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Communications
where h is the wavelength of the signal and Ae represents the efsective
antenna area which, for a parabolic dish, is given by equation 5-6.
where D is the dish diameter (as depicted in Fig. 5-4) and p is an antenna
eficiency factor (typically around 0.5-0.8). Notice that using the same
units for A, and h results in a dimensionless value for the gain.
As you can imagine, directions away from the boresight of a parabolic
dish antenna reduce the effective area that can be seen, decreasing the
associated gain of the antenna in that direction. The antenna pattern shown
for the parabolic dish in the sketch also represents the antenna’s gain.
Though it is easy to imagine this as similar to a flashlight beam emanat-
ing from the antenna (which it is), it must be realized that this same gain
applies to the antenna when it is receiving signals as well as when trans-
mitting them. Parabolic antennas send and receive signals best from the
boresight direction and do not receive signals as well from other direc-
tions. Numerically, the same relationship that gives the boresight gain for
a transmitting dish (GT), also applies to the antenna’s ability to receive
signals from the boresight direction, called the receiving gain (GR).
Power Budget
The amount of power that arrives at an antenna, and the ability of the
antenna and receiver to pick up and recognize that signal, determines if
successful communications will occur. This is determined by a link (or
power) budget analysis of the specific system, as depicted in Figure 5-5.
A certain amount of transmitter power (PT, usually described in watts [w]
or decibel-watts [dBW]) is delivered to the antenna. If this were an isotrop
ic radiator, the power would be radiated uniformly in all directions and at
some distance (R) from the antenna the amount of power detected would be:
