Microwave line of sight radio

A typical microwave radio relay system consists of two axi-symmetric parabolic reflector antennas on towers, Figure 17.12, with a spacing of the order of 50km apart in a line of sight path. The relationship between the transmitted and received powers and the antenna and path parameters is given by the Friis transmission formulae, Equation 17.2. The typical antenna gain is about 43dBi which means a diameter of about 3 metres at 6GHz.

In addition to the pattern envelope specifications (which must be low because two or more antennas are normally mounted next to each other on a tower), there are a number of other important criteria for microwave radio antennas. The front-to-back ratio must be high, and the cross-polar discrimination needs to be high for dual polari­sation operation, typically better than -25dB within the main beam region over a bandwidth of up to 500MHz. The VSWR needs to be low (typically 1.()6 maximum) in a microwave radio relay system in order to reduce the magnitude of the round trip echo. The supporting structure must be stable to ensure that the reflector does not move significantly in high winds. The reflectors must operate under all weather conditions, which means that a radome is often required. This poses extra design problems because inevitably it degrades the electrical performance. A long waveguide or coaxial cable feeder must be provided from the transmitter to the antennas. Not only must this be low loss but it must also be well made so that there is no possibility of loose joins introducing non-linear effects. Finally the cost must be relatively low because a large number of reflectors are required in a microwave communication system.

The majority of antennas in use are prime focus symmetric reflec­tors often with shields and radomes. The design of the prime focus reflectors follows the procedure discussed in earlier sections. The need to have a high front-to-back ratio means that either a low edge illumination must be used or baffles and shields must be used. The latter methods are preferable, but increase the weight and cost. The most common feed is a modified ТЕ circular waveguide, which is designed to have a low VSWR and good pattern symmetry. Sometimes operation in two frequency bands is needed in which case the feed must combine two waveguides and operate at the two frequencies. The VSWR can be reduced by replacing the centre portion of the paraboloid with a flat plate, called a vertex plate. This minimises the VSWR contribution from the dish although it also degrades the near-in sidelobes and reduces the gain.