how do i figure the distance from a dmx741 to 8' dish center? (1 Viewer)

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chaskuchar

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Apr 16, 2010
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saint charles, missouri
my 6' is 26". i read the post about resurrecting a 10' and that distance is about 58" i got the dish free. it is solid and not warped. it has 4 holes where the original legs for the lnb were but they didn't come with the dish. i have positioned the dish that is a few feet from my 6' dish and i am at 97.49w so i am aimed pretty good at 97w. i guess i could go to c-band with my dsr200c and tv and tune to c-band and start about 6' from the center of the dish and try to find signal. if not there, i will use spacers to get closer to the dish center and work my way in trying to find signal? any ideas will be appreciated. i did get some free 1/2 emt conduit from the electric shop so i have plenty to work with. charlie
 

cracklincrotch

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Sep 28, 2007
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You need to calculate the focal length of the dish. Each dish is unique.

The focal length is calculated as follows: f = D x D / 16 x C, where D is the diameter of the dish and "C" is the depth of the dish.

You want to position your LNB so that the focal length distance extends into the LNB's throat about 1/4".
 

chaskuchar

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i am trying to get a prime focus, 8' dish working. need an idea of the distance from the lnb to dish center.

i can't find any label or imprint on the dish itself. there are 4 holes where the orignal legs for the lnb were attached. i have 1/2 " emt conduit to fabricate legs with. my 6' dish with the dmx741 is 26". i am using a dmx741 on this dish also. any ideas? charlie
 

chaskuchar

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would 'c' be the depth from a line across the dish edges to the center. the dish is 95" wide and the curve is 91" wide. a line stretched across the dish has 17" in the cehter from the line to the dish center. so 95 times 95 is 9025. 9025 divided by 16 is 564. and then multiply this by 17? i am lost figuring out this distance... charlie
 

hwm

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Apr 29, 2008
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There is a way to figure the focal length of a parabola dish. I am not a math whiz, but found the information to do so with a Google search when I restored my old Winegard C-band dish. It was pretty easy to do. Try a google search for "focal length of parabola".

Here is a link to one search result that looks pretty good to me: Determining the focal length of a parabolic dish
 
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chaskuchar

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91 times 91 is 8281. 16 x 17 = 272. 8281 / 272 = 30.44 inches. i tryed the formula from someone on the c-band board but i thought i was supposed to multiply by the depth but you had the depth dividing the area. makes more sense. i will try that tomorrow. thanks, charlie
 

chaskuchar

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ok, i think i was supposed to divide the depth instead of multiplying it. the curve starts about 91" across. that gives 8281 area and then dividing 16 x 17 is 272 and that divided into 8281 gives a focal length of 30.44". thats a good place to start with. i have a vise and plenty of conduit. the dmx741 has room to move in and out around that focal point. thanks for the help. will let you know tomorrow how it works. charlie
 

Pixl

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Feb 27, 2010
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Charlie,

Why not just draw a string tight across the dish from edge to edge, tie it off on the back side somewhere. Then measure from the string to the face of the dish in the center. Post that value in here and lets see how many different answers come back. We already know the other dimension of 8ft diameter.
 

chaskuchar

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carefully measured the distances. 90.5 from ridge to ridge on the edges. 17.5 was the c distance. 29.25 is my calculated lnb distance. starting to make the legs now. charlie
 

chaskuchar

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my legs are done but i might have cut them short at the limit. i am receiving 72w to 135w but still adjusting the azimuth. i am getting more power in c band and a little more in k band but still adjusting. i had learned adjusting the 6' that up and down should be adjusted at the south satellite and azimuth adjusted on the track of the satellites. i get my exercise going from the basement to the dish a lot of times... charlie
 

chaskuchar

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success. i now have nbc on 105w coming in at 70. still trying to figure out nebraska pbs on g28. and still aligning it a bit. the 1/2 conduit was easy to fabricate the legs. i though i was a little too short but my dmx741 is at '6' on the lnb guide on the side of the lnb. thanks, charlie
 

brex2001

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Jul 14, 2009
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success. i now have nbc on 105w coming in at 70. still trying to figure out nebraska pbs on g28. and still aligning it a bit. the 1/2 conduit was easy to fabricate the legs. i though i was a little too short but my dmx741 is at '6' on the lnb guide on the side of the lnb. thanks, charlie
Did you extend the legs like I did when I goofed?
1"x1/8" flat stock is cheap enough at the hardware.:)
 
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