Super high frequency

Lore & Background

Microwaves in the SHF band propagate solely by line of sight; groundwave and ionospheric reflection do not occur due to the short wavelength. Unobstructed rights of way cleared to the first Fresnel zone are usually required, though in some cases building walls can be penetrated enough for useful reception. The small wavelength allows antennas to be much larger than a wavelength, enabling highly directional high-gain antennas that produce narrow beams, used in point-to-point terrestrial links limited by the visual horizon to 30–40 miles (48–64 km). Such high gain antennas also permit frequency reuse by nearby transmitters.

SHF waves create strong reflections from metal objects the size of automobiles, aircraft, and ships, and the narrow beamwidths possible with high gain antennas, combined with low atmospheric attenuation compared to higher frequencies, make SHF the main frequencies used in radar. Attenuation and scattering by moisture in the atmosphere increase with frequency, limiting the use of high SHF frequencies for long range applications. Small amounts of microwave energy are randomly scattered by water vapor molecules in the troposphere, a phenomenon used in troposcatter communications systems operating at a few GHz to communicate beyond the horizon, achieving distances of 300 km, mainly for military communication.

At SHF frequencies, efficient transmitting antennas are small enough to be mounted on handheld devices. Omnidirectional antennas such as the printed inverted F antenna (PIFA), sleeve dipoles, and quarter-wave monopoles are used. Directive antennas are mostly aperture types: parabolic antennas, lens, slot, and horn antennas. Large parabolic antennas can produce very narrow beams of a few degrees or less. Another type is the phased array, consisting of many dipoles or patch antennas on a flat surface, each fed through a phase shifter to steer the beam electronically. Because coaxial cable has high power losses at these frequencies, waveguide—a special metal pipe—is used to transport microwaves with low losses.

Reader's Guide

SHF frequencies occupy a 'sweet spot' in the radio spectrum exploited by many new radio services. They are the lowest frequency band where radio waves can be directed in narrow beams by conveniently sized antennas, allowing frequency reuse without interfering with nearby transmitters on the same frequency. At the same time, they are the highest frequencies usable for long distance terrestrial communication; higher frequencies in the EHF (millimeter wave) band are highly absorbed by the atmosphere, limiting practical propagation distances to one kilometer or less. The high frequency gives microwave communication links a very large information-carrying capacity (bandwidth). In recent decades many new solid state sources of microwave energy have been developed, and microwave integrated circuits for the first time allow significant signal processing to be done at these frequencies. Sources of EHF energy are much more limited and in an earlier state of development. The SHF band is used for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, satellite phones (S band), and numerous short range terrestrial data links. It is also used for heating in industrial microwave heating, medical diathermy, microwave hyperthermy to treat cancer, and to cook food in microwave ovens.

Did You Know?

• SHF wavelengths range from one to ten centimetres, giving it the name centimetre band.
• Microwaves in SHF propagate solely by line of sight; groundwave and skywave propagation do not occur.
• Troposcatter systems using SHF can communicate beyond the horizon up to 300 km.
• The printed inverted F antenna (PIFA) is a common omnidirectional antenna for SHF wireless devices.