Characteristics of beyond-the-horizon radio transmission

During the past five years, it has been definitely established that useful radio signals at all frequencies can be received consistently at distances far beyond the horizon. These facts have forced a considerable modification of the theories, concepts and charts found in most textbooks and handbooks. Even more important, these facts have opened up many interesting radio possibilities, particularly in the field of fixed point-to-point communication. This relatively long distance transmission has been called by various names, such as, beyond-the-horizon or extended range transmission, but perhaps it is most generally known by the term, "scatter". It is important to note that the word "scatter" is used in a specialized sense which is actually closer to the layman's concept of reflection than it is to the opposite concept of scattering in all directions. More recently, the term, "forward scatter" has been used in an attempt to minimize this language difficulty. There are two different types of beyond horizon or scatter transmission. One type is the ionospheric scatter which is useful for telegraph signals at frequencies below about 50 mc for distances up to a thousand miles or more. The second type is tropospheric transmission which is useful over a very wide band of frequencies but is limited for wide band transmission to distances of a few hundred miles. A review of the principal characteristics of tropospheric transmission shows that the differences between line-of-sight and beyond-the-horizon transmission are not as great as has been commonly believed. Until a few years ago, the general expectations were that, first, the beyond horizon signal would not be reliable, second, it could not be used effectively because antennas would not realize their free space gain, and, third, the useful bandwidth would be very small. All of these doubts had a logical basis and contained an element of truth. However, these fears have been exaggerated. It is now clear that reliable point-to-point radio circuits can be engineered for path lengths of 150 miles or more to provide good quality multichannel voice circuits or possibly even television for radio relay purposes. Such circuits do require high power and large antennas but they are technically feasible. It is expected that the use of beyond horizon circuits will supplement but not supplant line of sight systems. The principal applications will probably be for point to point circuits over difficult terrain.