question about F/D

IntelPennny4

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Oct 28, 2016
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so im setting up another dish image that. i need help with the F/D. i know what it is .42 but do i set it to the outside the ring or inside the ring? thanks
 

kdjuan

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Mar 5, 2006
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primestar31, what measurements on my dish do i need. Any way I can contact you personally.
 

FTA4PA

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whats the best/easiest way to find you F/D.
what measurements on my dish do i need.
You need the dish depth and the dish diameter. To get the dish diameter measure across the face of the dish. To get the dish depth put a string across the face of the dish and measure from the center of the dish to the string. If you can get them in centimeters just enter them into the calculator that primestar31 linked to and you are set. If you have them in inches multiply them by 2.54 first to get centimeters. :)

P.S. Here is the actual formula (using inches) to figure focal length and FD ratio:

C = dish depth
D = dish diameter
F = focal length

F = ( D x D ) / ( 16 x C )

FD Ratio = F / D

Here is an example using a 93 inch wide dish with a depth of 14.5 inches:

93 x 93 = 8649
16 x 14.5 = 232
8649 / 232 = 37.280172414

So the focal length = 37.280172414

To get the FD ratio:

37.280172414 / 93 = .400862069 or .40 :)
 
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primestar31

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kdjuan

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Mar 5, 2006
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iam getting the vertical channels but not getting the horizontal channels what do i need to do
 

KE4EST

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iam getting the vertical channels but not getting the horizontal channels what do i need to do
Please start a new thread with details of your system and what you are trying to do. What satellite? What kind of dish? LNBF? C or Ku? etc.
 
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Captain Midnight

SatelliteGuys Guru
Sep 16, 2019
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Greers Ferry, Arkansas
Not to resurrect an old thread, but I am using the calculator to make sure my feed is at the right height.

For focal length, do I measure from the center of the dish to the front of the scalar ring or to the opening of the LNBF with it set to the f/D given by the calculator? I want to fine adjust my feed.
 

arlo

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Dec 4, 2016
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My trials and tribulations. Use all of your math results as a good starting point.
Cross string your dish to verify that it's not sagged. I believe they say 1/4" max string gap at the dish center. Use that as a reference to aim your LNB at the dish center.
You don't want to peak signal with your LNB while your dish points off center of the sat you're trying to peak with.
Take the scalar distance with a grain of salt. Look at other peoples ideas for using stainless machine screws and nuts to make the scalar distance adjustable. My feed works best with the scalar a touch more towards the dish than what the numbers say.
Being I have a button hook feed made finding that out contrary to doing the math (and sticking with that) a reality.
In the end having the scalar adjustable will assist in getting that last little bit of signal.
And to aim the LNB dead-nuts at the dish center I used 2 spray paint can lids. One that fits snug in the LNB throat. Taped back to back. With the center dimple used to drill a hole to put a straight dowel through.
The LNB end of the dowel just through the hole and the total length enough to point at the dish center. DO not let the jig enter the LNB throat so far that it comes close to the probes.
I got signal. Tweaked the dish position THEN tweaked the LNBF/scalar.
Just a few tips from the school of hard knocks.
 

Captain Midnight

SatelliteGuys Guru
Sep 16, 2019
126
69
Greers Ferry, Arkansas
My trials and tribulations. Use all of your math results as a good starting point.
Cross string your dish to verify that it's not sagged. I believe they say 1/4" max string gap at the dish center. Use that as a reference to aim your LNB at the dish center.
You don't want to peak signal with your LNB while your dish points off center of the sat you're trying to peak with.
Take the scalar distance with a grain of salt. Look at other peoples ideas for using stainless machine screws and nuts to make the scalar distance adjustable. My feed works best with the scalar a touch more towards the dish than what the numbers say.
Being I have a button hook feed made finding that out contrary to doing the math (and sticking with that) a reality.
In the end having the scalar adjustable will assist in getting that last little bit of signal.
And to aim the LNB dead-nuts at the dish center I used 2 spray paint can lids. One that fits snug in the LNB throat. Taped back to back. With the center dimple used to drill a hole to put a straight dowel through.
The LNB end of the dowel just through the hole and the total length enough to point at the dish center. DO not let the jig enter the LNB throat so far that it comes close to the probes.
I got signal. Tweaked the dish position THEN tweaked the LNBF/scalar.
Just a few tips from the school of hard knocks.
And in assembling the dish, the feed arms should always attach to the rear of the scalar ring, and not the front, correct? I just want to confirm. My dish was found with the arms attached to the front of the ring... When I assembled the feed arms with them attached to the rear, my focal length is roughly 2 cm too short for my f/D. I have a good signal, but want to make sure I peak it to the max.
 

arlo

SatelliteGuys Pro
Dec 4, 2016
191
91
North Eastern
And in assembling the dish, the feed arms should always attach to the rear of the scalar ring, and not the front, correct? I just want to confirm. My dish was found with the arms attached to the front of the ring... When I assembled the feed arms with them attached to the rear, my focal length is roughly 2 cm too short for my f/D. I have a good signal, but want to make sure I peak it to the max.
That's where checking things out (doing the math) is paramount.
Could be mixed/matched parts. Could be the neighborhood handyman put it together initially.
Only makes sense that if you block the scalar rings, it can't do its job. So yes. Looks like some fabrication may have to be done.
Perhaps you could drill holes more towards the dish center to space the end of the arms where you attach the scalar to that 2cm longer you need. Or even a little bit farther out and use the spacer screws and locknuts to get it tweaked to perfection.
Making new arms isn't that hard. Finding the aluminum tubing would be. EMT sucks. It rusts. But hey. Any floating piece of wood when the lifeboats and preservers are gone...
Right?
 
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Captain Midnight

SatelliteGuys Guru
Sep 16, 2019
126
69
Greers Ferry, Arkansas
That's where checking things out (doing the math) is paramount.
Could be mixed/matched parts. Could be the neighborhood handyman put it together initially.
Only makes sense that if you block the scalar rings, it can't do its job. So yes. Looks like some fabrication may have to be done.
Perhaps you could drill holes more towards the dish center to space the end of the arms where you attach the scalar to that 2cm longer you need. Or even a little bit farther out and use the spacer screws and locknuts to get it tweaked to perfection.
Making new arms isn't that hard. Finding the aluminum tubing would be. EMT sucks. It rusts. But hey. Any floating piece of wood when the lifeboats and preservers are gone...
Right?
Yes, I know the old days involved a lot of iffy assembly, but analog was so forgiving. I will think of something. I noticed that the tek2000 guide to feed assembly mentions adding 1/2" to your focal length measurement if your dish has a center plate. Mine does, so when I add the 1/2" (1.27cm), that puts my length only 0.73cm (0.28") short... Seems like a pretty low margin of error...
 

arlo

SatelliteGuys Pro
Dec 4, 2016
191
91
North Eastern
Yes, I know the old days involved a lot of iffy assembly, but analog was so forgiving. I will think of something. I noticed that the tek2000 guide to feed assembly mentions adding 1/2" to your focal length measurement if your dish has a center plate. Mine does, so when I add the 1/2" (1.27cm), that puts my length only 0.73cm (0.28") short... Seems like a pretty low margin of error...
I read that. My center plate is not 1/2" thick. If it's warm(er) outside tomorrow I'll go do my actual measurements versus the calcs. I came up when coming up with F and f/d and all of that crap versus my final settings. My arc is from 133W to 47.5W and all things considered I get very good signal. For the other guy I found that looking for the weakest tp's (V&H) on a sat to tweak my skew was the way to go. Strong tp's kind of overwhelm the tweak process. Get a good Sharpie pen.
Also it was a bit of a revelation that if you have a steerable dish you have to hit a happy medium of skew adjustment.
Do that in mid arc. It was 101W for me.
 
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