Direct-Wave Propagation

Point-to-point transmission, as from one microwave radio-relay station (or repeater station) to another in a superhigh-frequency radio communication system such as exists from New York to Boston (page 527), is not exactly direct-wave propagation, but it approaches it.

Methods are available²⁸ for calculating the transmission to be expected at ultrahigh and superhigh frequencies. Many factors must be considered such as frequency, types, and heights of antennas, terrain including trees, hills, buildings, etc.

In the New York-Boston system, operating at about 4000 megacycles and using highly directional antennas located about 30 miles apart on hills, it has been found that ground reflection and variable atmospheric refraction affect transmission.³⁰ However, it is reported that it is possible during non-fading periods to obtain essentially free-space conditions.

Point-to-point radio transmission at ultrahigh (300 to 3000 megacycles, 100 to 10 centimeters) and superhigh (3000 to 30,000 megacycles, 10 to 1 centimeters) frequencies is possible beyond the optical, or line-of-sight, path, that is, beyond the horizon. This is due in a large measure to atmospheric refraction and diffraction. The "horizon" for the radio path, or the maximum possible "direct-ray" path is

where d is the distance in miles, and h_t and h_r are the heights in feet above the surface of the earth of the transmitting and receiving antennas.

Atmospheric "ducts" as they are called are caused by refraction in the troposphere and reflect superhigh-frequency radio waves back to the earth, where they are reflected back to the refracting region, and so on. Thus, radio waves are "trapped" within the duct, and peculiar transmission phenomena occur.³¹ Transmission at these frequencies is also affected quite adversely by rain, particularly at wavelengths below 5 centimeters.^32,33