Page 157 - Satellite Communications, Fourth Edition

Chapter
6

Antennas

6.1 Introduction
Antennas can be broadly classified according to function—as transmit-
ting antennas and receiving antennas. Although the requirements for
each function, or mode of operation, are markedly different, a single
antenna may be, and frequently is, used for transmitting and receiving
signals simultaneously. Many of the properties of an antenna, such as
its directional characteristics, apply equally to both modes of operation,
this being a result of the reciprocity theorem described in Sec 6.2.
Certain forms of interference (see Chap. 13) can present particular
problems for satellite systems which are not encountered in other radio
systems, and minimizing these requires special attention to those fea-
tures of the antenna design which control interference.
Another way in which antennas for use in satellite communications
can be classified is into earth station antennas and satellite or spacecraft
antennas. Although the general principles of antennas may apply to
each type, the constraints set by the physical environment lead to quite
different designs in each case.
Before looking at antennas specifically for use in satellite systems,
some of the general properties and definitions for antennas will be given
in this and the next few sections. As already mentioned, antennas form
the link between transmitting and receiving equipment, and the space
propagation path. Figure 6.1a shows the antenna as a radiator. The
power amplifier in the transmitter is shown as generating P W. A feeder
T
connects this to the antenna, and the net power reaching the antenna
will be P minus the losses in the feeder. These losses include ohmic
T
losses and mismatch losses. The power will be further reduced by losses
in the antenna so that the power radiated, shown as P rad , is less than
that generated at the transmitter.
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