C-Band Scalar Question(s)

[Titanium]

To further clarify, a measurement from the center of the reflector to the A or B points notated in your scalar photo is not calculated. The scalar's FD ratio position is referenced to the feedhorn opening and the feedhorn position is referenced to the reflector center.

 

[arlo]

Apologies. Really.
Familiarity and experience may make the verbiage seem clear to you all.
36 or .36 is just a number calculated and if a flat scalar with no rear flange was present. It's just a number.
Maybe I missed something. If what I understand is true, then.
1) Focal length is a hard factor found from measuring the dish dimensions. And so is f/D. Cool. Got that. Kind of like telescope science.
2) The LNBF performs best when the focal point is 1/4" INSIDE of the throat of the feed. Cool. Got that too. Mine varies a touch though to get best signal. But very close to the 37.1" I calculated.
3) The feedhorn throat, once positioned in an imaginary fixture in the ethers with focus at 1/4" inside will stay there. Now we adjust the rear scalar flange to align with the scale on the LNBF.
And that is where the scalar should be adjusted to. Being support rods or button hook feed. So now we lock the scalar down and tighten the LNBF right there

What I guess I'm not getting is what I am seeing. Keeping to suggestions. My scalar ended up being closer to focus for max signal and minimum AGC.
Min AGC tells me the amp doesn't have to crank up gain to compensate for a weak sig.
Again LNBF focus was close to the quarter inch inside the throat.
With it (scalar) being back and LNBF sticking way out (guesstimating .31 going by the scale). Signal sucked.
So what in the heck is the scalar doing by working better being being that much closer than the manual tells me?

It all relates to folks who need to build support arms and go on the hard rules. Install a new LNB. And never know the difference.
If I did and did not have the adjustability, I would have never known a signal gain could have been had.

Or looking at it another way. Do the math with an existing old dish. Scalar supported on the same arms or button hook it had from the get-go.
No need to change anything because that's the way it was made. So it must be right. Right?
Get a new LNB. Calculate and do the math. Instructions say you slap it on at the 36 mark. Do a skew tweak. And enjoy 10dB channels thinking you did it right.

 

[Titanium]

I may have installed and or serviced too many systems through the years... Simply shared the feedhorn install method that has been used by technicians for almost 40 years. These calculations and settings are estimations to place the feedhorn and scalar in approximate positions. Once the approximate settings are made, the installer optimizes the settings. If one is building feedhorn supports from scratch, I would suggest that they have a good grasp on parabolic calculations and feed install methods.

If on a motorized system, the optimization is made using multiple satellites and polarities to optimize and even detune an optimization on a single satellite to provide best overall performance across multiple satellites and polarities.

Scalar placement can be optimized for optimal performance on a single satellite and/or polarity, but that placement may not be the best for reception across the arc. As I mentioned in a previous post, the scalar placement has significant effects on the lobe(s) patterns (Shaping main lobe and attenuating or placing side lobes). Maybe share the reflector measurements and we can verify the math?

 

[arlo]

It's been awhile and winter has set in.
Getting back on this whole project finally.
I did all of my calculations and placed the Titanium lnb exactly where focus said it should be.
The lnb had to be butted against the rear of the scalar according to f/D of my dish. That put it a touch 'short' still.
Button hook bolted back to the dish I got signal real fast.
I did replace the stock aluminum tubing with thicker wall stock and it's more robust.
LNB pointing right at the dish center. 2 paint can tops back to back fit snug in the throat and a dowel rod works like a peach.

Trials and tribulations. Signal peak on 101W was where the scalar was moved in about an inch and the LNB was pulled out a little more from actual focus calcs.
Then I took 4 stainless bolts and a bunch of nuts for lock nuts and made the scalar mount adjustable in, out, up, down, etc.
Getting real good signal across the arc from 131 all the way out to 47.
But as you all said, skew doesn't actually follow the arc. Maybe time to refurb the old Chaparral C/kU feed.
So there you have it. Guys building new arms for their dishes probably should make them a touch longer than what calculations say and use long bolts and nuts for stand-offs for the scalar mounts for tweakability.
Like Brian said. What numbers you come up with need a little leeway. And 3, 4 fixed support arms fashioned up per hard calculations may actually need some flexibility.