f and f/D ratio adjustments

[esteveW]

I initially tested my new(ish) 3 meter solid with the C band LNB that came with it. I intentionally left all the setting the same since the move was only a 1/4 mile and had been working at that sit since installation in ~'90s until about 6 years ago. About all I did was rebuild the actuator which had been filled with mud during a flood several years ago. However, the feedhorn and LNB only had a few dead bees inside. I replaced all the old coax and control cables. I was able to receive a couple C band sat. channels on a very difficult anolog receiver. Signal quality was low but I could get a sharp black and white picture.

Next I installed the Corotor II Plus from my old(er) 10' mesh dish. I left the scalar as it was on the old dish and the "d" at the same distance as the original feed horn.

The reception was about the same as that of the original feed horn C-band LNB. Still no color and no Ku band. (In retrospect I don't think I ever got any Ku on my old(er) mesh dish. In fact I'm having about the same problems that I had on the mesh BUD).

I calculated the d distance and the f/D ratio for the solid BUD with the Corotor and here are my numbers:

D = 101.5"
d = 15.875"
f = 40.56"
f = ratio = .3996

I set the focal distance to 40.6 inches and set the scalar to .40

Now I get almost nothing on either C or Ku band.

Since I had nearly the same problems with my other dish, I'm wondering if I should look into replacing my LNBs. Especially the Ku since I have never receive any signal on either dish.

Please look a my math and see if I have been doing something wrong.

Comments and opinions welcomed.

Steve

 

[pendragon]

I calculated the d distance and the f/D ratio for the solid BUD with the Corotor and here are my numbers:

D = 101.5"
d = 15.875"
f = 40.56"
f = ratio = .3996

I set the focal distance to 40.6 inches and set the scalar to .40

Now I get almost nothing on either C or Ku band.

Since I had nearly the same problems with my other dish, I'm wondering if I should look into replacing my LNBs. Especially the Ku since I have never receive any signal on either dish.

Please look a my math and see if I have been doing something wrong.

Did you measure both the dish diameter and its depth? I only ask because it seemed ambiguous what was measured vs. calculated.

For starters, a diameter of 101.5" is a bit less than 2.6m. But if you measured D and d correctly, all of your calculations check out fine. Did you set the focal length a bit inside the feedhorn? Many C-band feeds don't like the focal point at the mouth's edge. Did you examine the Corotor for any pickup damage and verify you have enough adjustment range for the skew? Otherwise if you're sure that your peaked the dish for a given satellite and you're not getting much signal/quality, maybe it is time to try another LNB.

 

[esteveW]

Yes, I measured both. For the depth, I pulled a string across the dish at it's widest and measured to the dish center.

Steve

 

[gabshere]

another thing to consider is proper lnb feedhorn angle, i used a lazer pointer to ensure i was centered ( and yes mine was off by about 1 inch )

 

[esteveW]

Admittedly I haven't checked this.

Both of my dishes have the feed horn mounted on a Button Hook mount. On the mesh dish the 1" tubing come out of the center straight for about 18" then curves out to clear the scalar, something like a sheeperds staff with the flat plate bolted to the scalar. This arrangement defies alignment with a dowel pointer or laser since it won't extend beyond the straight portion of the button hook.

The second dish, the solid 9 meter. has a button hook as well but it is fabricated from 1" square tubing and comes out of the center of the dish at angle to a point where it has cleared the scalar and it clamped to the round portion of the feedhorn behind the scalar.
In this arrangement I could use a laser pointer if I make a bushing to fit inside the feed horn throat or clamp to the scalar some how.

Thanks for the suggestion,

Steve

 

[gabshere]

if you have an old feedhorn cap that will work or you might be able to use a cheap pvc cap and mount the pointer on that

 

[arcticracer]

Adjustments

Once you are in the ballpark you should fine tune your feed location. Not easy on a buttonhook feed. Vary the focal length. Measure from the feedhorn to 4 points on the edge of the dish to see if it is centered, the easiest method.