# 10.4 Antenna Beams

Whereas most Earth-station antennas are designed to provide highly directive beams (to avoid interference effects), the satellite antenna system is designed to produce beams tailored to match the areas served by the satellite. For global coverage, beamwidths of 17.4° are required. In contrast, for transmission to and reception from a small area, beamwidths on the order of 1° or less may be needed (Fig. 10-8).

# Figure 10-8 Spot and multibeam satellite antenna systems for coverage of defined areas on Earth’s surface.

(a) Zone coverage

(b) Multispot beams

An antenna with a beamwidth β of 1° would produce a spot beam on Earth covering an area approximately 630 km in diameter.

Beam size has a direct connection to antenna gain and, in turn, to transmitter power requirements. Antenna gain G is related to the directivity D by G=ξ D, where ξ is the radiation efficiency and D is related to the beamwidth β by the approximate expression given by Eq. (9.26). For a circular beam,

G=ξ 4 π β 2, (10.12)

where β is in radians. For a lossless antenna (ξ=1), a global beam with β=17.4° (=0.3 rad) corresponds to a gain G=136 or 21.3 dB. A narrow 1° beam, on the other hand, corresponds to an antenna gain of 41,253 or 46.2 dB.

To accommodate the various communication functions associated with satellite systems, four main types of antennas are used.*

1. Dipoles and helices at VHF and UHF are used for telemetry, tracking, and command functions.

2. Horns and relatively small parabolic dishes (with diameters on the order of a few centimeters) to produce wide-angle beams for global coverage.

3. Parabolic dishes fed by one or more horns provide a beam for zone coverage (Fig. 10-8(a)) or multiple spot beams (Fig. 10-8(b)).

4. Antenna arrays consisting of many individual radiating elements are used to produce multispot beams and for beam steering and scanning.

Concept Question 10-1: What are the advantages and disadvantages of elliptical satellite orbits in comparison to the geostationary orbit?

Concept Question 10-2: Why do satellite communication systems use different frequencies for the uplink and downlink segments? Which segment uses the higher frequency and why?

Concept Question 10-3: How does the use of antenna polarization increase the number of channels carried by the communication system?

Concept Question 10-4: What are the sources of noise that contribute to the total system noise temperature of a receiver?