U.H.F. Propagation

Author: J.B. Hoag

For frequencies above approximately 30 MHz, the energy is transmitted in a direct ray through the atmosphere. For greatest field strength there must be no obstruction in the path of the waves. The ground proves to be a fairly good reflector for these ultra-high frequencies, so that the received signal is a composite of the direct and a reflected ray. The effect of the latter is to weaken the signal because the two waves are, in general, slightly out of phase.

The waves can travel somewhat beyond the bulge of the earth because of a small amount of refraction in the atmosphere.

Fig. 9 E. Line of sight distance for u.h.f. transmission from antennas of various heights

The solid line in the graph of Fig. 9 E gives the line-of-sight distances for antennas of different heights, after allowing for atmospheric refraction (d = 1.41 √h). Distances are read off the chart (solid line) using first the height of the transmitting antenna, then that of the receiving antenna. The total distance will be the sum of the separate values.

The range of transmission of u.h.f. waves sometimes exceeds that of the direct ray just discussed because of an increased refraction in the troposphere or lower atmosphere caused by a " temperature inversion." This means that a layer of warm air exists above cooler air near the ground, a condition which is fairly common in the summertime. U.H.F. transmissions over distances of 3,000 miles have sometimes been observed, while sporadic ranges of several hundred miles are common.