Micro wave link

Most of the link uses 4 and 6 GHz. Short term fading in microwave communication links can be overcome by diversity reception and transmission.Ionosphere propagation is not possible in micro wave communication because it penetrate through ionosphere. In microwave communication links, the rain drop attenuation experienced is mainly due to absorption of microwave energy by water vapour. Rainfall is an important factor for fading in frequencies above 10 GHZ. Because of high loss coaxial cable is not used for micro wave transmission. wavelength of a wave in a waveguide is greater than that in free space.

Repeaters are placed in 48 km intervals. Frequency difference between transmitted and received signals are few hundred MHz. Schottky diode is used as the mixer in the receiver because of very low noise. Output of IF section is send to amplitude limiter to prevent spurious amplitude modulation, output of limiter is feed to varactor diode for mixing. Mostly VHF transistor crystal oscillator with varactor multiplier is used as source of micro wave

Repeater requires 200 to 300 watts power. Height of microwave towers are up to 75 meters. In Microwave take the elements as Distributed circuit elements. Intrinsic impedance of free space is 377 ohm or 120. Free space wave propagation is represented by Helmholtz equation

When a wave travels 1/a distance, the amplitude of the wave falls by 1/e times then 1/a is called skin depth or depth of penetration. Skin depth is very very small of the order of a few mm. Poynting vector shows the power per unit area in the wave which is carried in the direction of propagation