b)]. We recommend that the apertures for LOS microwave antennas not exceed 12 ft (3.7 m). Not only does the cost of the antenna get notably greater as aperture increases over the 12-ft (3.7-m) limit we have set, but the equivalent sail area of the dish starts to have an impact on system design. Wind pressure on large dishes increases tower twist and sway, resulting in movement out of the capture area of the ray beam at the receive antenna. This requires us to stiffen the antenna tower, which can dramatically increases system cost. Also, as aperture increases, gain increases and beamwidth decreases possibly to unmanageable small angles. Other measures we can take to overbuild a link are as follows: Insert a low-noise amplifier (LNA) in front of the receiver-mixer. Improvement: 6-12 dB. Use an HPA (high-power amplifier). Usually a traveling-wave tube (TWT) amplifier; 10-W output. Improvement: 10 dB. Implement FEC (forward error correction). Improvement: 1-5 dB. Involves adding a printed circuit board at each end. It will affect link bandwidth. (See Ref. 3 for description of FEC.) Implement some form of diversity. Space diversity is preferable in many countries. Can be a fairly expensive measure. Improvement: 5-20 dB or more. Diversity is described in Section 9.2.5. 9.2 RADIO SYSTEMS It should be appreciated that fading varies with path length, frequency, climate, and terrain. The rougher the terrain, the more reflections are broken up. Flat terrain, and especially paths over water, tends to increase the incidence of fading. For example, in dry, windy, mountainous areas the multipath fading phenomenon may be nonexistent. hot, humid coastal regions a very high incidence of fading may be expected. 9.2.5 Diversity and Hot-Standby Diversity reception means the simultaneous reception of the same radio signal over two or more paths. Each "path" is handled by a separate receiver chain and then combined by predetection or postdetection combiners in the radio equipment SO that effects of fading are mitigated. The separate di