Weatherproof your antennas

WEATHERPROOF YOUR ANTENNAS

The antenna plays a key role in a satellite communications system. When transmitting, the antenna concentrates radio frequency energy into a very narrow beam, reducing transmitter power required and preventing interference with adjacent satellites. During reception, it rejects signals from adjacent satellites and simplifies the receiver.

After installation, any deterioration in antenna performance produces dire consequences. Signal quality deterioration could make the earth station unusable. Worse yet, interference with other satellites or services could result in an order by the FCC to cease earth station operation.

Antennas for this application have two primary components: The reflector collects and directs energy toward its focus. The feed horn, located at the reflector focus, collects the concentrated energy and couples it to a waveguide.

The waveguide functions as a low-loss transmission line equivalent to a coaxial cable. The far end of the waveguide conencts the microwave transmitter and receiver components to the antenna.

Satellite communications started nearly 30 years ago. Originally, it supplemented terrestrial communications.

As technology improved, satellite communications evolved to its current utility status. Hub eart stations routinely achieve availabilities of 99.9% or better. This allows only seven hours of downtime each year.

Water, in any form, interferes with the normal operation of the antenna system. It absorbs radio frequency energy. Although not usually a critical problem at C band, it is at Ku and Ka band.

Water is a dielectric medium. All forms of precipitation, independent of density, have dielectric constants much greater than air.

Radio waves travel through water at lower velocity than through air. This causes bending and reflection of radio waves at the interface between air and water.

Reduced Efficiency

Accumulated snow or ice on antenna components reduces their efficiency. It attenuates both transmitted and received signals and alters antenna directionality in an unpredictable way. The weight of the snow and ice distort antenna reflector and structure, further reducing efficiency.

Snow and ice problems extend from top to near the bottom of the U.S. Icing during winter months occurs even in central Florida and the Gulf Coast. The mid-South frequently experiences freezing rain in winter.

Unlike snow, ice from freezing rain requires other than mechanical removal.

Achieving earth station availabilities well over 90% requires antenna de-icing.

Practical de-icing systems use heat for melting ice and snow. Most systems employ electric heaters for reasons of economy and reliability. Occasionally city gas, LNG, or propane offer advantages. Typical power densities range between 20 and 60 watts per square foot of reflector area. Exceptional applications may require higher densities.

De-icing requires heating both the reflector and the feed horn. some antenna designs mount the feed horn behind the reflector.

In this case, a sub-reflector mounted short of reflector focus directs energy to the feed horn. The sub-reflector also requires de-icing heaters. Feed horn heaters operate with those on the reflector.

When installed and operating, the de-icing heaters must not degrade antenna performance. Uniform reflector heating minimizes mechanical distortion, thus preserving antenna performance.

The heaters must provide a power density adequate for the climate. The de-icing heater system's weight and installation must not disturb the antenna alignment of limit antenna movements in azimuth and elevation. Well engineered de-icing systems satisfy these needs.

Packaged De-Icing

Antenna manufacturers and third parties provide packaged de-icing systems offering heat by conduction, convection, or radiation.

Conduction systems bond heaters directly to the reflector. These employ either constant-wattage of self-regulating heaters. The self-regulating heater's output power varies with thermal load (i.e., snow load, ambient temperature, etc.). This saves energy and provides extra power at low temperatures and under windy conditions when needed. Support structures attached to the rear of the reflector limit the application of conduction heaters.

Although more costly, self-regulating heaters provide better reliability than do constant-wattage units.

After heater installation, foam insulation applied to the rear of the reflector improves thermal efficiency by minimizing heat losses.

Convection, often referred to as "hot air" systems, can provide uniform heating. Although more complex than either conduction or radiation systems, this technique owes its popularity to low acquisition cost. Convection systems generally employ a doubled walled refector assembly. Constant-wattage heaters in conjuction with blower provide heated air. Introducing the heated air between the reflector walls accomplishes de-icing. Employing an insulating material for the rear wall improves thermal efficiency.

A radiation system looks similar to a typical convection system. It employs a self-regulating heater as a long-wavelength infrared generator.

The rear cover combines the functions of infrared reflector and insulator.

This improves themal efficiency.

The self-regulating heater adapts to thermal load, thus reducing de-icing energy costs. The advantages of this system include simplicity, reliability, and uniform heating

Antenna de-icing systems consume large amounts of electric power.

For example, a typical 1.8-meter small-aperture antenna requires 2200 watts, nominally.

COPYRIGHT 1990 Nelson Publishing
COPYRIGHT 2004 Gale Group