Specifications

CONSTRUCTION
receives the signals from the satellite and
focuses them to the focal point where the
feed horn is positioned. The focused sig-
nal picked up by the feed horn is fed into
the LNB, which is mounted on feed horn
itself. The dish antenna kits are now-a-
days readily available and can be easily
assembled.
LNB assembly. The LNB assembly
consists of the following three parts: the
scalar ring, polarator, and LNB.
(a) The scaler ring: There are two
types of scalar rings, namely, adjustable
and fi
(i) Adjustale scalar ring: In this, the
scalar ring slides on the feed horn and can
be positioned to suit the focal distance to
diameter ratio (F/d) of the dish (refer
Fig. 2). The focal distance ‘X’ is related to
the F/d ratio, as shown in Table I.
ii) Fixed scalar ring: Here, the scalar
ring is an integral part of the feed horn
and the distance ‘X’ is fixed. It may not
always suit the F/d ratio of the dish be-
ing used. Hence it is not preferred.
(b) Pol-
arator: In-
side the
feed horn,
there is a
probe,
which is re-
quired
to move ac-
cording to
the polari-
sation of the
satellite sig-
nals.
For
large dish assemblies, a motorised
polarator is prefered. The motor used in
a polarator has three terminals: +5V,
ground, and pulse.
The motor responds to the width of
the pulse supplied by the receiver. The
pulsewidth can be varied from 0.8 to 2.8
ms with the help of the ‘trim’ controls in
the receiver and the position of the V/H
(vertical/horizontal polarisation) switch. In
‘H’ position, the probe can rotate by al-
most 140
o
. For a stream of pulses with
fixed width, the probe position will be
fixed. The probe will move only when the
pulsewidth is changed. Keeping the ‘trim’
control in mean position and changing the
V/H switch from V to H or H to V results
in pulsewidth changes in one step, en-
abling the probe to rotate through 90
o
. To
know as to why the probe movement is
required, we have to know about the
‘polarisation’.
There are three types of polarisations:
(a) vertical, (b) horizontal, and (c) circu-
lar. Electromagnetic waves have two
fields—electric field and magnetic field—
which are mutually at right angle to each
other and also at right angle to the direc-
tion of motion.
In vertical polarisation, the electric
field is along the North-South axis of the
satellite.
In horizontal polarisation, the electric
field is at 90
o
to the North-South axis of
the satellite.
In circular polarisation the electric
field advances like a cork screw. It can be
either left-hand circular (LHC) or right-
hand circular (RHC).
To convert this circularly polarised
waves into linearly polarised waves (ver-
tical or horizontal), a 6.4mm thick fibre
glass piece is fixed in the feed horn along
its diameter. For example, CNN trans-
mission has circular polarisation, and
therefore the fibre glass piece is essential
to get maximum signal pick-up. On the
other hand, Star TV signal is vertically
polarised, and hence the fibre glass piece
is not required. It should in fact be re-
moved to avoid 3dB loss.
For maximum signal pick-up, the
probe should be in line with the
polarisation of the signal it is receiving.
Probe movement is therefore required for
alignment purpose. If the probe is aligned
to receive a horizontally polarised signal
and the signal being received is vertically
polarised, the probe has to be moved
through 90
o
for maximum signal pick-up.
This will give a crystal-clear picture. This
can be done either by trim control or with
the help of V/H switch, with the trim con-
trol in its centre position.
(c) LNB. The LNB stands for low-noise
block converter. LNB comprises an ampli-
fier and a frequency converter. The sig-
nals in C-band (3.7 GHz to 4.7 GHz), which
are received and reflected by the dish, are
fed to the amplifier inside LNB via the
feed horn probe, as mentioned earlier. The
signal-to-noise ratio of the amplifier has
to be rather good because the received
signals are very weak. The lower the noise,
the better will be picture quality.
The converter inside the LNB com-
prises a fixed frequency oscillator running
at 5150 MHz, which beats with the in-
coming signal frequency. The difference
frequencies obtained range from 5150 -
4200 = 950 MHz, to 5150 – 3700 = 1450
MHz, i.e. the input frequency range of
4.2 GHz to 3.7 GHz is converted to 950 to
1450 MHz range. The gain of LNB is typi-
cally around 50 dB.
Fig. 3(a): Block diagram of C-band satellite receiver
Fig. 3(b): Pin out of DBS timer with FM demodulator
104