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Naijatop.com » Africa's No.1 Free-To-Air Satellite Forum »   » Tutorials » HOW TO CALCULATE FOCAL LENGTH OF A DISH

HOW TO CALCULATE FOCAL LENGTH OF A DISH

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1 HOW TO CALCULATE FOCAL LENGTH OF A DISH on 1st June 2014, 1:52 pm

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some find it had knowing or measuring the focal length
of big dish sizes like those used for nilesat, hotbird etc,
and you know that without getting the correct
measurement of the focal length you won't get the full
signal so in this tutorial will learn how to measure it and
get correctly.so if you are with me let's go there. Calculating Focal Length Of Your Dish
First check here if youf dish is listed.
If the curve of your dish exactly matches an equation,
and
the feedhorn and LNB sit exactly at the focal point, then
performance is at an optimum. To obtain this you have
to
do a few things.
First check the curve of your dish (and diagnose any
warp
in the mesh) which is called cross-stringing. Beg borrow
and steal some lightweight yarn and stretch it across
the
dish, starting from one side and crossing over exactly
the
opposite point. Now do this again and again starting at
different points, so that you divide up the face of the
satellite into six or eight pieces, somewhat like a pie. All
the
strings should just touch at their midpoints. At most
there
should be a one quarter inch gap between two strings
at the
intersection. Any other gaps that are larger indicate a
serious warp or twist in the satellite that must be fixed.
Not
all warps can be fixed, so you might have to purchase a
new satellite.
Next we make a small hole (enough to see through)
exactly
at the centre of the dish if your satellite doesn't have a
hole
all ready. Then you look through this hole from the rear
of
the satellite towards the feedhorn, and you should see
the
intersection of the strings and the centre of the
feedhorn all
lined up. Over time, the feedhorn usually settles slightly,
so
carefully re-aim it so that it's pointed exactly at the
centre
of the satellite, where all the energy is concentrated.
To check the focal length you will need to do a little
math.
F = the focal distance in inches, or the distance from
the dish
surface to a point one quarter inch inside the feed horn
opening. D = the diameter of the dish in inches and d =
depth
in inches, from the intersection point of all those strings
to
the surface of the satellite. Therefore the equation you
would use is F=(D*D)/(16*d)
For example if a satellite is 10 feet or 120 inches this
would
be D . Then the depth from the intersection point of the
strings to the back of the satellite is 25 inches this
would be
d . Therefore F = (120*120)/(16*25) which equals 36
inches.
Adjustments of the focal distance could be made faster
with
any dish aiming meter, available at RV supply shops for
the
small dish crowd. Also you can purchase one at
skyvision.
To adjust the feedhorn it can be done two different
ways:
By twisting the entire feedhorn so that it moves in or
out or
with washers. But first you'll need to find out wether
your
feedhorn is adjustable or fixed. As an adjustable
feedhorns
have two or three setscrews holding the inner cylinder
of
the feedhorn to the scalar ring. Before going any further
you'll need to calculate the ratio between the focal
distance
and the diameter explored previously. To calculate this
you
take F which you have from before and divide by D or F/
D .
Your answer should typically be between 0.30 to 0.37 .
As
adjustable feedhorns have numbers on the inner
cylinder.
All you do is set it to the F/D ratio that you calculated.
Adjusting the focal distance by twisting the feed will
change
the polarization settings for all of your satellites. If you
use
an aiming meter remove the polarization motor and
hold the
polarization steady with a large straight screwdriver
while
twisting the feed in and/or out.
Also you should check and repair any smaller dints in
the
dishes mesh. You can purchase individual sections of
mesh
for your satellite if needed. If you satellite is a solid one
then there is not much you can do, if your satellite is
damaged, besides hammering out minor dints. Some
solid
satellite dishes are made out of fiberglass and when
they
become worn over time the fibers start to show and this
will
cause signal loss, which is especially noticeable on all
digital signals. If your satellite is like this you'll need to
purchase a new one unfortunately.
Hope after you've done all of this your satellite is all
better
and working.
NB . ->This is sort of a complicated and long process to
make sure your satellite is in an optimum condition so it
might be best to call your satellite installer to perform
the
above adjustments.
Note : You can make a template from bristal board or
card
board and place your F/D ratio on one corner of bristal
board making sure you have a 90 degree angle to work
with
measure square from the scalar rings to end of the feed
horn minus cover. The following numbers in inches
equal
your F/D ratio.
F/D .42 = .00 inches
F/D .40 = .20 inches
F/D .38 = .40 inches
F/D .36 = .60 inches
F/D .34 = .80 inches
F/D .32 = 1.0 inches
Now that this is done you must align your feed horn

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