Page 379 - Satellite Communications, Fourth Edition

The Space Link 359

origins of the noise in either case are similar, the magnitudes of the
effects differ significantly.
The antenna noise can be broadly classified into two groups: noise orig-
inating from antenna losses and sky noise. Sky noise is a term used to
describe the microwave radiation which is present throughout the uni-
verse and which appears to originate from matter in any form at finite
temperatures. Such radiation in fact covers a wider spectrum than just
the microwave spectrum. The equivalent noise temperature of the sky, as
seen by an earth-station antenna, is shown in Fig. 12.2. The lower graph
is for the antenna pointing directly overhead, while the upper graph is
for the antenna pointing just above the horizon. The increased noise in
the latter case results from the thermal radiation of the earth, and this
in fact sets a lower limit of about 5° at C band and 10° at Ku band on the
elevation angle which may be used with ground-based antennas.
The graphs show that at the low-frequency end of the spectrum, the
noise decreases with increasing frequency. Where the antenna is zenith-
pointing, the noise temperature falls to about 3 K at frequencies between

Figure 12.2 Irreducible noise temperature of an ideal, ground-based antenna. The
antenna is assumed to have a very narrow beam without sidelobes or electrical
losses. Below 1 GHz, the maximum values are for the beam pointed at the galactic
poles. At higher frequencies, the maximum values are for the beam just above the
horizon and the minimum values for zenith pointing. The low-noise region between
1 and 10 GHz is most amenable to application of special, low-noise antennas. (From
Philip F. Panter, “Communications Systems Design,” McGraw-Hill Book Company,
New York, 1972. With permission.)