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56 E L EC TR I C AL CONNEC T I ON
AU T UMN 20 1 6
is beamed up from transmitters on
earth and the transponders rebroadcast
100-plus channels, multiplexed and at
a different frequency so as to avoid
blanking out the incoming signal.
Separate transponders serve different
areas on earth with slightly overlapping
footprints, providing complete coverage
of the assigned geographic region.
Installation of a satellite dish is
straightforward except for the aiming
process, which is highly exacting. It’s
like hitting a rubbish bin lid at a
distance of five miles. If the dish is not
pointed correctly, performance will
be compromised.
You can use the on-screen signal
strength meter or a portable instrument
made for the purpose.
It’s not feasible to point the dish in
various directions and watch the meter
for a response. You probably wouldn’t
find a signal, or you would lock onto a
signal from the wrong satellite.
The correct procedure is to look up
online the setting for your location. Aim
the dish in that direction then tweak it in
to lock onto the strongest response.
If the dish is pier-mounted, the
metal post should extend well below
grade (depending on local soil, wind
and frost conditions) and set in
concrete. If it’s roof- or wall-mounted,
it is important to lag-screw the mount
securely into sound timber framing. If it
is spongy or loose, long-term alignment
will not happen.
Apart from pointing the dish,
installation is not difficult but you need
to understand the circuitry. The signal
from the satellite is high-frequency
electromagnetic radiation that carries
audio, video, synch and metadata for
the programming.
Because the frequency is high, parallel
capacitive and series inductive losses
are great, and transmission via ordinary
cable is precluded.
For that reason, the signal at the
dish’s focal point must travel through a
waveguide to the location of the down
converter. A waveguide is a bolted and
gasketed pipe of rectangular cross-
section with a polished reflective inner
surface that channels the signal about 1m
to where it enters the low-noise block.
Inside the feedhorn is a polariser.
Its purpose is to double the number
of available channels by sending two
separate signals on each frequency.
These signals are either vertically or
horizontally polarised, and they occupy
the same frequency. The desired signal
is chosen in the feedhorn.
In early satellite dishes a servo motor
rotated a polariser to select the channel,
but now the mechanism is electronic.
Because the low noise block contains
semiconductors, power supply voltages
are needed. A DC supply voltage
originates in the building, and travels
over the coaxial transmission line to
the site of the dish. Two voltage levels
(typically 13V and 17V) select the type of
polarisation (horizontal or vertical).
In circular polarisation, clockwise
or counter-clockwise rotation is
selected, doubling the number of
channels available.
The signal, reduced to a frequency
compatible with coaxial cable and
correctly polarized, travels to the
receiver in the building where four
operations are performed.
First, the desired channel is selected.
Some signals are encoded to prevent
unauthorised access. A smartcard,
inserted into a slot in the receiver,
performs the decoding function.
Proprietary microchips enable menu
interactivity with suitable on-screen
graphics.
The signal frequency is reduced to
RF compatible with the television set
and fed into it through a short coaxial
cable. When there is stereo capability,
colour-coded cables with RCA jacks
are used.
Satellite dish systems are also used
for internet access where cable is not
available. This is a more expensive
and elaborate system compared with
TV, and more expertise is required for
installation and troubleshooting.
For one thing, the dish has to be
aimed more accurately, because any
attenuation will show up in the quality
of the internet connection. Also, unlike
TV, an internet installation involves
transmission up to the satellite so that
there can be interactivity.
Two coaxial cables connect the
modem in the building to the low noise
block that is part of the feedhorn/dish
assembly. This is usually a twin cable
with a grounding wire included.
When working on an internet dish,
you must beware of radiation burns,
which can occur if the system goes into
transmission mode while you are at the
focal point.
Then there’s the sometimes difficult
matter of configuring the owner’s
computers and mobile devices, which
involves installing cabling or enabling
wireless with the network name and
password in each device.
TV and internet satellite dish
providers offer extensive online training
with examinations and certification.
Lots of technical data is included, plus
practical information on installation
with particular emphasis on mounting
and aiming the dish.
Communications circuitry is one area that has a lot in
common with power and light.