focal length?
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i didnt get a sinal anywhere around the 41.14 inches i went bacjk to the 34 i had and moved it in and out a hair at a time till i got a satisfatory picture mines still not good but havent messed with it i been busy trying to stop oil leaks on a russian tracter i bought grrrr good luck
Where are you measuring to?rover54 said:
My dish didn't include the feedhorn supports so I needed to build my own. The diagrams I was able to find weren't 100% clear as to where you measure the focal length to. I measured from the face of the dish at the center up to the flat surface (facing the dish) of the scaler ring.
You're talking about a difference of 7"! That is a HUGE difference. Is it possible that any of your measurements of the dish were off? The formula was dead on for me.
measurement errors, adjusting focal distance...
As others have pointed out elsewhere, the diameter of a dish is often not what it is advertised to be. An eight foot dish may actually be seven and a half. That would throw the focal distance calculation way off, so don't just assume the dish diameter is what you think it is. Probably goes without saying, but just in case...
I think you want to measure from the center of the dish to the Ku pickup element inside the feedhorn throat (underneath the plastic cap). The C-band pickup is farther back, but it is also much less finicky.
I am going to measure my 12' mesh dish and see how the focal length is adjusted on it. The documentation says the focal length is "approximately 53 inches". Adjusting it a little bit at a time is somewhat problematic since the feedhorn is under tension from guy wires. There is a large U-bolt welded to the frame that clamps the shepherds-hook support for the feedhorn, but the way it is built I can't see the end of the pipe to see if there is any room to shorten it without cutting down.
I am thinking it would almost be easier to shim the feedhorn on the mounting plate to bring it closer to the dish and see if I get an improvement. It would still be a couple hour job to fiddle with the guy wires, remove the mounting bots, add some spacers, re-center the feedhorn, etc. Probably worth doing, but not easy to tweak. Trouble is that it adds a lot of other variables to the process that might confound the test results.
A nice screw adjuster on the back side of the dish center mounting plate for the shepherd's-hook mast would be very handy. Almost worth fabricating it. (hmmmm...). With a 12' dish, it is very hard to reach the center of the dish from the front side. I will have to study the construction there and see what I can come up with...
As others have pointed out elsewhere, the diameter of a dish is often not what it is advertised to be. An eight foot dish may actually be seven and a half. That would throw the focal distance calculation way off, so don't just assume the dish diameter is what you think it is. Probably goes without saying, but just in case...
I think you want to measure from the center of the dish to the Ku pickup element inside the feedhorn throat (underneath the plastic cap). The C-band pickup is farther back, but it is also much less finicky.
I am going to measure my 12' mesh dish and see how the focal length is adjusted on it. The documentation says the focal length is "approximately 53 inches". Adjusting it a little bit at a time is somewhat problematic since the feedhorn is under tension from guy wires. There is a large U-bolt welded to the frame that clamps the shepherds-hook support for the feedhorn, but the way it is built I can't see the end of the pipe to see if there is any room to shorten it without cutting down.
I am thinking it would almost be easier to shim the feedhorn on the mounting plate to bring it closer to the dish and see if I get an improvement. It would still be a couple hour job to fiddle with the guy wires, remove the mounting bots, add some spacers, re-center the feedhorn, etc. Probably worth doing, but not easy to tweak. Trouble is that it adds a lot of other variables to the process that might confound the test results.
A nice screw adjuster on the back side of the dish center mounting plate for the shepherd's-hook mast would be very handy. Almost worth fabricating it. (hmmmm...). With a 12' dish, it is very hard to reach the center of the dish from the front side. I will have to study the construction there and see what I can come up with...
(eight foot dish might be seven and a half ok maybe im messuring wrong i mesured from out side edge to out side edge is this wrong? i didnt know about messuring inside of scaler ring to the pickup tho thanks)
To have a 34 inch focal length you would need a 17 inch depth for a 96 inch dish. For a 14 inch depth and a 34 inch focus you would need to have an 87 inch dish. I can't believe you could have mismeasured that much. I can't adjust my buttonhook out far enough to be 7 inches past the focal point, I can only do about 2 1/2 inches longer. The focal length is measured from the deepest point on the dish face (the center) to about 1/4 inch farther then the plastic cover on the feed horn. I set mine from the formula I gave you a while back and it worked. I tweaked it in and out and ended up at the calculated point. It sounds like that is how you measured it too.
On dish diameter measurement, if you are within an inch or two it is going to get you in the right ballpark, as you can see from the data above, and you can tweak from there. To be technical about it, if there is a rim or structure on the dish edge that doesn't look to be part of the reflecting surface you should not include it in the diameter, nor should it be in the depth. The depth needs to be a pretty good measurement but it sounds like you measured it right.
There is something going on here that apparently none of us has grasped yet. Have you gone through the exercise to see if your dish is flat across the face and if the feed horn is centered? If these two aren't close it can do some strange things.
To have a 34 inch focal length you would need a 17 inch depth for a 96 inch dish. For a 14 inch depth and a 34 inch focus you would need to have an 87 inch dish. I can't believe you could have mismeasured that much. I can't adjust my buttonhook out far enough to be 7 inches past the focal point, I can only do about 2 1/2 inches longer. The focal length is measured from the deepest point on the dish face (the center) to about 1/4 inch farther then the plastic cover on the feed horn. I set mine from the formula I gave you a while back and it worked. I tweaked it in and out and ended up at the calculated point. It sounds like that is how you measured it too.
On dish diameter measurement, if you are within an inch or two it is going to get you in the right ballpark, as you can see from the data above, and you can tweak from there. To be technical about it, if there is a rim or structure on the dish edge that doesn't look to be part of the reflecting surface you should not include it in the diameter, nor should it be in the depth. The depth needs to be a pretty good measurement but it sounds like you measured it right.
There is something going on here that apparently none of us has grasped yet. Have you gone through the exercise to see if your dish is flat across the face and if the feed horn is centered? If these two aren't close it can do some strange things.
Focus confusion...
I know this is crazy, but this is not an off-center fed dish is it? That would totally invalidate the formula for focal distance. An off-center fed dish is a segment of one side of a larger dish, so to apply the formula for focus, I think you would need to know the depth and diameter of the complete centered parabola.
Of course a dish that was mis-shapen would also be an explanation.
Other than those two alternatives, I am befuddled, a parabola is a parabola is a parabola. How could the formula for focal length not work in this case?
I know this is crazy, but this is not an off-center fed dish is it? That would totally invalidate the formula for focal distance. An off-center fed dish is a segment of one side of a larger dish, so to apply the formula for focus, I think you would need to know the depth and diameter of the complete centered parabola.
Of course a dish that was mis-shapen would also be an explanation.
Other than those two alternatives, I am befuddled, a parabola is a parabola is a parabola. How could the formula for focal length not work in this case?
dish focus regression...
Okay, now I am even more confused.
I thought that the formula for the focus of a parabolic dish was:
(D x D) / (16 x d) .... (or "D squared, divided by sixteen times d")
Where:
D = dish diameter
d = the depth of the dish measured between the line used to measure the diameter and the center of the dish.
Well my "Twelve foot dish" measures 11' 6" diameter at the smallest point (squarish scalloped edge design) and 21" deep from that diameter line, which yields a calculation of about 56.68" for a calculated focus, but the manufacturer says the focus is "about 53" and my measurement of where it is set to gives me about 54." Even if I assume a quarter inch error in my depth measurement, I still come up with more than 56" for the focus. Or if I was an inch off in my diameter measurement, I would still get 55.86" for the focus. Both errors combined would yield more than 55" focus! I am pretty sure my measurements are more accurate than that.
I find it hard to believe that I could be getting some Ku channels with 100/85 signals if I was that far off in focus?
The trouble is that changing the focus is not that easy to do on my dish without affecting other adjustments, particularly the centering and perpendicularity of the feed horn. The dish has a shepherd's hook pipe running out from the center with a "U" bolt clamp to adjust it, and four guy wires to keep the horn positioned in the center. These guy wires are under some tension. If I am going to unclamp the center pipe and push the feed horn away from the dish, I am going to have to loosen the guy wires to almost limp and then readjust the whole thing, center the horn, adjust elevation, etc. because even a small change in centering can cause a big change in elevation. This could be a long project just to make a trial re-adjustment in the focus.
I am thinking that a screw adjuster for focus distance would make this a lot more controlled process.
All of which brings me back to my opening comments about the focus calculation. Have I got this calculation wrong somehow? I was pretty sure that was what I had read someplace (wrote it down on my sat dish folder). Could these dishes be built that innaccurately? This dish has a massive frame made from triangulated one square inch box extrusions. I don't see how it could have more than 1/4" mechanical error from edge to edge. So where is this two or three inches of discrepancy coming from?
Hummmm.... Should I try to adjust the focus better? Or should I leave it alone?
Okay, now I am even more confused.
I thought that the formula for the focus of a parabolic dish was:
(D x D) / (16 x d) .... (or "D squared, divided by sixteen times d")
Where:
D = dish diameter
d = the depth of the dish measured between the line used to measure the diameter and the center of the dish.
Well my "Twelve foot dish" measures 11' 6" diameter at the smallest point (squarish scalloped edge design) and 21" deep from that diameter line, which yields a calculation of about 56.68" for a calculated focus, but the manufacturer says the focus is "about 53" and my measurement of where it is set to gives me about 54." Even if I assume a quarter inch error in my depth measurement, I still come up with more than 56" for the focus. Or if I was an inch off in my diameter measurement, I would still get 55.86" for the focus. Both errors combined would yield more than 55" focus! I am pretty sure my measurements are more accurate than that.
I find it hard to believe that I could be getting some Ku channels with 100/85 signals if I was that far off in focus?
The trouble is that changing the focus is not that easy to do on my dish without affecting other adjustments, particularly the centering and perpendicularity of the feed horn. The dish has a shepherd's hook pipe running out from the center with a "U" bolt clamp to adjust it, and four guy wires to keep the horn positioned in the center. These guy wires are under some tension. If I am going to unclamp the center pipe and push the feed horn away from the dish, I am going to have to loosen the guy wires to almost limp and then readjust the whole thing, center the horn, adjust elevation, etc. because even a small change in centering can cause a big change in elevation. This could be a long project just to make a trial re-adjustment in the focus.
I am thinking that a screw adjuster for focus distance would make this a lot more controlled process.
All of which brings me back to my opening comments about the focus calculation. Have I got this calculation wrong somehow? I was pretty sure that was what I had read someplace (wrote it down on my sat dish folder). Could these dishes be built that innaccurately? This dish has a massive frame made from triangulated one square inch box extrusions. I don't see how it could have more than 1/4" mechanical error from edge to edge. So where is this two or three inches of discrepancy coming from?
Hummmm.... Should I try to adjust the focus better? Or should I leave it alone?
Ooooops...
Using your description. I came up with 47 and 3/4 inches.
Al
Edit: Ooooops, I did it again and got 53.7 inches. Sorry.
Using your description. I came up with 47 and 3/4 inches.
Al
Edit: Ooooops, I did it again and got 53.7 inches. Sorry.
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Well don't know for sure, but the discrepancy might come from whether the sat is 22700 miles out or 23300 miles out, or something like that.trscott said:
if I wasn't pleased, I would try more adjustments, if I was pleased, I'd leave it alone. Regardless of what you do, you'll never get it perfect. I've had it so close, just one more will do it. And, screw it up. Be careful and most of all, be patient.
Al
No, good ole' math. It's time to ignore me, done it again and got 59 and 2/3rds. I give up. Now I'm confused. Sorry.....
Al
Edit: your question, I am very curious how you got 53.7?
my answer, me stupid, when I multiplied to get your squared inches, I come up with 18044, should have been 19044, therefore, i used a 3 instead of 9 in my division.
no matter, my latest may be wrong too, me stupid. I quit.
Al
Edit: your question, I am very curious how you got 53.7?
my answer, me stupid, when I multiplied to get your squared inches, I come up with 18044, should have been 19044, therefore, i used a 3 instead of 9 in my division.
no matter, my latest may be wrong too, me stupid. I quit.
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A starting point only...
Whatever number the formula yields is a starting point! Then you move the LNB in and out a little at a time for best signal. my Winegard instructions are very explicit about that! (yes, I still have the instructions and everything else that came with it!) With electronics. I always keep everything!
Jeff
voomvoom said:
Whatever number the formula yields is a starting point! Then you move the LNB in and out a little at a time for best signal. my Winegard instructions are very explicit about that! (yes, I still have the instructions and everything else that came with it!) With electronics. I always keep everything!
Jeff
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