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How to measure the focal lenght of your dish

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Xmonks
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PostXmonks on Mon 24 Feb 2014, 8:53 pm

To be able to calculate Focal Length Of Your Dish

Firstly, check to see if you dish is listed.

Secondly, if the curve of your dish exactly matches an equation, and the feed horn 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 ntersection 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

This is how it is done, you must align your feed horn.

Source:
http://www.nigerianforum.forumotion.com/t3-how-to-measure-the-focal-lenght-of-your-dish#3
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Decency
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PostDecency on Tue 25 Feb 2014, 10:46 am

This is very long post

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