Einstein College of Engineering
The existence of electromagnetic waves, of which microwaves are part of the frequency spectrum, was
predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to
demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected
microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a
horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic crosssection worked as a reflection antenna. In 1894 J. C. Bose publicly demonstrated radio control of a bell
using millimetre wavelengths, and conducted research into the propagation of microwaves.
Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire gigahertz
range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)
The microwave range includes ultra-high frequency (UHF) (0.3–3 GHz), super high frequency (SHF)
(3–30 GHz), and extremely high frequency (EHF) (30–300 GHz) signals.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is
effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical
window frequency ranges.
Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence
of controlling electric or magnetic fields, and include the magnetron, klystron, travelling wave tube
(TWT), and gyrotron. These devices work in the density modulated mode, rather than the current
modulated mode. This means that they work on the basis of clumps of electrons flying ballistically
through them, rather than using a continuous stream.
A maser is a device similar to a laser, except that it works at microwave frequencies.
Solid-state sources include the field-effect transistor, at least at lower frequencies, tunnel diodes and
Before the advent of fiber optic transmission, most long distance telephone calls were carried via
microwave point-to-point links through sites like the AT&T Long Lines. Starting in the early
1950's, frequency division multiplex was used to send up to 5,400 telephone channels on each
microwave radio channel, with as many as ten radio channels combined into one antenna for the
hop to the next site, up to 70 km away.