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Naijatop.com » Africa's No.1 Free-To-Air Satellite Forum »   » Tutorials » Mounting The LNB And Feedhorn

Mounting The LNB And Feedhorn

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1 Mounting The LNB And Feedhorn on 1st June 2014, 2:01 pm

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Extreme care should be taken when bolting the LNB to the feedhorn.
Do not
touch the probe inside the mouth of the LNB. This probe can be
adversely
affected by grease or dirt. Also 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. Otherwise, moisture can seep into this opening
and
disrupt your reception.
The center of the feedhorn must be supported exactly at the focal
point of the
dish. The focal distance between the center of the dish and the mouth
of the
feedhorn will vary between different models of antennas, depending on
whether the manufacturer has used a deep or shallow dish design. The
exact
distance will be provided in the instructions.
There are two fed support styles available, one of which will be supplied
with
your antenna. A “button hook” support, sometimes called an LNB tube,
is a
single piece of tubing which extends from the center of the dish
outward. The
button hook support, which is curved so that it resembles a hook,
allows the
feed to be mounted looking back at the center of the dish. Guy wire
kits are
available for button hook supports that will provide the additional
stability
required for reception of both C- and Ku-band signals. Ku-band signals
are
much higher in frequency and there fore are beamed Earthward in
much
smaller wave lengths. Consequently, the antenna curvature and
location of the
feed for Ku-band reception must be much more precise than what C-
band
systems commonly require.
We recommend that you check to that the feed is centered over the
dish by
measuring from the lip of the antenna to the edge of the feedhorn’s
opening at
four equally spaced intervals around the antenna’s rim. This is
especially
important if you are using guy wires to anchor the button hook
support. If the
feed is centered, these measurements will all be equal. If they are not
equal,
you will have to adjust the tension if the guy wires until the feed is
properly
centered.
The second type of feed support uses a multi-legged structure to hold
the
feedhorn and LNB (or LNBs for dualband systems). These are made up
of
three (“tri-pod”) or four (“quad”) straight, equal length pieces of
aluminum or
steel. Quad supports are inherently more stable than button hook
supports,
offering better Ku-band reception. When mounted at the
recommended locations on the antenna’s surface, these supports
should accurately position
the feed at the correct focal length: the distance between the center of
the dish
and the opening of the feedhorn. The correct focal length for your dish
is
provided in the manufacturer’s assembly manual.
Once your system is up and running, you can fine tune the focal
length for its
optimum position by moving the feed in and out in small increments
while
watching the receiver’s signal strength meter. This is easier to do with
a button
hook than a quad support. This adjustment is particularly important if
you are
having trouble receiving Ku-band satellite signal.
You can compute the focal length if you know the diameter of the dish
along
with its f/D ratio. Focal length = the antenna diameter times the f/D
ratio. For
example, the focal length of a 10 foot antenna with an f/D ratio of .45
equals 10
x .45 = 4.5 feet (54 inches).
To determine the antenna’s diameter, measure across the surface of
the dish
from one side to the other. The radius equals one half the diameter.
The depth
of the dish is the distance from the center of the dish to the plane of
the rim.
Stretch a string across the antenna’s rim so that it crosses in the
center of the
dish. The depth will be the distance from the antenna’s center to the
string.
Many feeds today have adjustable scalar rings. These feeds can be
broken
down into two parts: a round flat “scalar” plate with concentric circles
on its
surface and the wave guide onto which the LNB is mounted. This wave
guide
fits into the center of the scalar plate and can be adjusted inward and
outward.
The distance that the wave guide extends beyond the surface of the
scalar
plate must be set correspond to the f/D of the antenna. Consult with
the
manufacturer’s assembly directions or use the formula provided above
to
determine the correct f/D ration of your antenna.
The wave guide may be marked to indicate the various f/D ratio
settings.
Alternatively, the feedhorn may come with an adjustment gauge for
setting the
correct location of the scalar rings.
Another thing to check: the plane of the feed opening should be the
same as
the plane of the rim of the dish. You can use your inclinometer to
check to be
sure that both the feed opening and the antenna’s rim are parallel with
each
other.

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