Page 262 - Satellite Communications, Fourth Edition

242 Chapter Eight

Huck and Day (1979) have shown that satisfactory reception can
be achieved with reflector diameters in the range 0.6 to 1.6 m
(1.97–5.25 ft), and the two nominal sizes often quoted are 0.9 m (2.95 ft)
and 1.2 m (3.94 ft). By contrast, the reflector diameter for 4-GHz
reception can range from 1.83 m (6 ft) to 3 m (10 ft). As noted in Sec.
2
6.13, the gain of a parabolic dish is proportional to (D/l) . Comparing
the gain of a 3-m dish at 4 GHz with a 1-m dish at 12 GHz, the ratio
D/l equals 40 in each case, so the gains will be about equal. Although
the free-space losses are much higher at 12 GHz compared with 4 GHz,
as described in Chap. 12, a higher-gain receiving antenna is not
needed because the DBS operate at a much higher EIRP, as shown
in Table 1.4.
The downlink frequency band of 12.2 to 12.7 GHz spans a range of
500 MHz, which accommodates 32 TV/FM channels, each of which is
24-MHz wide. Obviously, some overlap occurs between channels, but
these are alternately polarized left-hand circular (LHC) and right-hand
circular (RHC) or vertical/horizontal, to reduce interference to accept-
able levels. This is referred to as polarization interleaving. A polarizer
that may be switched to the desired polarization from the indoor con-
trol unit is required at the receiving horn.
The receiving horn feeds into a low-noise converter (LNC) or possibly
a combination unit consisting of a low-noise amplifier (LNA) followed
by a converter. The combination is referred to as an LNB, for low-noise
block. The LNB provides gain for the broadband 12-GHz signal and
then converts the signal to a lower frequency range so that a low-cost
coaxial cable can be used as feeder to the indoor unit. The standard fre-
quency range of this downconverted signal is 950 to 1450 MHz, as shown
in Fig. 8.1. The coaxial cable, or an auxiliary wire pair, is used to carry
dc power to the outdoor unit. Polarization-switching control wires are
also required.
The low-noise amplification must be provided at the cable input in
order to maintain a satisfactory signal-to-noise ratio. An LNA at the
indoor end of the cable would be of little use, because it would also
amplify the cable thermal noise. Single-to-noise ratio is discussed in
more detail in Sec. 12.5. Of course, having to mount the LNB outside
means that it must be able to operate over a wide range of climatic con-
ditions, and homeowners may have to contend with the added prob-
lems of vandalism and theft.

8.2.2 The indoor unit for analog (FM) TV
The signal fed to the indoor unit is normally a wideband signal cov-
ering the range 950 to 1450 MHz. This is amplified and passed to a
tracking filter which selects the desired channel, as shown in Fig. 8.1.