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a33

SatelliteGuys Pro
Feb 4, 2015
578
377
netherlands europe
Yes you understand rightly. :)
The calculation is done on the triangle: top of dish - middle of dish - bottom of dish - back to top of dish.
With cosine rule and Pythagoras you get the depth.

I believe that method is easier and more accurate than measuring the depth, because:
- you don't need to 'construct' the flat face, from where to measure the depth. If that bends a little (0,5 mm), the error further on in the calculation is much bigger.
- you don't have to compensate separately for dish rim height.
- you can check easily that you have the center, by comparing measurement from top and measurement from bottom. They should be exactly the same, or you aren't at the center point.
- by measuring from the exact triangle points, when you make a small error, the error applies to the whole triangle, and is in end-effect smaller I guess.
- the measurement can be done easily with erected dish or with dish lying down, as you wish.

But I am not a technician or mathematician, so these are just my thoughts and considerations.
All calculations are done with sound mathematical (goniometric and parabolical) formulas, though.
If you give me the exact input measures, I can let my calculator calculate your dish specs for you!


BTW:
In addition to what I wrote earlier about wider than high dishes:
Of course they can also be calculated by the deepest point calculations.
However with my Triax-115 I tried to find the deepest point, using a marble, and a trackerball from a computer mouse (worked a little bit better), but I didn't get an unequivocal point as the deepest point.
So I used the calculation outcomes from that method mostly for comparison with the parabola Calculator 2.0. But if someone has a bright idea how to simply determine the exact location of the deepest point (more precise than 1 millimeter), I would be very interested!

greetz,
A33
 
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a33

SatelliteGuys Pro
Feb 4, 2015
578
377
netherlands europe
Just remembered:
I made a drawing with indication of the points, used in the calculator, couple of months ago.

See here:
A33 sat dish calculator 3 met rechthoek.png

greetz,
A33
 

harshness

SatelliteGuys Master
May 5, 2007
16,825
2,879
Salem, OR
The picture looks like your LNB is skewed way too far to the right. Move it back to the center.
Given that much of the focus of the Slimline is concentrated on 99W and 103W, this may be misleading advice. The Ku comes in at the center focus but I suspect that much of the dish effort may be concentrated at the Ka slots. Absent the Ka slots, an 18" dish works just fine for 101W.

Another reason not to use multi-focal dishes for FTA.
 
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a33

SatelliteGuys Pro
Feb 4, 2015
578
377
netherlands europe
If that is correct, isn't that more sensitive to slight measurement errors than measuring the depth against a straight edge going across the dish? (I have not done the math, just wondering though)

I've just tested that, and you were totally right!
Adding 0.1 cm (so 1 millimeter) to the distance of top to center, gives dish depth of 9.15 cm (instead of 8.5), and moves the focal point 4.5 cm towards the vertex!

So I was totally wrong there! :(
I guess I should delete my method D from the calculator.
Measuring depth itself is the far better way there!


I also tried adding a 1 mm error in method A (in the triangle: top - deepest point - bottom); the effect there seems much less bad; though the error affects both the offset angle and the focal distance in that calculation method.
But maybe, when there is no sound and easy way of finding the exact deepest point of an offset dish, method A, B and C should disappear anyway. Nice mathematical exercition, but very 'unpractical' in reality.


So, I'm curious if methods F and G can remain?
I started off with these methods, trying whether the effective focal distance (distance F-G, needed for multifeed-calculations) could be calculated from the top and bottom string length (together with dish hight). A bit to my surprise, these three input data were sufficient to calculate ALL parabolic dish specs for an (offset) dish.
I'll have to compare now for these two methods, which one is most sensitive to measuring errors; or that both (F and G) can remain.


For the calculation method I have in my mind for a multifeed dish: That can be done on the basis of method E. :)
So no problem there, that will remain on my to-do list.


greetz,
A33
 
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