Page 123 - Intro to Space Sciences Spacecraft Applications

110 Introduction to Space Sciences and Spacecraft Applications
na concentrates its attention horizontally with decreased receiving capa-
bilities in other directions.

Parabolic Dish. In many applications, it is desired to concentrate a signal
as much as possible in a single direction. For these applications, a para-
bolic dish antenna is most commonly used. Figure 5-4 depicts a parabol-
ic dish with a representative antenna pattern, and also delineates some of
the terms with which we will be dealing.
When transmitting, the signal is either emitted by, or reflected off, a
structure at the focal point of the antenna paraboloid. The parabolic shape
of the antenna reflects the signal mainly along the boresight, concentrating
most of the signal power in this direction. When receiving, the incoming
electromagnetic wave “reflects” off the dish and is concentrated onto the
focal point of the parabola where (either by pickup or secondary reflection)
the signal is delivered to the receiving components. An antenna has the
same propensity for receiving a signal from a certain direction as it has for
transmitting in that direction, as quantified in the following section.

Antenna Gain. The ability for an antenna to concentrate a signal in a cer-
tain direction is given by the antenna’s gain in that direction. This prop-
erty is dependent on the size and shape of the antenna structure and on the
frequency of radiation as well. We will be dealing mainly with parabolic
dish antennas in this chapter, and a simple relationship for gain in the
boresight direction for such an antenna is given by equation 5-5.

PARABOLIC
DISH
I RECEIVEW VLP BORESIGHT 2
REFLECTOR

TRANSMllTER

ANTENNA
PAlTERN
(locus of 1/2 power)

Figure 5-4. Parabolic antenna. Parabolic antennas are used to concentrate
transmitting or receiving energy in a particular direction.