Chapter 2 - Microwave Propagation

Abstract

Microwave signals are electromagnetic (EM) waves that propagate at a velocity of 3 x 108 m/s in free space. Polarization of an EM wave is the direction of the electric field vector. The magnetic field vector is always orthogonal to (90° from) the electric field vector. EM waves exhibit four major propagation paths: surface wave, space wave, tropospheric wave, and ionospheric wave. The space wave has two components: direct and reflected. Ground wave communications are affected by wavelength, height of both antennas, distance between antennas, terrain, and weather along the transmission path. When both direct and reflected space wave signals reach the receiver antenna, either a constructive or destructive interference will be exhibited. Tropospheric propagation occurs mostly by refraction and takes place 4 to 7 miles above the surface. Various tropospheric propagation modalities are recognized as simple refraction, superrefraction, ducting, and subrefraction. Because of temperature and humidity dependencies, tropospheric propagation phenomena show diurnal variation of up to 20 dB. Ionospheric scatter occurs when regions are heavily ionized locally and reflect signals that propagate to the outer space. Ionization caused by solar radiation and charged particles can increase the microwave background noise level.