GLENAIR
ANTENNAS
Satellite antennas, combined with transmitters, receivers, and transponders turn orbiting satellites into radio relay stations whose primary mission is the uplink and downlink of RF communications from one point on earth to another. Transponders are sophisticated on- board processing units used to demodulate, decode, re-encode, and re-transmit RF traffic captured via on-board satellite antennas.
Modern communication satellites utilize four types of antennas for RF, Microwave and mmWave communications. The physical structure of the antenna defines its name and type: wire, horn, reflector, or array. Let’s take a brief look at each.
Wire antennas were commonly employed on large form-factor GEO satellites such as those launched for INTELSAT, the international telecom consortium. Wire antennas are omnidirectional; that is, they transmit and receive signal strength in all directions. Monopole and dipole are two common configurations. Wire length is critical to managing gain and impedance performance. On early versions of INTELSAT, antenna gain in the neighborhood of 4 dBi for receive and 9 dBi for transmit was typical. In today’s modern satellite networks, wire antennas are considered suitable for use in tracking, telemetry, and command (TT&C) functions. Horn or aperture antennas are employed on satellites for microwave frequency signals. The antenna consists of a flared metal waveguide that concentrates the radio wave in a beam. For transmission, radio waves are introduced into the waveguide via a coaxial cable attached to the side, with the central conductor projecting into the waveguide to form a quarter-wave monopole antenna. The waves then radiate out of the horn end in a focused beam. At the receiving end, horns are frequently employed as feeds for reflector antennas. The reflector antenna is most popular format used in communications satellites due its rugged structure, small form- factor, and high-gain performance. Types include paraboloid, hyperboloid, and spheroid. A reflector antenna consists of a curved reflecting surface and a feed system. Curved reflectors increase the strength of a signal for all wave forms, including lower frequency RF, microwave as well as optical energy. Deployable reflector antenna utilizing collapsed
aluminum or mesh sides are widely used to provide larger aperture and increased gain strength. Array antennas generate directional beams via a patterned array or grouping of individual radiators, called elements. The many individual elements of the array antenna allow flexibility in modulating frequencies, phases as well as steering the signal. An array antenna is essentially a set of connected antennas which work together to transmit or receive radio waves. The individual elements are typically connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship.
Third-Generation Tracking and Data Relay Satellite (TRDS) Capabilities. Note the numerous antenna types and functions. (Source: NASA)
QwikConnect • January 2023
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