10 foot offset c band dish question (1 Viewer)

newbyyou

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New Member
Jul 15, 2020
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NW MONTANA
New to satellite completely, however in about the last month and half, I have put up a 39" ku band dish with a motor and having no issues with it at this point.
My next challenge is with a c band antenna, which I plan to be stationary on one satellite, the issue I am having is, right now with all the covid stuff going on, pretty much every place is out of any decent sized and priced dishes, from China, that may not happen for quite some time, so I do have an older c band fiberglass dish, the problem is, it is just the fiberglass dish, no mounting brackets on the back, no lnb and no lnb support brackets.
I have been running the internet to see if I can identify the make and model but to no luck, finally found something I cannot locate, not even a picture on the web.
I can fabricate a mount of some sorts and bolt it to the dish to mount on pole, but since the lnb and supports are missing, I have no idea how to figure out the correct positioning of the lnb since I would have to also fabricate the support arms.
Roughly, the dish means top to bottom about 9 feet and side to side 100", the holes are in the sides of the dish for the support arms, there are three, holes at the very bottom and one on each side not quite up to half the dish moving toward the top, so I figure it is and offset dish.
I did find a website that give explanation and formulas for figuring this out, but, to be honest, it seems to be a little beyond me, if I could identify the dish and find some place that has replacement lnb support arms that would take care of it.
I could add photos of the dish if anyone thought that might be helpful in figuring out the make.
Thanks for any help someone could offer.
 

newbyyou

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Jul 15, 2020
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NW MONTANA
Here are photos of this dish, I realize it will need a bit of TLC, a little fiberglass repair and paint.
If anyone could help identify the maker or model or lead me in a useful direction, that would be great. The location that has four or five holes next to each other is the bottom of the dish.
Thank,
 

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a33

SatelliteGuys Pro
Feb 4, 2015
516
330
netherlands europe
I did find a website that give explanation and formulas for figuring this out, but, to be honest, it seems to be a little beyond me

I have my own calculator for that on my PC.
So if you haven't figured out what dish it is via the photoos, I can calculate for you; assuming it is a normal paraboloid dish.

Need width and height of the dish's working area (so without the rim!), and depth at the center of the working area (also without the rim). Especially the depth as accurate as you can (more accurate than millimeter, if possible).
From that you get the exact location of the focal point, relative to the dish sides.

(BTW I'm not so familiar with non-metric measures; so do me a favour.... ;) )

Greetz,
A33
 

Inclined Orbit

SatelliteGuys Pro
Jan 2, 2018
376
201
Los Angeles
If this was a prime focus dish its not that bad to calculate the F/D ratio and fabricate spars to hold the feed with some adjustment range for focus. If its an offset dish, I know one person that was actually successful in getting a feed in the exact right spot and he was an expert and it took a really long time measuring, calculating and testing on a home made test range.

In my opinion, if its an offset then throw it away, you will probably never get the feed in the right location. Ever.

I have a beautiful 1.2M carbon fiber Ka rated offset reflector with no brackets here and I have some experience with this and I dread the day I finally have to find the focal point on this dish for a hamster project. I would be surprised if I can pull it off in less than a weeks labor if at all.
 

a33

SatelliteGuys Pro
Feb 4, 2015
516
330
netherlands europe
In my opinion, if its an offset then throw it away, you will probably never get the feed in the right location.

Well, once you know what parabolic equations you have to use, it's not difficult anymore; and the outcome is not an approximation, but accurate for the parabola.
So my opinion is exactly the opposite. :)

BTW. What amazed me most about paraboloids, is that only three measurements are sufficient to determine all dish specs. Not only with deepest point calculation or with depth at the center calculation, but also with calculation from feedhorn-position (needing topstring length, bottomstring length, and dish height as inputs). Parabolas really are special things!
And I'm by far not the only one that does/did calculations on offset dish's geometry...

Greetz,
A33
 
Last edited:

Inclined Orbit

SatelliteGuys Pro
Jan 2, 2018
376
201
Los Angeles
C-band is not as critical as what I have worked on, but an offset dish is still very difficult to work with finding the focal point and angle. You will never know if you have it right unless you have a perfect working sample to compare with.

Well, once you know what parabolic equations you have to use, it's not difficult anymore; and the outcome is not an approximation, but accurate for the parabola.
So my opinion is exactly the opposite. :)

BTW. What amazed me most about paraboloids, is that only three measurements are sufficient to determine all dish specs. Not only with deepest point calculation or with depth at the center calculation, but also with calculation from feedhorn-position (needing topstring length, bottomstring length, and dish height as inputs). Parabolas really are special things!
And I'm by far not the only one that does/did calculations on offset dish's geometry...

Greetz,
A33
 

a33

SatelliteGuys Pro
Feb 4, 2015
516
330
netherlands europe
... but an offset dish is still very difficult to work with finding the focal point and angle. You will never know if you have it right unless you have a perfect working sample to compare with.

Well, who says the 'working model' has it right?
I've seen examples where the calculations give a better result than how the dish comes from the factory. Arm too low, arm too long, bad LNB-aiming, that sort of things.

Only assumption for the calculation is that the dish has, in fact, a proper paraboloid form.
That, of course, isn't guaranteed: you never know how thorough it is manufactured.
And, I agree, there will always be the risk of small measurement errors...

Greetz,
A33
 

newbyyou

Thread Starter
New Member
Jul 15, 2020
3
0
NW MONTANA
First off, I would like to graciously thank all of you that have taken the time to look at my situation.
Another question I have is: I am assuming that this is an offset dish due to the fact that the dish is wider top to bottom then it is from side to side, also, just glancing at the location of the holes for the lnbf support arms, the two for the sides are slightly lower that the center from bottom to top of the dish.
I just assumed that any dish that is not exactly round and was ovaled like this one, is an offset dish. Does my reasoning sound accurate? Or can a prime focus dish also be oval shaped, I just assumed that all prime focus dishes were round dishes, perhaps my thinking is wrong.
In any event, I will try to get as accurate of measurements as I can and post them later this day.
Again, thanks to all.
 

a33

SatelliteGuys Pro
Feb 4, 2015
516
330
netherlands europe
I just assumed that any dish that is not exactly round and was ovaled like this one, is an offset dish. Does my reasoning sound accurate?

If the dish (or rather: its working area) has a flat face, then YES.
You'll also notice that the deepest point of such a dish is not at the center of the dish, then, but somewhat below the center.

Or can a prime focus dish also be oval shaped, I just assumed that all prime focus dishes were round dishes,

In theory one could give a PF dish a non-flat dish face, so that it is not round anymore. But I don't see much sense in that.

However, not all round dishes are PF. It happens that they give an offset dish a 'round(er)' appearance, by giving it an uneven dish rim, or by using a non-flat dish face.

Does this answer your questions?

Greetz,
A33
 

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