Noise in Antennas
Thus far we have examined how to calculate the power radiated from an antenna and, using
the Friis transmission formula, how to calculate the received power at the other end of
a communication link. However, the received signal power is meaningless unless compared
with the power received from unwanted sources over the same bandwidth. Such noise sources
include thermal radiation from the earth and sky, cosmic background radiation, and random
thermal processes in the receiving system. In today’s wireless environment, additional noise
due to nonstationary radio frequency interference from pagers, cellular phones, etc., often
needs to be considered, but in this analysis we will concentrate on natural sources only.
1 Natural Sources Characterized
Consider the source/receiver configuration shown in Fig. 1. The brightness of the source is
1 sq m
Figure 1: Radiation from a natural source.
defined to be the electromagnetic flux density (power per unit area) at the receiver per unit
solid angle of source. By dimensional analysis,
Power · R2
m2 · m2 = Brightness.
From the dimensional analysis we can see that an equivalent definition of brightness is power
per unit area of source per unit solid angle of receiver. Monochromatic brightness is brightness
per unit frequency. Often the term “brightness” only is used, without the monochromatic
qualifier; the meaning is usually clear from the context.
Contrast the definition of brightness with that of flux density: power flow per unit area.
Brightness is suitable for extended sources, while flux is suitable for point sources, as shown
in Fig. 2. Each cross-sectional area (S, S ′,S ′′) intercepts the same total power, but the flux
flux = P/S P/S’ P/S"
Figure 2: Power flux from a point source.
density decreases as energy propagates away from the source. The power intercepted by a
receiver at any poin