Microwave and Radio - Based Systems

Microwaves are radio waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF, and various sources use different boundaries. In all cases, microwave includes the entire SHF band at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Why use microwaves?
Communication using electromagnetic radiation (except for light) began early in this century, and most early practical systems used very long wavelengths (low frequencies) which traveled great distances. Eventually, electronics were developed, including the vacuum tube (or "valve") which allowed controlled frequencies and modulation. This led to the use of higher frequencies, many channels, and commercial and industrial radio. During the 1930's and 1940's various experimenters discovered that higher frequencies could bring other advantages to communications. Some of these experimenters were government agencies and the military - some were universities, and some were private individuals.

Among these discoveries were that microwaves are easier to control (than longer wavelengths) because small antennas could direct the waves very well. One advantage of such control is that the energy could be easily confined to a tight beam (expressed as narrow beamwidth). This beam could be focused on another antenna dozens of miles away, making it very difficult for someone to intercept the conversation. Another characteristic is that because of their high frequency, greater amounts of information could be put on them (expressed as increased modulation bandwidth). Both of these advantages (narow beamwidth and modulation bandwidth) make microwaves very useful for RADAR as well as communications.

Eventually, these qualities led to the use of microwaves by the telephone companies. They placed towers every 30 to 60 miles each with antennas, receivers and transmitters. These would relay hundreds or even thousands of voice conversations across the country. The ability to modulate with a wide bandwidth permitted so many conversations on just one signal, and the reduction in beamwidth made this reasonably secure. In the 1950s experiments were conducted that showed the potential to connect the two coasts of the US via these microwave circuits to produce television programming on a continental basis, and true television networks were born.

Amateur radio interests in microwaves have mostly been for the challenge of working with such esoteric frequencies that require specialized techniques in design, fabrication and testing. Furthermore, in order to reach beyond LOS (line-of-sight) amateurs have spent countless hours carefully measuring propagation phenomena. Amateurs have carried on conversations using 10GHz well over 1,000 miles, and have bounced signals at that frequency off the moon. For more information about amateur radio uses of microwaves set your browser to www.wa1mba.org, contact a local VHF/Microwave Amateur radio club, or contact the ARRL.

Early Microwaves
Surprisingly enough, some of the first electromagnetic experiments conducted by Heinrich Hertz in 1886 and also by Marconi used frequencies near the microwave region - some around 500 MHz and some even in the multiple GHz (Gigahertz) region. By the way, the current unit frequency "Hz" is the "Hertz" named after Heinrich. GHz is pronounced "Giga Hertz". There were some very interesting experiments conducted in 1895 with frequencies/wavelengths that are even today considered a challenge. For a reference to this work see http://www.tuc.nrao.edu/~demerson/bose/bose.html