Measuring the focus of the Dish Paraboloid

Discussion of large prime focus satellite dishes used for C-Band signal reception.
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tek2000
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Measuring the focus of the Dish Paraboloid

Post by tek2000 » Mon Mar 27, 2017 5:46 pm

All,

Regarding this post:

http://www.satelliteguys.us/xen/threads ... fy.368412/

This poster (bpalone) did email us with his concerns as described above. I became alarmed that something was amiss, so I opened several 10ft mesh panel boxes in the warehouse to see what the fuss was all about. :thinking

The panels all looked fine and so did the LNBF struts. The manufacturer also confirmed that the geometry of the panels was not altered for this production run either.

bpalone claims he measured the focal length to be 43.14 inches. Did he measure correctly? With reference to the diagram below, the correct focal length is the distance from p to the parabola vertex v. It is NOT the distance from p to the centre disc (which in all likelihood he was measuring). You have to add an additional 2 inches from the disc to the vertex. :wink

The 48.5 inches quoted on our website is actually the distance from the paraboloid vertex (v) to the front (corrugated) part of the scalar ring. The open throat of the LNBF projects another 1 - 1.5 inches out of the scalar. That puts the actual focal length around 47 inches or so and gives us an f/D of 0.4

You will receive signals if your LNBF is situated anywhere between 45 - 49 inches from the vertex (convergence region). It seems bpalone is probably just within this convergence region now (43.14 + 2 = 45.14), but needs to come out a bit to maximize his signal gain.

As bpalone confirms in his post, even his slightly detuned 10ft mesh dish will pick up all North American DVB-S2 / 8PSK signals. If he uses a spectrum analyser and fine tunes the feedhorn position, he will start to pick up 16/32APSK signals and the weaker Latin American transponders (40.5W, 43.1W).

https://en.wikipedia.org/wiki/Parabola
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Re: Measuring the focus of the Dish Paraboloid

Post by C-Band Lover » Tue Mar 28, 2017 8:55 pm

If I were you I would post this over at satellite guys. why post it here? seems you would help sales by posting there to let more people know.

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Re: Measuring the focus of the Dish Paraboloid

Post by dishcrank » Wed Mar 29, 2017 10:48 pm

Satelliteguys is hostile to every c band vendor except Titanium. :wink

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Re: Measuring the focus of the Dish Paraboloid

Post by LATINOTV » Fri Mar 31, 2017 2:46 pm

Try this [codehttp://www.satlex.it/es/fdratio.html[/code]

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Re: Measuring the focus of the Dish Paraboloid

Post by fatso » Sat Apr 01, 2017 10:28 pm

I think I understand the confusion. Most of us measure the focal length of the dish from the dish surface (not vertix) to the front of the feed throat. Also most installers I knew from back in the day measured 0.5, 0.75 or even 1 inch into the throat of the feed depending on the dish/LNBF manufacturer.

This is all a waste of time if you ask me. Once you lock your first signal, you will need to play around with both the focal length and skew to optimize reception. I would also ignore the f/d calibration settings on the LNBF because they were never tested for your particular dish.
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