hough for compactness small-diameter open-ended waveguides are preferred. The radia- tion patterns of the array are mainly determined by the element spacing and the amplitudes and phases of the signals sent to the individual elements. In addition to being able to form a wide range of multiple or shaped beams, the array has the advantage that the cross- polarization of the total array is lower than that of an individual element. However, the closeness of the array elements gives rise to mutual coupling between the aperture fields, which can distort the radiation patterns (Clarricoats, Tun, & Brown, 1984; Clarricoats, Tun, & Parini, 1984). A significant disadvantage of an array is that a beam-forming network of waveguide components must be used behind the array elements to produce the correct amplitudes and phases for the array. For large arrays this can be heavy, expensive, and significant part of the design of the complete antenna system. Microwave Line-of-Sight Radio A typical microwave radio relay system consists of two axisymmetric parabolic reflector antennas on towers (see Figure 9.14), with a spacing of the order of 50 km 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 formula (equation 9.2). The typical antenna gain is about 43 dBi, which means a diameter of about 3 meters at 6 GHz. 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 polarization operation-typi- cally better than -25 dB within the main beam region over a bandwidth of up to 500 MHz. The VSWR needs to be low (typically 1.06 maximum) in a microwave radio relay system to Reflective Antenna Types Fig. 9.14 Microwave line-of-sight reflector antenna. reduce the magnitu