One of the scarcest commodities in this country today is space in the radio spectrum. The increasing requirement for broadcasting services, public and private communications, aircraft guidance, military applications and a thousand other uses of the radio spectrum has created a demand for available frequencies that far exceeds the supply. This over population of the radio spectrum has caused many to look toward the new frontier as an area for expansion of the useful radio spectrum.
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The part of the spectrum, known as extremely high frequencies (EHF), forms such a frontier. Its bounds are from one millimeter wave-length to one centimeter wavelength with corresponding frequencies of 300,000 megacycles per second and 30,000 megacycles per second. This frequency range of 270,000 megacycles is nine times that of all the other bands combined. It is no wonder, then, that millimeter radio waves are being investigated more and more frequently for their application potential.
Millimeter radio waves, however, do not present a panacea for all of the radio requirements, but they, like most frontiers, have their hardships which may be quite discouraging to the explorer. It is the purpose of this paper to describe the unique characteristics of millimeter radio waves with the hope that it will be useful in evaluating their adaptation for specific purposes.
The two features of millimeter radio waves which separate them form their longer wavelength cousins are that they are in a frequency region where they suffer absorption by atmospheric gases and that their very short wavelength makes them have many of the characteristics of light waves. A successful use of EHF will be one for which these characteristics will be an advantage or one for which they will present little disadvantage.
Absorption of radio waves by the gases in the atmosphere is almost non-existent for wavelengths longer than a few centimeters. Furthermore, optical wavelengths are also essentially free of this loss factor. The short centimeter, the millimeter and the infrared frequencies are, however, subject to this loss which may be very minor at certain frequencies and very severe at others.
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