question about F/D

[Captain Midnight]

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.

Now before I get myself totally screwed up, the diameter measurement used is straight across the dish width, and not a measurement of the curved surface of the dish from rim to rim? My dish diameter straight across is 7.5' but the curve is 8' rim to rim. I receive 133w to 43w as it is, so I'm not too concerned, but I like to have to peak performance.

 

[arlo]

Now before I get myself totally screwed up, the diameter measurement used is straight across the dish width, and not a measurement of the curved surface of the dish from rim to rim? My dish diameter straight across is 7.5' but the curve is 8' rim to rim. I receive 133w to 43w as it is, so I'm not too concerned, but I like to have to peak performance.

Correct. Stretch a string line across the diameter of the dish. 2 to make an X would be better.
Where the strings intersect that's where you measure the depth of it.

 

[Captain Midnight]

Correct. Stretch a string line across the diameter of the dish. 2 to make an X would be better.
Where the strings intersect that's where you measure the depth of it.

Ok, so by definition is my dish 7.5ft instead of 8ft? I rather be sure of my equipment. It is a SAMI dish. I tried searching here for proper dish sizing, but most threads were for Ku reflectors. tek2000 doesn't even state how they size their 8ft mesh in the product description...

 

[arlo]

Ok, so by definition is my dish 7.5ft instead of 8ft? I rather be sure of my equipment. It is a SAMI dish. I tried searching here for proper dish sizing, but most threads were for Ku reflectors. tek2000 doesn't even state how they size their 8ft mesh in the product description...

Fuzzy logic. Your measurements will put you right in the ball park. Some dishes have somewhat of a flat rim for rigidity. It's where the parabola starts is where it matters. You can set it all up by the book with lasers and micrometers in the shop and still have adjustments when it gets on site. I still get screwed up by pipe and tubing diameters. One is OD with wall thickenss and the other ID with wall thickness.

 

[Captain Midnight]

Fuzzy logic. Your measurements will put you right in the ball park. Some dishes have somewhat of a flat rim for rigidity. It's where the parabola starts is where it matters. You can set it all up by the book with lasers and micrometers in the shop and still have adjustments when it gets on site. I still get screwed up by pipe and tubing diameters. One is OD with wall thickenss and the other ID with wall thickness.

Now that I think back on my geometry and calculus, what I am really asking is whether dishes are sized strictly by diameter or by the length of the parabolic curve? That might better define the question. I do understand that f/D is calculated strictly with Diameter.

 

[arlo]

Now that I think back on my geometry and calculus, what I am really asking is whether dishes are sized strictly by diameter or by the length of the parabolic curve? That might better define the question. I do understand that f/D is calculated strictly with Diameter.

correct me if i'm wrong. there is only 1 correct formula for a parabola. but some manufacturers make reflectors with a deeper or more shallow focus. that could be a factor why my 80's vintage dish doesn't follow F and f/D calcs.
and why an x band antenna doesn't play nice with ku lnb's. don't sweat the little things. want to get real confused?
research on how a scalar works....or a chaparal scalar....or whatever the scalar du' jour is called.
many different theories, many different explanations, and how they actually work.
if you do the math and place the feed components right where you're supposed to and go from there you've done all you can do.

 

[(((Garyd)))]

whats the best/easiest way to find you F/D.

Peak it for best digital signal, so you don't see sidelobes off another bird

 

[a33]

correct me if i'm wrong. there is only 1 correct formula for a parabola. but some manufacturers make reflectors with a deeper or more shallow focus.

And they would still be parabolas, but with a different focal distance f , and so a different f/D-ratio.

The parabola-formula has the f in it as a variable: x = y² / (4 * f )
(Or, if you like this equivalent better: y² = 4 * f * x )

Greetz,
A33

 

[Captain Midnight]

I just wanted to chime in with my results of fiddling with my feed. I'll post my calculator results, but my focal length was 85.4cm instead of the 85.9cm the calculator gave me.

So 0.38 didn't work nearly as well as the 0.40 setting. I figure this is because it offsets the 0.5cm that I am short...

Would adding a small spacer to the base of my feed legs to reach the correct focal length and setting the LNBF to 0.38 give me much more gain?

 

[Comptech]

I am late to this post, but might have saved you some time had I posted this. Focal Length Distance

 

[arlo]

I just wanted to chime in with my results of fiddling with my feed. I'll post my calculator results, but my focal length was 85.4cm instead of the 85.9cm the calculator gave me.

So 0.38 didn't work nearly as well as the 0.40 setting. I figure this is because it offsets the 0.5cm that I am short...

Would adding a small spacer to the base of my feed legs to reach the correct focal length and setting the LNBF to 0.38 give me much more gain?View attachment 144187

Maybe this will help. Maybe it won't. My dish is considered a "shallow" parabola (?).
After all of the string and tape measurements my LNB was supposed to be stuffed in to the very last (and then some) line on the scale.
I at first tried the original settings that the Corotor was set to. What I found out was the dish was under illuminated by the scalar.
Signal sucked. The "beamwidth was very narrow.

But that was where the original installer placed the Corotor and scalar.
This is where I ended up and signal is great.
It's a button hook feed and I replaced all of the aluminum support tubing with thick wall. It's stiff!
But simply tightening down the LNB set screw where signal was maxed out caused it to drift. Grrrr.
So I grabbed a handful of stainless hardware and made the LNB stationary and the scalar moveable for that last little bit of signal.

 

[Captain Midnight]

Maybe this will help. Maybe it won't. My dish is considered a "shallow" parabola (?).
After all of the string and tape measurements my LNB was supposed to be stuffed in to the very last (and then some) line on the scale.
I at first tried the original settings that the Corotor was set to. What I found out was the dish was under illuminated by the scalar.
Signal sucked. The "beamwidth was very narrow.View attachment 144190 But that was where the original installer placed the Corotor and scalar.
This is where I ended up and signal is great.
It's a button hook feed and I replaced all of the aluminum support tubing with thick wall. It's stiff!
But simply tightening down the LNB set screw where signal was maxed out caused it to drift. Grrrr.
So I grabbed a handful of stainless hardware and made the LNB stationary and the scalar moveable for that last little bit of signal.

That's interesting. I have very good signal reports as is, so I'm not looking to totally overhaul anything right now. But it gives me food for thought.
I have 12.8dB on 105w NBC feeds, 14.7dB on CTV (107.3w), 15.1dB on 97w CBS Encoder (4120V), and 16.1dB on 97w (3940H).

When I changed my LNBF to 0.38, signals suffered, so I went back to 0.40.
I just wonder how much of my dish is under-illuminated, because it is only 7.5' diameter.

 

[Titanium]

Are you also setting the Focal Length setting to be measured approx. 1/4" inside the feedhorn opening?

How are you setting the FD? The scalar looks to be set too far forward for a .38 setting.

Is the button hook mount clamped to the feedhorn and the scalar independently slides fore/aft? Hard to tell from the photo how the feedhorn is mounted.

Looking at your photo, the scalar and feedhorn do not appear to be correctly aligned. The scalar doesn't appear to be parallel to the dish surface and feedhornlooks to be tilted up and not aimed into the center of the dish.

 

[Captain Midnight]

Are you also setting the Focal Length setting to be measured approx. 1/4" inside the feedhorn opening?

How are you setting the FD? The scalar looks to be set too far forward for a .38 setting.

Is the button hook mount clamped to the feedhorn and the scalar independently slides fore/aft? Hard to tell from the photo how the feedhorn is mounted.

Looking at your photo, the scalar and feedhorn do not appear to be correctly aligned. The scalar doesn't appear to be parallel to the dish surface and feedhornlooks to be tilted up and not aimed into the center of the dish.

I know you are referring to Arlo's photo, but I will say I am measuring my focal distance based on the distance from the center plate to 1/4" inside the feedhorn +1cm to account for the center plate being flat and stopping my measurement from the true vertex of the parabola. I also add that extra 1cm to my dish depth.

If that is incorrect, please let me know.

 

[Titanium]

Sorry, didn't notice that the photo was not yours.

FD: 1/4" inside the feedhorn opening is a typical convergence point.

 

[arlo]

Are you also setting the Focal Length setting to be measured approx. 1/4" inside the feedhorn opening?

How are you setting the FD? The scalar looks to be set too far forward for a .38 setting.

Is the button hook mount clamped to the feedhorn and the scalar independently slides fore/aft? Hard to tell from the photo how the feedhorn is mounted.

Looking at your photo, the scalar and feedhorn do not appear to be correctly aligned. The scalar doesn't appear to be parallel to the dish surface and feedhornlooks to be tilted up and not aimed into the center of the dish.

Hey Brian. Nah most of this is covered in my other post. f/D is an unbelievable .32. No sh*t.
The appearance you see (misalignment) is actually casting slop. A square placed across the yoke of the casting and extended to dish center doesn't point right at the center. Even the original install had washers added for shims.
Aluiminum castings and bending don't play nice together. So there you have it.
On the bench I shimmed the scalar bore with nylon strips and guaranteed the LNB is only going to slide straight in and out.
Then with the assy. mounted and it all pointing dead nuts at the dish center and F was in fact that magic 1/4" inside the LNBF throat. That's when I started tweaking for a sig.
Moved the dish first (knew the LNBF was pointing right at the center) for max signal then used the 3 bolts/jam nuts to squeeze that last little bit out of it.
It got cold out. I left it that way. So far so good.

BTW That LNBF has a weak port. One side is a few dB less than the other. Thought it was a cable until it was swapped over with the other.