ections. The need to have a high front-to-back ratio means that either a low edge typical microwave radio relay system consists of two axi-symme- illumination must be used or baffles and shields must be used. The tric parabolic reflector antennas on towers, Figure 17.12, with a latter methods are preferable, but increase the weight and cost. The spacing of the order of 50km apart in a line of sight path. The most common feed is a modified TE, 11 circular waveguide, which relationship between the transmitted and received powers and the is designed to have a low VSWR and good pattern symmetry. antenna and path parameters is given by the Friis transmission Sometimes operation in two frequency bands is needed in which formulae, Equation 17.2. The typical antenna gain is about 43dBi case the feed must combine two waveguides and operate at the two which means a diameter of about 3 metres at 6GHz. frequencies. The VSWR can be reduced by replacing the centre In addition to the pattern envelope specifications (which must be portion of the paraboloid with a flat plate, called a vertex plate. This low because two or more antennas are normally mounted next to minimises the VSWR contribution from the dish although it also each other on a tower), there are a number of other important criteria degrades the near-in sidelobes and reduces the gain. Antennas used in communication systems 17/13 Earth's atmosphere. A minimum isotropic background radiation of about 3K is always seen by any antenna. The value of the other VENTEX or factors depends on frequency. The spillover noise is the only com- PARABOLOIU ponent under the control of the antenna designer. Its value can be reduced by designing an antenna with very low sidelobes. The receiver noise is normally the dominant noise factor. It depends on the method of amplification and detection. Early earth stations all PAHABOLIC AXIS used cooled receivers which have low noise temperatures. Modern earth stations use uncooled receivers which are dependent on the noise performance of the fr