Page 169 - Satellite Communications, Fourth Edition

Antennas 149

feeder losses. With the receiving antenna aligned for maximum recep-
tion (including polarization alignment, which is described in detail
later), the received power will be proportional to the power density of
the incoming wave. The constant of proportionality is the effective aper-
ture A eff which is defined by the equation

P rec A (6.13)
eff
For antennas which have easily identified physical apertures, such as
horns and parabolic reflector types, the effective aperture is related in
a direct way to the physical aperture. If the wave could uniformly illu-
minate the physical aperture, then this would be equal to the effective
aperture. However, the presence of the antenna in the field of the incom-
ing wave alters the field distribution, thereby preventing uniform illu-
mination. The effective aperture is smaller than the physical aperture
by a factor known as the illumination efficiency. Denoting the illumi-
nation efficiency by gives
I
A (6.14)
A eff I physical
The illumination efficiency is usually a specified number, and it can
range between about 0.5 and 0.8. Of course, it cannot exceed unity, and
a conservative value often used in calculations is 0.55.
A fundamental relationship exists between the power gain of an
antenna and its effective aperture. This is

A eff l 2
(6.15)
G 4

where l is the wavelength of the TEM wave, assumed sinusoidal (for
practical purposes, this will be the wavelength of the radio wave carrier).
The importance of this equation is that the gain is normally the known
(measurable) quantity, but once this is known, the effective aperture is
also known.

6.10 The Half-Wave Dipole
The half-wave dipole is a basic antenna type which finds limited but
essential use in satellite communications. Some radiation occurs in
all directions except along the dipole axis itself, and it is this near-
omnidirectional property which finds use for telemetry and command
signals to and from the satellite, essential during the launch phase
when highly directional antennas cannot be deployed.
The half-wave dipole is shown in Fig. 6.8a, and its radiation pattern
in the xy plane and in any one meridian plane in Fig. 6.8b and c.Because