y of (3.11) a signal in the medium The velocity of the signal will be different in different media. Refractivity of the earth's atmosphere has been found to be approximately 1.000 3000 n units. As this number is difficult to use for mathematical calculations, another unit N for radio refractivity is used in propagation theory. As pressure, temperature and humidity all change with height, the refractive index also changes with height, as refractivity varies with pressure, temperature and humidity. This also means that the radio refractivity N also changes with height: (3.12) n 1.000 300 n units (3.13) Hence, N is defined as units (3.14) (3.15) (3.16) where P = pressure (hPa) T = temperature (K) e = water vapour pressure (hPa) Under average conditions, P = 1017 hPa, e = 10 hPa (50 9 % relative humidity) and T = 291.3 K(18 °C). This equation has essentially two components, dry and wet, with the former representing Ndry and the latter Nwet. Ndry has a fairly constant value of about 256 N units, but it is the Nwet term that is responsible for the variability of the equation. When a microwave signal travels in the atmosphere, these electromagnetic waves travel faster in the medium of the lower dielectric constant and the upper part of the wavefront tends to travel with greater velocity than the lower part, causing a downward deflection of the beam. In a homogeneous atmosphere vertical change in the refractivity is gradual; hence the microwave also bends gradually away from the rarer medium towards denser media, thus making the beam bend to follow the earth's curvature, as shown in Figure 3.39. However, the radius of the microwave signal (kR) from the transmitting antenna to the receiving antenna makes an arc whose radius is different from the radius of the earth (R). In a real scenario, both the radio signal and the earth surface have a curvature, which makes the path profile calculations difficult. The possible solution of this problem can be modification of either of the two curvatures in such a way that one of them 