F/D Ratio Question for 12 Ft Dish

[jsattv]

Calculated F/D Ratio Versus Actual or what works:
I measured my 12 Foot Dish Diameter last November and it was 145 7/8 inches (145.875 inches). My dish depth = 23 3/4 inches. Using the Formula:
F/D Ratio = D*D/ 16 X d I came up with: 12.15625 X 12.15625/ 16 X 23.75 =
147.77 / 380 = 0.3888. But when I was adjusting my Corotor II LNB I found out that when I adjusted F/D Ratio for 0.38 I couldn't get a signal. When I set F/D back to 0.36 I was receiving the Sat Signals. At that time my declination Angle was at 10 degrees when it should be at 6.61 degrees for my location.

My Question: Now that I have re adjusted my Declination Angle should I try to re adjust the F/D Ratio to 0.38 to get a better Sat Signal?? Any replies would be appreciated.

 

[Hermitman]

There are several factors related to the proper positioning of a lnb. F/D ratio is only one. You need to make sure the distance between the lnb and the dish face is set as calculated. You also need to make sure the lnb is pointing directly to the middle of the dish and parallel to the dish face. It sounds like the lnb distance to the dish face isn't set properly. Declination and F/D ratio have no relationship. You should set the declination once to the proper degrees and then forget about it. Good luck.

 

[jsattv]

There are several factors related to the proper positioning of a lnb. F/D ratio is only one. You need to make sure the distance between the lnb and the dish face is set as calculated. You also need to make sure the lnb is pointing directly to the middle of the dish and parallel to the dish face. It sounds like the lnb distance to the dish face isn't set properly. Declination and F/D ratio have no relationship. You should set the declination once to the proper degrees and then forget about it. Good luck.

Thanks very much for the reply Hermitman. When you say "It sounds like the lnb distance to the dish face isn't set properly". - I'm not sure what you mean, that is, do you mean the LNB assembly may NOT be parallel or needs to be set closer to the center of the Dish?? With my setup you can reposition the Depth of where the LNB is sitting in the Scalar Ring if my terminology is correct.

On Friday carefully standing on a ladder in the snow with my Trimax Satellite meter (with the Strap around my neck) I checked a couple of Tp's on G25 - 97.0W. I also tried to readjust and re position the entire Corotor II LNB but nothing changed much. I still couldn't pickup Tp's 12166, and 12155 which is a strong Tp. There was one spot where when I set the LNB a bit deeper - the signal seemed to get a bit better, but rotating the whole LNB away from 11 and 5 o'clock didn't seem to help much. Maybe I'll take some closup pictures of the LNB today. Any replies would be much appreciated.

 

[Hermitman]

I think I've found a couple of problems with your math. First, the formula you used is for the focal distance. And the units in the formula need to all be the same (all inches or all feet).

Focal Distance = (DxD)/(16xd)
f/D Ratio = Focal Distance/Dish Diameter

Using your numbers, focal distance = (145.875x145.875)/(16x23.75) = 55.998 inches

f/D Ratio = 55.998/145.875 = .3838

The f/D ratio is the number you set your lnb to in the scalar ring. This is important when it comes to antenna match, gain, cross-polarization and pattern symmetry.

The focal distance is measured from the deepest part of the dish to 1/4 inch inside the lnb throat. This is also important because if you're off by as little as 1/2 inch, your signal strength can be reduced to the lnb by 50%.

Beside these measurements, you should make sure the lnb is pointed directly toward the center of the dish and the scalar ring face is parallel with the dish rim. More info can be found at the below webpage. Good luck.

C-band Satellite Installation

 

[jsattv]

I think I've found a couple of problems with your math. First, the formula you used is for the focal distance. And the units in the formula need to all be the same (all inches or all feet).

Focal Distance = (DxD)/(16xd)
f/D Ratio = Focal Distance/Dish Diameter

Using your numbers, focal distance = (145.875x145.875)/(16x23.75) = 55.998 inches

f/D Ratio = 55.998/145.875 = .3838

The f/D ratio is the number you set your lnb to in the scalar ring. This is important when it comes to antenna match, gain, cross-polarization and pattern symmetry.

The focal distance is measured from the deepest part of the dish to 1/4 inch inside the lnb throat. This is also important because if you're off by as little as 1/2 inch, your signal strength can be reduced to the lnb by 50%.

Beside these measurements, you should make sure the lnb is pointed directly toward the center of the dish and the scalar ring face is parallel with the dish rim. More info can be found at the below webpage. Good luck.

C-band Satellite Installation

Thanks a lot for the correction Hermitman that is a great webpage. What I don't understand is when I adjusted the Corotor II LNB to the mark at 0.38 previously I couldn't get a signal at all!! But when I set it for 0.36 I got Satellite Signals. Maybe I need to try it again at 0.38 and re adjust the LNB Depth in the Scalar Ring???

 

[Hermitman]

Here's some of the theory. When you place the lnb in the scalar ring at the .3838 setting and lock it down, the lnb and scalar ring become one electronic unit. This unit is now ready to receive the satellite signals reflected off the dish and most effectively and efficiently guide those signals down the throat to the lnb probe. The dish is designed to reflect satellite signals to a single focal point about the size of a golf ball. And this focal point on your dish was calculated to be 55.998 inches. The idea is to now move the lnb/scalar ring unit so the focal point is just inside the entrance of the lnb opening. The signals are then guided down to the lnb probe and then to your receiver. Its quite possible that when you moved the lnb from .38 to .36, you actually moved the lnb throat to a position that now is closer to the focal point and signals are now going down the lnb throat to the probe. In my opinion, I would try setting the lnb to .3838, the focal distance to 55.998 inches and see what happens. To check for a better signal, the first thing I would change is the focal distance slightly in or out while leaving the lnb/scalar ring setting unchanged. Good luck.

 

[Smith P.]

I don't see what's wrong with the adjustment in real environment, theoretical number should be used as preliminary values for start tweaking to obtain better results. I wouldn't worried about calculated values ( is your dish's parabola is 100% correct ? ) and will use adjusted position.

 

[Anole]

agreed

True, the calculations should get you very close, but then it's up to you to optimize in real time.

I get a chuckle out of the many newbie users setting up their Ku dish for the first time.
They dial in the elevation, skew, and azimuth, and expect the bird to be right there.
Some dishes have an elevation error.
It's easy to misunderstand which way the skew goes (or ignore it all together).
Many seem to easily run afoul of a compass, too. -

I'm all for doing some calculations to find out where you should be, but then you tune for maximum smoke! - :up

 

[jsattv]

Thanks guys, with some warm weather scheduled for Thursday and Friday I'll climb the ladder and try to check the focal distance for 55.9 inches and then move that LNB setting back to 0.38 and see what happens.

 

[jsattv]

Here's some of the theory. When you place the lnb in the scalar ring at the .3838 setting and lock it down, the lnb and scalar ring become one electronic unit. This unit is now ready to receive the satellite signals reflected off the dish and most effectively and efficiently guide those signals down the throat to the lnb probe. The dish is designed to reflect satellite signals to a single focal point about the size of a golf ball. And this focal point on your dish was calculated to be 55.998 inches. The idea is to now move the lnb/scalar ring unit so the focal point is just inside the entrance of the lnb opening. The signals are then guided down to the lnb probe and then to your receiver. Its quite possible that when you moved the lnb from .38 to .36, you actually moved the lnb throat to a position that now is closer to the focal point and signals are now going down the lnb throat to the probe. In my opinion, I would try setting the lnb to .3838, the focal distance to 55.998 inches and see what happens. To check for a better signal, the first thing I would change is the focal distance slightly in or out while leaving the lnb/scalar ring setting unchanged. Good luck.

Got Some Improvement Today:
Hermitman, took your advise and moved the lnb setting to 0.38 today and also moved the LNB assembly lower and had some success. The first 2 pics show where the position of the LNB was when it was set for 0.36, and the last 2 pics show where it is now set for 0.38. S has improved about 2 points but the all important Q has improved about 10 - 12 points on ALL Tps. From looking at the pictures and looking at the LNB from the Ground I don't think it is completely square in the Ring so I guess I could work to improve that.

Unfortunately I'm still not getting 11966..... but what is confusing is that where its Q was intermittant at 0 to 10 before, it is now jumping from Q = 0 to 35 ?? So the Signal for this Tp looks like it is getting stronger, the same can be said for Tp 12155, still can't pull it in. Or would it help to run another Blind Scan for G25 / 97.0W?? Also it seems pressing the Skew for my Polorotor Control on my old Uniden 9900 Receiver is NOT working any more. Any replies would be very much appreciated.

 

[Hermitman]

If your skew adjustment isn't working, that's a big problem. Ku signals tend to be more sensitive to the proper skew setting than c band. Check your connections and maybe have someone listen for the servo movement outside while you activate the controls inside. Or you could bring the corotor unit inside, connect it directly to the receiver and look down the throat to see if the wire hook in there is rotating. Good luck.

 

[gpat]

heres how I squared mine up initally.
set it up with the correct spacers under the feedhorn for the focal distance needed.
loosen the set screws on the co rotor
push down on corotor(plastic cap removed) until it sits square with flat surface its on.
tighten set screws.

then when you bolt it up to the feedhorn supports use something to slide into the throat of feedhorn(careful not to hit the probe).
I used a piece of EMT that fit thru the hole in dish and snuggly into the feedhorn on the first step (it wouldnt go far enough in to hit the probe anyway).
On my old feedhorn the same pipe would go all the way in.

adapter I made for 1-1/4 EMT to feedhorn throat size(2 inch threaded pipe)

everything is now squared up and perfectly aligned.


Also want to add that if you unhook the actuator the dish is easy to work on
just leave it hanging by the pivot bolt

 

[gabshere]

one thing i noticed in your photos is your scaler isn't square/level

i know we discussed on this site about using a laser pointer and pointing back to the feedhorn cover to ensure it is correctly centered i hope to make a cheap one soon. and mount it to either a smal carpenters square or something flat metal that i can glue magnets to and get it square.

sounds like your getting close

gpat hopefully when i veiw the photos at home i can see your level device / sounds like we are trying to achive the same thing

 

[truckracer]

So above what you are saying is -

You can set the F/D ratio on the feedhorn to scalar first (even if it is not on the dish yet)

then adjust the whole assembly for best signal (scalar and feedhorn as one unit)
kind of treating it like the non adjustable ADL feedhorns.

Instead of just screwing the scalar to the legs and then trying to move the feed in and out to optimize the signal potentially never getting the best since the scalar position could be wrong for the proper f/d of the dish.

On my dish I would have to use longer bolts and jamb nuts I think. Does that aluminum ring come with the corotor?
My corotor should be here tomorrow.