During World War II, RF and microwave systems proved to be of major importance for both communication and radar. Consequently, intensive research was conducted in this field, resulting in major breakthroughs. The introduction of novel communication and sensing equipment continued after the war for both military and civilian applications. As a result, the frequency spectrum up to 10 to 20 GHz became increasingly crowded, thus limiting the introduction of new and advanced applications.
Ultimately, discussions came up quite early on the use of frequencies beyond 30 GHz, allowing huge bandwidths, but meeting limitations in other respects, as was discussed by W. Straiton and C.W Tolbert in “Millimeters—The New Frontier,”1 originally published in Microwave Journal in 1960 and reprinted in this issue. A major limitation was the fact that the absorption of electromagnetic waves by water vapor and oxygen results in a general increase of atmospheric attenuation and reduces the range of systems at mm-wave frequencies.
The first maxima of the atmospheric attenuation exists around 23 and 60 GHz. Rain leads to an increased scattering of waves as the wavelength reaches the range of raindrop dimensions, resulting in further attenuation. In addition, for radar, reflections by raindrops led to unwanted clutter. However, compared to optical systems, fog has a much lower impact on mm-waves. Another major advantage of increased frequencies is the fact that, for a required antenna gain or beam width, the antenna area scales down with the square of the wavelengths.
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