Microwave

Lore & Background

The prefix 'micro-' in microwave indicates that these waves are small (having shorter wavelengths) compared to earlier radio technology. The boundaries between far infrared, terahertz radiation, microwaves, and ultra-high-frequency (UHF) are fairly arbitrary and differ between fields. Frequencies in the microwave range are often referred to by IEEE radar band designations such as S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

Frequency bands

The letter system originated in World War II with a top-secret U.S. classification of bands used in radar sets, which became the IEEE radar bands. The term P band is sometimes used for UHF frequencies below the L band but is now obsolete per IEEE Std 521. When radars were first developed at K band during World War II, it was not known that there was a nearby absorption band due to water vapor and oxygen; to avoid this, the original K band was split into Ku and Ka bands.

Propagation

Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves, they do not travel as ground waves or reflect off the ionosphere. On the Earth's surface, microwave communication links are limited by the visual horizon to about 30 to 40 miles.

At the high end of the band, microwaves are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer. Beginning at about 40 GHz, atmospheric gases begin to absorb microwaves, and above 100 GHz the atmosphere is effectively opaque until the infrared and optical windows. A technique called tropospheric scatter (troposcatter) allows communication beyond the horizon at distances up to 300 km using frequencies between 0.45 and 5 GHz.

Antennas

At microwave frequencies, transmission lines such as coaxial cable have excessive power losses, so waveguides are used to carry microwaves when low attenuation is required. Due to the high cost and maintenance of waveguide runs, in many microwave antennas the transmitter output stage or receiver front end is located at the antenna.

The short wavelengths allow omnidirectional antennas for portable devices to be very small (1 to 20 cm), and also allow narrow beams from conveniently small high-gain antennas (0.5 to 5 meters in diameter). Parabolic dish antennas are the most widely used directive antennas, but horn, slot, lens, and flat microstrip antennas are also used. Phased arrays produce electronically steerable beams.

Reader's Guide

Microwaves have become indispensable across numerous fields of technology. Their ability to carry large amounts of information over line-of-sight paths makes them the backbone of point-to-point communication links, microwave radio relay networks, and satellite and spacecraft communication. The short wavelengths enable compact antennas for portable devices such as cell phones, cordless phones, wireless LANs (Wi-Fi), and Bluetooth earphones, while also allowing narrow, high-gain beams for radar and long-distance links. The narrow beams permit frequency reuse by nearby transmitters without interference.

In medicine, microwaves are used for diathermy and cancer treatment. In industry, they serve for heating, remote sensing, and spectroscopy. Microwave ovens use them for cooking food.

The atmospheric absorption at higher frequencies, while limiting communication range, also creates spectral bands that influence system design. The legacy of microwave technology includes the development of specialized vacuum tubes for high-power sources, such as the Gunn diode used in radar speed guns. The design and analysis of microwave circuits require distributed-element and transmission-line theory because wavelengths are comparable to circuit dimensions, leading to the use of waveguides, stripline, cavity resonators, and resonant stubs instead of discrete components. At even higher frequencies, optical methods replace microwave techniques.