ch a way that cable lengths are as small as possible to reduce losses. Circular Waveguides These have the lowest loss of all the waveguides and can support two orthogonal polarisations in one guide. They can carry more than one frequency band in the same guide; e.g. WC-281, which is used with the horn reflector antenna, can provide two polarisations at 4 GHz and 6 GHz. However, these circular waveguides have the following disadvantages: Practically they can only be used for small runs. They can have problems related to waveguide modes. Microwave Link Planning Figure 3.37 Feeder connected to the antenna Elliptical Waveguides Semi-flexible elliptical waveguides are available in sizes comparable to most of the standard rectangular waveguides. The most distinctive feature is that it can be installed as a single continuous run with no intermediate flanges. It provides good voltage standing wave ratio (VSWR) performances, but relatively small deformations can introduce enough impedance mismatches to produce severe distortions. A couple of examples can be: 11 GHz: EW-107 (loss: 3.70 dB/100 inch) 12-13 GHz: EW-37 (loss: 4.50 dB/100 inch) Due to their characteristics, elliptical waveguides are usually used in microwave links. However, they are more expensive than rectangular waveguides. Calculation of Feeder Loss Consider a microwave cable connected from the IU to the OU as shown in Figure 3.37. The feeder loss in this case can be calculated as Feeder loss Lcon (3.10) where a = loss (dB/m) Lhor = horizontal length of the feeder Lver = vertical length of the feeder Lcon = connector losses The size and attenuation of the waveguides are determined by the frequency band: the higher the frequency, the smaller the waveguide and the higher its attenuation for each unit length. The number of bends, twists and flexible sections should be kept to a minimum with great care taken during the installation. Transmission Network Planning and Optimisation (a) Pole (b) Guyed Tower (c) Metallic self-supporting Tower Figure 3.38 Different ty