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