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The
home satellite TV receiving systems of the
twenty-first century will be the on-ramps of the
global information super highway, delivering a
plethora of multimedia productions including
hundreds of digital TV and radio services,
personal Videoconferencing and high speed
Internet access to the WorldWide Web. The only
question is: just who is going to be installing
all those dishes? |
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To explain all the
technical ins and outs of satellite installations, we turn to an
internationally recognized author and consultant, Mark Long, who
regularly presents technical seminars and advanced satellite
technology workshops throughout the Asia/ Pacific region and the
world at large. |
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The
azimuth coordinate represents the bearing of the
satellite from the site location, while the
elevation is the tilt angle at which the dish
looks up at the satellite. Every satellite within
view from the site location has its own unique
pair of azimuth and elevation coordinates which
can be calculated using a computer program or a
simple az/el chart. |
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Once
you know the azimuth coordinate for any
satellite, you can use a compass to determine its
direction from the site location. To achieve a
high degree of accuracy however, you must first
correct the compass readings from magnetic north
to true north. Magnetic correction maps are
available for different regions from a variety of
sources. |
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Mark
Long demonstrates the use of an
inclinometer. ". . .we can point the
inclinometer in the direction of the corrected
compass bearing and then tilt it back until it
reads out the satellite's elevation angle. We
therefore can visually confirm whether or not any
satellite will be available before we even begin
constructing the dish." |
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Terrestrial
interference from land-based telephone
transmitting stations, or airport, shipyard, and
military radar installations can restrict or even
preclude satellite reception. The spectrum
analyzer displays the interference source when
the LNB is pointed in the direction of your local
microwave telephone relay station. |
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Larger
dishes require a concrete pad as a supporting
foundation for the mount. It is best to pour the
pad the day before you intend to begin installing
the dish so it has time to cure. Be sure to pour
the amount of concrete recommended by the
manufacturer into a form that extends well below
the local frostline. |
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Assembling
The Dish The dish antenna should come
with a detailed set of instructions covering all
the essential construction information. When you
first receive the dish, check to see that all
parts listed in the manual are present and
accounted for. Read over the instructions to see
what tools and other materials will be required. |
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Begin
assembly early in the day to allow enough time to
complete the project. |
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Solid
petalized antennas are best assembled face down
on a flat surface such as a concrete driveway or
outdoor patio, while mesh antennas should be
assembled face up to prevent damage to the mesh
material. |
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Antenna System Fundamentals
Technical
author Bob Cooper explains the details of correct
feedhorn placement.
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Geostationary
satellites which uplink and downlink signals
across a vast distance of more than 22,300 miles,
generate enough power to overcome background
noise, but only when the correct antenna size and
electronics are used to receive their signals. |
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Thermal
noise also is internally generated within the
LNB's first stage of electronic amplification.
C-band LNBs are graded according to their t noise
temperature performance in degrees Kelvin, while
Ku-band LNB's are rated according to an
equivalent scale in dB/K. The lower the LNB's
noise rating, the better its performance. |
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Another
potential source of damage to the LNB is
lightning. Be sure that you connect the dish to a
ground rod pounded into the earth next to the
pad. Surge protectors also are available to
protect the indoor electronics from damage. |
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When
assembling the LNB and feedhorn, make sure that
the neoprene gasket that goes between the
feedhorn's flange and the mouth of the LNB is
seated properly in the groove provided. |
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Many
feedhorns for prime focus antennas can be broken
down into two parts: one is a scalar ring plate
which can be adjusted up and down the throat of
the feedhorn to match the focal length to antenna
diameter ratio of the dish. To determine the
f-to-D of the dish, simply divide the focal
length by the antenna diameter. |
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Adjusting
the feedhorn to the correct f-to-D value sets the
feedhorn's 'illumination taper.'
Under-illumination of the dish will result in a
loss of some signal amplification that the
antenna would otherwise provide. Over
-illumination causes the feedhorn to 'look' over
the rim of the dish and 'see' the 'hot' noise
source of the ground below. |
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Running
the Cable In addition to the coaxial
cable, two additional cables are needed to
connect the dish to the indoor receiver. The
first cable Supplies a dc voltage and a pulse
count to the receiver so that it knows how far to
travel from satellite to satellite. The second
cable provides the five volts dc pulse and ground
required by the feedhorn's servo motor. |
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A shielded
wire cable called coax is used to connect the LNB
to the receiver. Coax is made up of an inner wire
covered with a plastic or foam sheath, and an
outer mesh that is in turn surrounded by an outer
plastic covering. An 'F' connector is attached to
each end of the coaxial cable. |
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The
feedhorn cable is comprised of three color-coded
22 gauge (or larger) stranded wires. The three
wires provide power, pulse, and ground
connections for the feedhorn. Each of these wires
connects to a corresponding terminal on the back
of the receiver. |
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The
feedhorn's servo motor rotates the pick-up probe,
which swings back and forth while switching
between the horizontally and vertically polarized
transponders (odd and even channels). |
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Correct
tracking of the total geostationary satellite arc
is only possible when the declination has been
properly set. Too much declination loses the
medium elevation satellites while two little
declination will cause the loss of the low
elevation satellites. |
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Move the
dish east or west until you reach arc zenith for
your location. Select an active transponder on
the satellite closest to the east or west of arc
zenith. Now move the dish in the direction of the
nearest satellite. If all the settings have been
done right you should have no problem finding it. |
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For peak
reception of satellites that are in the upper
section of geostationary arc, adjust the
elevation bracket up slightly and then jog the
motor drive east and west. If reception is worse,
try going down slightly, then jog the motor drive
east and west again. You should not have to move
the elevation setting very much because you
already have pre-set it for your location. |
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The rule of
thumb here is: Rotate the dish on the pole and
use the motor drive to receive the lower
satellites, and adjust the elevation of the dish
and use the drive to receive upper satellites,
and not vice versa. |
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For those
situations where the installer can't afford to
sacrifice ANY C-band signal, a second Ku-band
feed and LNB can be attached to one side of the
C-band feed along the plane of the dish that is
perpendicular to the mount's polar axis. |
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Rooftop
mounts are available that either penetrate the
roof and attach to the buildings rafters or mount
directly on a flat roof or peak. Above all,
select the type of mount which will give the
antenna a clear, unobstructed view of the
satellite. |
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Footprint
Maps are available to give you the signal levels
expected from any satellite in any given area.
Rain, fog. snow or
even rain filled clouds rolling overhead can
reduce the intensity of any Ku-band satellite
signal.
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The
spectrum analyzer is the single most powerful
tool available to the professional satellite
installer. It is a specialized receiver that
continuously sweeps across an entire band of
frequencies and presents the output as a video
display of signal amplitude against frequency.
The result is a panoramic view of the amplitudes
and frequencies of all the signals present in the
band of interest. |
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As we move
the dish through the satellite arc, we can easily
detect and peak the settings for each and every
available satellite. By adjusting the analyzer's
frequency marker, we can display the center
frequency for any one signal on the analyzer's
digital readout. |
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We can also
adjust the bandwidth of the spectrum analyzer to
look at an entire satellite frequency band. or at
just one satellite signal. |
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The shape
of the signal displayed on the analyzer's screen
also gives us clues as to what type of
transmission is taking place. Conventional analog
satellite TV signals have one kind of
characteristic shape, while digital signals,
including compressed digital video, have another. |
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"With this videotape, you have purchased
a ticket to a destination from which YOU can
contribute to both the technology and its
beneficial effects. Welcome to the twenty-first
century!" |
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