I can't make the numbers work (1 Viewer)

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wvman

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A friend of mine gave m a chart that shows the focal distances for various dishes, like KTI, Uniden and so forth. Using the formula, F=(D*D) / (16*C) I can't make the numbers work on the chart he gave me. His dish is a 10 foot diameter Unimesh and its 19.25 inches deep. The chart says the focal distance for his dish is 48 inches. Using the formula above, I come up with 46.75, not 48.

I've been out of school for 46 years and my math skills may be a little rusty. I'm curious, whose right, the charts or me. :) The focal lengths on the chart he gave me are supposed to be manufacturer specifications. :confused:
 
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primestar31

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A friend of mine gave m a chart that shows the focal distances for various dishes, like KTI, Uniden and so forth. Using the formula, F=(D*D) / (16*C) I can't make the numbers work on the chart he gave me. His dish is a 10 foot diameter Unimesh and its 19.25 inches deep. The chart says the focal distance for his dish is 48 inches. Using the formula above, I come up with 46.75, not 48.

I've been out of school for 46 years and my math skills may be a little rusty. I'm curious, whose right, the charts or me. :) The focal lengths on the chart he gave me are supposed to be manufacturer specifications. :confused:

Prime Focus Satellite Antenna reflector calculation

46.75 is correct, if you measured right. HOWEVER, the TRUE focal point is actually located from 1/2 the rib depth point between the top and bottom center plates of the dish, and to 1/4inch inside the feedhorn after it's set to it's FD ratio. NOT just to the surface of the topside center plate... Also, factory posted specs have been known to be WRONG, or to show from the surface of the center plate and to the front edge of the scalar when set to it's proper FD ratio. That's why we measure and calculate ourselves.
 
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wvman

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Prime Focus Satellite Antenna reflector calculation

46.75 is correct, if you measured right. HOWEVER, the TRUE focal point is actually located from 1/2 the rib depth point between the top and bottom center plates of the dish, and to 1/4inch inside the feedhorn after it's set to it's FD ratio. NOT just to the surface of the topside center plate... Also, factory posted specs have been known to be WRONG, or to show from the surface of the center plate and to the front edge of the scalar when set to it's proper FD ratio. That's why we measure and calculate ourselves.

"HOWEVER, the TRUE focal point is actually located from 1/2 the rib depth point between the top and bottom center plates of the dish, and to 1/4inch inside the feedhorn after it's set to it's FD ratio." That's exactly what I told him. I didn't think my math skills were that rusty. :D He couldn't understand measuring from 1/2 the rib depth, but he did understand measuring 1/4 inch inside the feed throat. But in any case, I couldn't make the numbers on the chart work. :confused:

At least the FD ratio on the chart was correct. I'm guessing the chart he gave me was someone's best guess rather than actual manufacture's data.
 

ancient

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I don't think I've ever done those calculations and then found that they actually gave the best signal - I usually just wind up sliding the feedhorn in and out of the scalar ring until I see the best readings on the receiver. Sometimes I've only gained a percentage point or two but wherever you get the peak signal is where you want it. It's the same thing with the skew, I just rotate the feedhorn one direction or the other until I see the best readings. It's a little hard with some LNB's because the scalar rings are so "sloppy" (the hole in the ring is quite a bit larger than the body of the feedhorn, relatively speaking) and you are trying to keep the LNB pointed at the center of the dish while still getting the best skew and focal depth positions but if you can watch the receiver, or failing that a good signal strength meter while you are doing the positioning, you will find the point where the best signal strength and SNR is achieved. That's really the best you can hope for if you don't have expensive tuning equipment or some mechanical aid to assist you in precisely pointing the feedhorn at the exact center of the dish.
 

wvman

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Those are technically correct calculations but, as I recall, you'll probably have to fine tune the feed.

As with anything, there's always a certain amount of tweaking that needs to be done. :) I've been through that with one of my dishes recently. Part of my mistake was assuming that the other guy got it right on a dish I recently picked up. I'm sure you're aware of what "Assume" does to you. :D
 

wvman

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I don't think I've ever done those calculations and then found that they actually gave the best signal - I usually just wind up sliding the feedhorn in and out of the scalar ring until I see the best readings on the receiver. Sometimes I've only gained a percentage point or two but wherever you get the peak signal is where you want it. It's the same thing with the skew, I just rotate the feedhorn one direction or the other until I see the best readings. It's a little hard with some LNB's because the scalar rings are so "sloppy" (the hole in the ring is quite a bit larger than the body of the feedhorn, relatively speaking) and you are trying to keep the LNB pointed at the center of the dish while still getting the best skew and focal depth positions but if you can watch the receiver, or failing that a good signal strength meter while you are doing the positioning, you will find the point where the best signal strength and SNR is achieved. That's really the best you can hope for if you don't have expensive tuning equipment or some mechanical aid to assist you in precisely pointing the feedhorn at the exact center of the dish.

It has been my experience that the signal quality is not consistent between different brands of receivers. right now, I am running an Amiko and it shows a signal of 98/87 on Grit. I can unhook it and put the receiver I was using before in its place, and it will show 79/67 on the same channel. Since I installed the Amiko, I've been watching the signal on Grit and it varies from 98/87 to 98/72 without doing a thing to the dish.

Through the day with a clear sky, it will show 98/87, but the picture breaks up. At other times when the signal is 98/72, it doesn't break up at all. :confused: I've been putting off buying a meter off Titanium, but I really need to do that so I can actually see how the signal stacks up to the reading supplied by the receivers.

"BTW, the Amiko has the absolute best HD video of any receiver I've ever used."
 

wvman

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I don't think I've ever done those calculations and then found that they actually gave the best signal - I usually just wind up sliding the feedhorn in and out of the scalar ring until I see the best readings on the receiver. Sometimes I've only gained a percentage point or two but wherever you get the peak signal is where you want it. It's the same thing with the skew, I just rotate the feedhorn one direction or the other until I see the best readings. It's a little hard with some LNB's because the scalar rings are so "sloppy" (the hole in the ring is quite a bit larger than the body of the feedhorn, relatively speaking) and you are trying to keep the LNB pointed at the center of the dish while still getting the best skew and focal depth positions but if you can watch the receiver, or failing that a good signal strength meter while you are doing the positioning, you will find the point where the best signal strength and SNR is achieved. That's really the best you can hope for if you don't have expensive tuning equipment or some mechanical aid to assist you in precisely pointing the feedhorn at the exact center of the dish.

"some LNB's because the scalar rings are so "sloppy" (the hole in the ring is quite a bit larger than the body of the feedhorn" You may want to try what I did. Get three radiator hose clamps with the worm gear. Hook them together so the worm gears are 120 degrees apart. Slide them over the outside of the feed, figure out where you want the feed set and slide the clamp in place and tighten slightly.

This gives you three anchor points that allow you to rotate the feed while keeping it centered on the dish. Once you're done, just leave the clamp there after you're done. It's a easy fix to the problem of sloppy scalars.
 

johnnynobody

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It has been my experience that the signal quality is not consistent between different brands of receivers. right now, I am running an Amiko and it shows a signal of 98/87 on Grit. I can unhook it and put the receiver I was using before in its place, and it will show 79/67 on the same channel. Since I installed the Amiko, I've been watching the signal on Grit and it varies from 98/87 to 98/72 without doing a thing to the dish.

Through the day with a clear sky, it will show 98/87, but the picture breaks up. At other times when the signal is 98/72, it doesn't break up at all. :confused: I've been putting off buying a meter off Titanium, but I really need to do that so I can actually see how the signal stacks up to the reading supplied by the receivers.

It would be nice if these receivers could be properly calibrated so that signal strength and signal quality would be more accurate. Or have an IF loop so that you could attach a calibrated instrument, such as my "fancy-dancy" XR-3. But, if you're at the pad with an XR-3 then there would be no need to have a receiver at the pad. But, the XR-3 is $$$.
 
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wvman

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It would be nice if these receivers could be properly calibrated so that signal strength and signal quality would be more accurate. Or have an IF loop so that you could attach a calibrated instrument, such as my "fancy-dancy" XR-3. But, if you're at the pad with an XR-3 then there would be no need to have a receiver at the pad. But, the XR-3 is $$$.

As far as the calibration, with some made in Korea, other made in China and still more made somewhere else, an industry standard for reading signal levels probably isn't going to happen. It's an added cost that can't be applied to a $25 or $50 receiver. Well, I guess they could, but it would unrealistic when you're selling something as inexpensive as they're selling. I guess we should be grateful they work as well as they do. :)

My biggest beef with most satellite receivers is their relatively short and narrow IR range.
 

johnnynobody

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As far as the calibration, with some made in Korea, other made in China and still more made somewhere else, an industry standard for reading signal levels probably isn't going to happen. It's an added cost that can't be applied to a $25 or $50 receiver. Well, I guess they could, but it would unrealistic when you're selling something as inexpensive as they're selling. I guess we should be grateful they work as well as they do. :)

My biggest beef with most satellite receivers is their relatively short and narrow IR range.

You get what you pay for. Since I've paid $1000 for a 4DTV it's logical to assume that I'm willing to pay a lot more for a receiver that is properly designed. I don't remember what I paid for the HD decoder for the 4DTV STB. But, I'm not going to pay $1000 for another STB unless there's support for at least 10 years and if there are subs available.
 
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wvman

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You get what you pay for. Since I've paid $1000 for a 4DTV it's logical to assume that I'm willing to pay a lot more for a receiver that is properly designed. I don't remember what I paid for the HD decoder for the 4DTV STB. But, I'm not going to pay $1000 for another STB unless there's support for at least 10 years and if there are subs available.

It's not likely there will ever be home based programming available again. Those days are long gone. I got in on the ground floor in the satellite industry and they sold so fast, the program providers saw a great market potential and they offered packages at a reasonable price. Then along came Dish and DirecTV and the BUD market dwindled to near nothing. I happened to get in at the right time, and we did great.

Finding a ground floor market these days is tough. Living in rural WV didn't hurt either. We got in BUDs by the truckload. Most of the time they were sold before they arrived. We saw the writing on the wall when the small dishes came out and we sold everything we could get our hands on before they took over the market. We had 3800 Dish and DirecTV customers as well as the BUDs. Those were the good old days.
 
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johnnynobody

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It's not likely there will ever be home based programming available again. Those days are long gone. I got in on the ground floor in the satellite industry and they sold so fast, the program providers saw a great market potential and they offered packages at a reasonable price. Then along came Dish and DirecTV and the BUD market dwindled to near nothing. I happened to get in at the right time, and we did great.

Finding a ground floor market these days is tough. Living in rural WV didn't hurt either. We got in BUDs by the truckload. Most of the time they were sold before they arrived. We saw the writing on the wall when the small dishes came out and we sold everything we could get our hands on before they took over the market. We had 3800 Dish and DirecTV customers as well as the BUDs. Those were the good old days.

That's too bad since it's generally known that a C-band BUD will outperform a Ku-band pizza dish - especially during rain fade or snow/ice in the Ku dish. BUD's are expensive (and more time consuming to install) but so is the programming for the pizza dish especially if you subscribe to the higher tier subs - which a lot of people do. And those pizza dish introductory pricing don't last for long. Like I said, you get what you pay for. Oh well, I'm done with this thread.
 
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primestar31

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I totally agree. It's very unlikely there will ever be another subscription service for the few BUD users that are left. It doesn't make all that much difference anyway, as there's so much FREE stuff up there to watch, why would I ever want to pay for anything again?
 
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wvman

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I totally agree. It's very unlikely there will ever be another subscription service for the few BUD users that are left. It doesn't make all that much difference anyway, as there's so much FREE stuff up there to watch, why would I ever want to pay for anything again?

I'm done with paying for channels I don't watch. That's the problem with the small dish, you get 300 channels, or so they call them in some instances, and you watch 10 or 12. People are under the impression when they purchase a 150 channel package, they're getting 150 watchable channels, and soon find out 7/8 of them are filler channels like music channels, shopping networks or some other channels that don't amount to a hill of beans for most people.

I'd consider paying for TV when someone steps up to the plate and gives you the option to purchase only the channels you want. I've looked over Sling TV's packages and they offer very little I'd be interested in. Even when DISH had their Dish Pix package, they limited what you had to choose from and it didn't last long either. Oh well, eventually streaming will replace both Dish and DirecTV before it's over, but they'll have to get off their butt and get better internet coverage in rural areas before that will happen. :)
 

sgs

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"some LNB's because the scalar rings are so "sloppy" (the hole in the ring is quite a bit larger than the body of the feedhorn" You may want to try what I did. Get three radiator hose clamps with the worm gear. Hook them together so the worm gears are 120 degrees apart. Slide them over the outside of the feed, figure out where you want the feed set and slide the clamp in place and tighten slightly.

This gives you three anchor points that allow you to rotate the feed while keeping it centered on the dish. Once you're done, just leave the clamp there after you're done. It's a easy fix to the problem of sloppy scalars.

If it's not too much to ask, could you share a pic or two? I'm trying to visualize this.

Thanks!
sgs
 
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wvman

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If it's not too much to ask, could you share a pic or two? I'm trying to visualize this.

Thanks!
sgs

Since I posted this, the LNBF I had this set upon went bad and I put an old LNBF that I bought years ago on in its place that didn't have all the slop between the scalar and feed, but it's not hard to duplicate. All you need is three hose clamps, just like the ones you use of heater or radiator hoses. Get the shorter ones and screw them together end to end, spacing each one 1/3 the way around the feed throat.

Tighten the clamps slightly, just enough to keep it in place. You'll need to know the focal distance for your dish. Make sure your scalar is centered properly on the dish. With the clamps on the feed, slide it through the scalar, checking to make sure that its set at the proper focal distance for your dish. If its too short or long, move the clamps up or down the feed throat until your focal distance is correct.

Now that it's set to the proper focal distance, you can rotate the feed left or right to adjust the polarity without losing center on the scalar. Be sure to do the final tightening on the hose clamps and scalar screws to keep it in place. I left the clamps on mine. That way of you have to change an F-Connector or check for moisture, you won't have to go through the process again. Be sure to mark the scalar and feed throat with a perminant marker so you have a reference point if you need to take the feed off again.

I'd take a picture, but my wife threw the old feed in the trash before I had a chance to remove the clamps. I think I can dig up some clamps tomorrow to show you how I had them positioned. It's really not as complicated as it sounds. :)
 
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sgs

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Ahhh I get it now... you used the hose clamps to get the feed positioned at the focal distance, and then you're free to adjust the polarity. Makes sense... thanks!

sgs
 
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