 antenna is limited by the horizon. Given an LOS microwave antenna of hft or h'm above ground surface, the distance dmi or dkm to the horizon just where the drops Terminal Radio repeater Exchange Figure 9.2 A sketch of an LOS microwave radio relay system. Millimeter-wave region is where the wavelength of an equivalent frequency is less than 1 cm (i.e., >30 GHz). CONCEPTS IN TRANSMISSION TRANSPORT ray beam from the transmitting antenna will graze the rounded surface of the horizon can be calculated using one of the formulas given below: To the optical horizon (k = 1): d =V3 (9.1a) and to the radio horizon (k = 4/3): d=V2h, = (9.1b) d' = 2.9(2h')¹/², (9.1c) where k expresses the bending characteristic of the path. These formulas should only be used for rough estimates of distance to the horizon under smooth earth conditions. As we will find out later, the horizon clearance must be something greater (n feet or meters) of grazing. The difference between formulas (9.1a) and (9.1b) and (9.1c) is that formula (9.1a) is "true" line-of-sight and expresses the optical distance. Here the radio ray beam follows a straight line. Under most circumstances the microwave ray beam is bent toward the earth because of characteristics of the atmosphere. This is expressed in formulas (9.1b) and (9.1c), and it assumes the most common bending characteristic. Figure 9.3 is a model that may be used for formulas (9.1). It also shows the difference between the optical distance to the horizon and the radio distance to the horizon. The design of an LOS microwave link involves five basic steps: 1. Setting performance requirements. 2. Site selection and the preparation of a path profile to determine antenna tower heights. 3. Carrying out a path analysis, often called a link budget. Here is where we dimension equipment to meet the performance requirements set in step 1. 4. Physically running a path/site survey. 5. Installation of equipment and test of the system prior to cutting it over to carry traffic. In the following paragraphs we review the