F/D Calculations? HELP!

[T4Runner]

My C band dish brand unknown, is ten ft (120") diameter and while doing my string test I measured the depth as 20" on the existing dish. However, I will be removing the old fixed feed horn and California LNB and replacing it with a DMX242 and new feed horn. Which means I will be bolting in a new feed horn at a new focal distance? Or the same focal distance as the old one?

i measured the old one from the dish center face plate to the edge of the old LNB was 43 5/8". Somebody here posted a focal distance table which I printed for myself, but nothing matches even close?

as Michael Douglas said to the president's girlfriend "my anxiety rises on several levels".

After replacing the new feed horn, do I set the new DUAL LNBF at 43 5/8"? Or do I calculate a new Focal length? And what's the F/D, so I can set the new LNBF?

 

[primestar31]

Your calculations come out to 45" for focal point. F=(D*D)/(16*d)

So F=(120*120)/(16*20) or 14400/320 = 45

That's more than likely (depending on your lnbf) where it should meet at least 1/4 inch INSIDE of the feedhorn.

Then for F/D you take F (45) and divide by D (120) = 0.375

Don't stress over this stuff, it's TV. It's supposed to be FUN and keep your brain engaged a little. If you just can't wrap your head around something, ask a question and continue to HAVE FUN!

P.S. Give me a pic of the dish polar mount. and the back center of the dish. Maybe I can id it.

 

[T4Runner]

Thanks for the statistics. Fun? Fun? Oh yeah, I remember now. Thats when it all comes together again.

the picture of the dish on the pole is before I removed the dish from the Craigs list seller, and the picture on the ground is here on the ground.

 

[primestar31]

Ok, it would be easier with a pic of the top polar bar aimed from the back on the dish, but from what I can see, it's a dead ringer for a "Perfect Ten" dish. "Perfect Ten" polar bars are very narrow and long (compared to everybody elses), and really should have been beefed-up a little better.

If it is a PT dish, Phlatwound end of last year sent me a pristine full size 11x17" assembly manual for those that covers the 7.5, 10ft, and 12ft versions, and I imaged it full-size, and uploaded it here. That manual has everything in it you need, and if that IS a Perfect Ten, you'd be wise to follow the directions exactly the way they lay them out (I tried doing it MY way, and it just wouldn't work right. Follow their exacting directions, and it all falls into place nicely). IF you set everything absolutely according to their directions, you will have a superior working dish, that's well worth the trouble.

Here's the manual: http://www.satelliteguys.us/downloads.php?do=file&id=329

 

[T4Runner]

Ok, it would be easier with a pic of the top polar bar aimed from the back on the dish, but from what I can see, it's a dead ringer for a "Perfect Ten" dish. "Perfect Ten" polar bars are very narrow and long (compared to everybody elses), and really should have been beefed-up a little better.

If it is a PT dish, Phlatwound end of last year sent me a pristine full size 11x17" assembly manual for those that covers the 7.5, 10ft, and 12ft versions, and I imaged it full-size, and uploaded it here. That manual has everything in it you need, and if that IS a Perfect Ten, you'd be wise to follow the directions exactly the way they lay them out (I tried doing it MY way, and it just wouldn't work right. Follow their exacting directions, and it all falls into place nicely). IF you set everything absolutely according to their directions, you will have a superior working dish, that's well worth the trouble.

Here's the manual: http://www.satelliteguys.us/downloads.php?do=file&id=329

Thank you for taking the time to address this. Ive been having some trouble lately with downloading to my IPAD. May have to go to my desktop. Thanks again.


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[T4Runner]

Downloaded the file and it looks pretty close. Ill look tomorrow during daylight.

 

[T4Runner]

Okay, downloaded the manual for the Perfect 10 and read it closely. There is a confusing statement on it about a difference between measured distances and actual mathematical focal points?

The Old EXISTING actual measurement is 43 5/8" to the tip of the LNB from the dish center.

The manual says the actual mathematical focal point is 45" which matches the above calcs.

But the manual states the actual measured distance should be 44 5/8".

I'll post a better picture of the back of the dish when I get a chance to have someone hold it up for me. But it looks a lot like the Perfect 10 manual.


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[FaT Air]

The manual says the actual mathematical focal point is 45" which matches the above calcs.
But the manual states the actual measured distance should be 44 5/8".

Focal length calculates to 45. Measured distance from dish to feed throat should be 44 5/8"
Yep. that should be correct. Puts the focal point 3/8" inside the throat.
Although the manual, and calculations, say some distance it's best to double check the accuracy and test the focal length by fine tuning in/out for best performance.
Maybe that was done already, and is the reason for it being an inch less, in it's present form.

 

[primestar31]

It's because the calculated focal point is from the backside of the 1/8" thick middle plate on the dish. Just set it according to the drawings, and it'll be perfect. They obviously did it wrong when it was first installed for the original owner. The focal point itself is inside the waveguide a bit, like FaT Air says.

 

[T4Runner]

It's because the calculated focal point is from the backside of the 1/8" thick middle plate on the dish. Just set it according to the drawings, and it'll be perfect. They obviously did it wrong when it was first installed for the original owner. The focal point itself is inside the waveguide a bit, like FaT Air says.

Okay, thank you gentlemen. I'll stick to the manual.


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[T4Runner]

Okay, new question:

I have two pipes that could be used for my ten ft C band dish, a 9ft long 1/4" thick heavy pipe, and a 12ft 1/8" thick,pipe.

which one is recommended for a ten ft dish, or does it matter?

No snow here, so no problems with having the dish too low, except I mow the weeds....

 

[primestar31]

How is your wind load out there? High winds frequently? 4 ft in the ground, and 6 feet above for a 10 ft dish gives you about 1.5 foot clearance between the ground and the absolute lowest you could aim it with a motor.

Can you live with only 1~ foot clearance, and stay away from it with lawnmowers and such? Any critters that can jump that high? I used to have a cat that could jump 6 feet straight UP when she had a mind to do so. Not that a cat could cause much damage. It's things like deer, livestock with itchy skin, and larger you'd have to watch out for.

p.s. Yet of course you don't want it so high off the ground, that you can't reach the feed or lnb's with a stepladder.

 

[T4Runner]

How is your wind load out there? High winds frequently? 4 ft in the ground, and 6 feet above for a 10 ft dish gives you about 1.5 foot clearance between the ground and the absolute lowest you could aim it with a motor.

Can you live with only 1~ foot clearance, and stay away from it with lawnmowers and such? Any critters that can jump that high? I used to have a cat that could jump 6 feet straight UP when she had a mind to do so. Not that a cat could cause much damage. It's things like deer, livestock with itchy skin, and larger you'd have to watch out for.

p.s. Yet of course you don't want it so high off the ground, that you can't reach the feed or lnb's with a stepladder.

Okay, got to chew on this awhile.... Thanks all.



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[T4Runner]

Okay, I removed the old LNB and feed horn and as stated above, previously measured the focal length of the existing before I did so. But it was off by an inch, so I set the new one to the exact measurements in the PerfectTen manual recommended.

its now set at 44 5/8" + 1/8" thickness of face plate on dish + 1/4" insidethe LNBF=45"

Working on installing the new pole now.

 

[primestar31]

Okay, I removed the old LNB and feed horn and as stated above, previously measured the focal length of the existing before I did so. But it was off by an inch, so I set the new one to the exact measurements in the PerfectTen manual recommended.

its now set at 44 5/8" + 1/8" thickness of face plate on dish + 1/4" insidethe LNBF=45"

Working on installing the new pole now.

Ok, let's clarify that: the measurement from the face of the middle plate, to the outer edge (what the arrow in the drawing is pointing at) of the feedhorn should be exactly 44-5/8".

Is that what you meant? Because if not, it's not set right.

That measurement already accounts for the thickness of the middle plate AND the fact that the actual focal point is slightly inside the feedhorn.

 

[T4Runner]

Ok, let's clarify that: the measurement from the face of the middle plate, to the outer edge (what the arrow in the drawing is pointing at) of the feedhorn should be exactly 44-5/8".

Is that what you meant? Because if not, it's not set right.

That measurement already accounts for the thickness of the middle plate AND the fact that the actual focal point is slightly inside the feedhorn.

Yes. I stand corrected. 44 5/8" to the face. Incidentally, the dish must be a PERFECT TEN as the manual looks exactly the same.

 

[AntAltMike]

I'm skeptical about the advice that the compensation for the faceplate is its thickness, because the faceplates I've installed were flat, rather than parabolic, and therefore the difference in distance between the front, center surface of the faceplate and the parabola itself was surely more than half an inch. So I'd add half an inch or more to the depth dimension used in the focal length calculation, and then I'd subtract that same amount from the calculated focal length.

Exact locating of the waveguide opening is much more important on a deep dish, like a Winegard Pinnacle (F/D .28, as I recall) than on shallow Ku dishes (around .6). I remember that Pinnacle owners loved those dishes, but I was always skeptical about their inefficiency in collecting signal from the outer edge region of their reflectors using conventional feedhorns. As I recall, Seavey made a feedhorn where the probe was right out there at the wave guide opening, that looked like it would "illuminate" a deep dish better than the Chaparral feedhorn did... but it probably cost ten times as much.

 

[T4Runner]

I'm skeptical about the advice that the compensation for the faceplate is its thickness, because the faceplates I've installed were flat, rather than parabolic, and therefore the difference in distance between the center of the faceplate and the parabola itself was surely more than half an inch. So I'd add half an inch or more to the depth dimension used in the focal length calculation, and then I'd subtract that same about from the calculated focal length.

Exact locating of the waveguide opening is much more important on a deep dish, like a Winegard Pinnacle (F/D .28, as I recall) than on shallow Ku dishes (around .6). I remember that Pinnacle owners loved those dishes, but I was always skeptical about their inefficiency in collecting signal from the outer edge region of their reflectors using conventional feedhorns. As I recall, Seavey made a feedhorn where the probe was right out there are the wave guide opening that looked like it would "illuminate" a deep dish better than the Chaparral feedhorn did... but it probably cost ten times as much.

you may very well be right. The original measurements were an inch different. I will find out within a week or so, as i will set the dish on the pole as soon as my concrete cures.

 

[primestar31]

I'm skeptical about the advice that the compensation for the faceplate is its thickness, because the faceplates I've installed were flat, rather than parabolic, and therefore the difference in distance between the center of the faceplate and the parabola itself was surely more than half an inch. So I'd add half an inch or more to the depth dimension used in the focal length calculation, and then I'd subtract that same about from the calculated focal length.

Exact locating of the waveguide opening is much more important on a deep dish, like a Winegard Pinnacle (F/D .28, as I recall) than on shallow Ku dishes (around .6). I remember that Pinnacle owners loved those dishes, but I was always skeptical about their inefficiency in collecting signal from the outer edge region of their reflectors using conventional feedhorns. As I recall, Seavey made a feedhorn where the probe was right out there are the wave guide opening that looked like it would "illuminate" a deep dish better than the Chaparral feedhorn did... but it probably cost ten times as much.

Do not follow these directions for a Perfect Ten dish. It won't be right. Follow the manual EXACTLY. It's true it's not exactly the thickness of the center plate, but since the manual was written for an "average guy" that doesn't need deep thought involved, just use the install manual instructions and think happy thoughts.

Trust me, I tried it different ways on my Perfect Ten, and it doesn't work correctly any other way.

 

[T4Runner]

Do not follow these directions for a Perfect Ten dish. It won't be right. Follow the manual EXACTLY. It's true it's not exactly the thickness of the center plate, but since the manual was written for an "average guy" that doesn't need deep thought involved, just use the install manual instructions and think happy thoughts.

Trust me, I tried it different ways on my Perfect Ten, and it doesn't work correctly any other way.

Are you trying to tell me pain and suffering are forthcoming if I deviate from the manual? LOL. I haven't even given you a picture of the back of the dish yet to double verify it is a PERFECT ten. But I get your point. Its set according to the manual to start with.

Im actually more concerned about the Zero SKEW setting as I was hoping to set this at the 12 O'clock position (or 3 or 6 or whatever this LNBF is set at) before I set it on the pole, but the dish was pointed over at 91W or something when I pulled it and I didn't bother paying attention to the top of the dish at true South, so I'll end up setting it after it gets on the pole, and after I put the actuator back on and run it to due South.




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