Page 378 - Satellite Communications, Fourth Edition

358 Chapter Twelve

lossy components of antennas, and thermal-like noise is picked up by
the antennas as radiation.
The available noise power from a thermal noise source is given by

(12.14)
P N kT N B N
Here, T is known as the equivalent noise temperature, B is the equiv-
N
N
alent noise bandwidth, and k 1.38 10 23 J/K is Boltzmann’s con-
stant. With the temperature in kelvins and bandwidth in hertz, the
noise power will be in watts. The noise power bandwidth is always
wider than the 3-dB bandwidth determined from the amplitude-fre-
quency response curve, and a useful rule of thumb is that the noise
bandwidth is equal to 1.12 times the 3-dB bandwidth, or B ≈ 1.12
N
B 3dB . The bandwidths here are in hertz (or a multiple such as MHz).
The main characteristic of thermal noise is that it has a flat frequency
spectrum; that is, the noise power per unit bandwidth is a constant. The
noise power per unit bandwidth is termed the noise power spectral den-
sity. Denoting this by N , then from Eq. (12.14),
0
P N
5 5 kT J (12.15)
N 0 N
B N
The noise temperature is directly related to the physical temperature
of the noise source but is not always equal to it. This is discussed more
fully in the following sections. The noise temperatures of various sources
which are connected together can be added directly to give the total noise.

Example 12.5 An antenna has a noise temperature of 35 K and is matched into
a receiver which has a noise temperature of 100 K. Calculate (a) the noise power
density and (b) the noise power for a bandwidth of 36 MHz.

Solution
(a) N 0 (35 100) 1.38 10 23 1.86 10 21 J
6
(b) P N 1.86 10 21 36 10 0.067 pW
In addition to these thermal noise sources, intermodulation distortion
in high-power amplifiers (see Sec. 12.7.3) can result in signal products
which appear as noise and in fact is referred to as intermodulation
noise. This is discussed in Sec. 12.10.

12.5.1 Antenna noise
Antennas operating in the receiving mode introduce noise into the satel-
lite circuit. Noise therefore will be introduced by the satellite receive
antenna and the ground station receive antenna. Although the physical