finding dish focal point. ( in Hindi)

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freeisforme

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Feb 21, 2017
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found this video about finding solid dish focal point. Its in Hindi but you get the idea. FF to 7:20 minutes. i gave it a fast try, it should work. opinions/ideas/suggestions..

 
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a33

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Feb 4, 2015
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The method with the wet dish is nice, indeed. Did you try that also?
The method is not a calculation, though, as the title suggests. Is seems more suitable for checking the LNB position, than for finding the proper spot for the LNB, I would think.

For calculating: I know of 3 calculators on the internet for offset dishes.
Alas the satsig calculation website gives incorrect top string and bottom string values, I noticed; and the "Parabola calculator 2.0" program, the part for the offset dishes, also gives incorrect results, due to the not-so-correct assumption of the bottom of the dish always being at the vertex. I wouldn't use or advice both of them.

The program Parabola6 by strannik gives correct values though (apart from one confusing but not so important string value), but is in russian (or some similar language). As it mainly works with pictures, that isn't a real problem though.
BTW I checked these outcomes with my own multi-method calculator, but that is still in the development stadium.

greetz,
A33
 
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Brct203

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Dec 24, 2016
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Interesting point A33... I used the SatSig calculator on a 120cm offset and the results did not make sense to me and were definitely not the correct focal point.

I will try the Parabola6 when I get a chance
 
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a33

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Feb 4, 2015
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Parabola6 uses dish height, dish width, and depth at the middle as inputs.
(The values for side strings CF and DF in the calculator are not right; the calculated value seems in fact to be EF in the first picture, as can be seen and calculated in the second picture/drawing: it is the mean of AF and BF.)

When you cannot use width of the dish because the dish probably isn't elliptical (higher than wide) and/or not paraboloid (parabolical both vertically and horizontally), you can also calculate all the dish specifications by deepest point data: dish height, depth at deepest point, and distance from bottom of dish to deepest point, measured at dish face (as, btw, are also 3 of the 4 inputs for Parabola calculator 2.0!).
Or the data of this triangle: height (top to bottom), top to deepest point, and deepest point to bottom; via goniometrics that specifies also the needed deepest point position relative to the dish face.
If you give me either three of those input data, I can let my calculator calculate for you. (That part is as good as finished.)

Bonus: I'll give you the f/D ratio, belonging to the LNB viewing angle, which isn't in Parabola6 (I'm not sure if I share stranniks reasons for that ~ though I haven't studied all of his explanation text about that yet).

greetz,
A33
 
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freeisforme

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Feb 21, 2017
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After a few days of clouds, i gave it a try and it does focus the sun on the LNB. For a quick, easy, free way to find the focal point, i would give it a thumbs up for my hughes net dish. I get 68% quality on PBS create AMC 21.
 
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a33

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Feb 4, 2015
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After a few days of clouds, i gave it a try and it does focus the sun on the LNB.

Come to think of it further:
Just a white spot on your LNB doesn't mean you have "the" focal point, though.
Does the film mention this?
If the LNB is positioned too high or too low, you'd also get a big white spot on your LNB, but at a wrong offset angle.
You'd have to find the place where the dot is as sharp and as little possible, changeing both the dish elevation and the offset angle for the LNB.
Not really an easy procedure...

Greetz,
A33​
 
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freeisforme

Thread Starter
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Feb 21, 2017
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south jersey

Come to think of it further:
Just a white spot on your LNB doesn't mean you have "the" focal point, though.
Does the film mention this?
If the LNB is positioned too high or too low, you'd also get a big white spot on your LNB, but at a wrong offset angle.
You'd have to find the place where the dot is as sharp and as little possible, changeing both the dish elevation and the offset angle for the LNB.
Not really an easy procedure...

Greetz,
A33​
I only speak english. yes he does show you, if not in this one he has anther on where he put mirrors along the perimeter and show with a cardboard to get a nice tight circle not skewed in any direction.
 
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a33

SatelliteGuys Pro
Feb 4, 2015
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netherlands europe
Did you also vary the dish elevation, and try higher and lower LNB positions in your tests?
Did that affect the sharpness of the light reflections, where they focussed?

Greetz,
A33
 
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