maximum front-to-back ratio than to achieve maximum gain. Polarisation Polarisation is defined as the direction in space along which the electric vector points. The polarisation of an antenna is described by the polarisation of its radiated field. Basically polarisation is of two types, vertical and horizontal, though some antennas are also called dual polarised antennas, i.e. the design of the antenna permits it to transmit in both polarisations. Cross Polarisation Discrimination (XPD) Suppose a transmitting antenna is oriented vertically and a receiving antenna has a similar orientation. Then in the absence of fading the transmitted signal will be completely received at the receiving antenna. However, due to the presence of multipath fading effects and atmospheric fading there is an apparent change in the phase of the signal. The greater the phase angle, the lower is the power received in the vertical component. Therefore, Power in desired orientation (3.8) Power in undesired orientation Parabolic Antenna Gain The most widely used antenna used to design microwave links is the parabolic antenna. The gain of the parabolic antenna is given by G = 17.5297 + 201og(Dm)+20log(FGHz) (3.9) where Dm = antenna diameter (m) FGHz = frequency (GHz) Beam Width The beam width of an antenna is the angular separation between the two half power points on the power density radiation pattern. It is also the separation between the two 3-dB down points on the field strength radiation pattern of the antenna. The term is used more frequently with narrow-beam antennas than with others and refers to the main lobe. Transmission Network Planning and Optimisation Standing Wave Ratio (SWR) The standing wave ratio (SWR) can be defined as the ratio of the maximum current (or voltage) to the minimum current (or voltage) along a transmission line. The SWR is a measure of the mismatch between the load and the line. In the case of perfect matching, the SWR is unity. Antenna Mounting Antennamounting is required for easy alignment of the