# Calculate

Discussion in 'C-BAND Satellite Discussion' started by elcheca1, Apr 16, 2019.

1. ### elcheca1Topic StarterActive SatelliteGuys Member

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someone could tell me from a website that calculates by the size of my antenna how long the arms holding the lnb should be

#1
2. ### primestar31SatelliteGuys Master

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#2
Last edited: Apr 16, 2019
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3. ### harshnessSatelliteGuys Master

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Not aimed at the TS, but for those in North America (where pizza dishes seem to garner an unfathomable fascination), the parabolic formulas only work with prime focus dishes. If you have a multi-focal dish (such as a DISH or DIRECTV dish), they don't apply all that well.

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4. ### elcheca1Topic StarterActive SatelliteGuys Member

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my parabolic is 3.7 primary focus Orbitron

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5. ### a33SatelliteGuys Pro

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netherlands europe

Though TS doesn't have an offset dish, so for him this isn't relevant: alas the satsig calculation website gives incorrect top string and bottom string values for offset dishes, I noticed.

The only 100% correct offset dish calculator I know off is Parabola6.exe, needing hight and width of the dish surface, and depth at the middle of the dish as inputs. I have a spreadsheet calculator that does calculate exactly the same.

When you know the exact location of focal point, you can also calculate the offset dish specs from the inputs hight of dish, topstring, and bottomstring (my spreadsheet calculator can also calculate that).
The satsig method with laser pointer can also be used, but I would use the deepest point of the dish instead of the centre of the dish as mirror location, as the only point on the dish that is parallel to the front of the dish is the deepest point of the dish. (So I doubt that the satsig method gives a 100% accurate result.)
If only the offset angle of the dish is needed (without other dish specs), hight and width of the offset dish are enough as inputs; when the dish is elliptical (higher than wide) and the dish face is flat.
That value is calculated correctly at the satsig website.

greetz,
A33

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6. ### mikekohlPrehistoric Satellite GuruSupporting Founder

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This was the Orbitron SX-12 antenna (140 inch or 11.7 foot diameter).
F/D ratio on feedhorn should first be set for 0.36
Actual focal distance for this antenna was 50-1/8 inches.

#6
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