Distance from scalar ring face to feed horn face?

[SatScanner]

I am using a DMX741 LNBF with my Chaparral scalar ring on my 10' mesh dish. I'm trying to get the focal length set properly.

I measured the diameter of the dish at 119" the dish depth at 24" which calculates to a focal length of 36.8776". Or a setting of 0.3098 on the LNBF scale.

My dish came with a "C" shaped mounting plate that the three support struts are attached to. The scalar ring is attached to the plate by three plastic thumb screws in a triangle pattern that have a bolt going through the outer groove of the scalar plate through the center of each of the thumb screws. I can loosen the lock nut on top of the plastic thumb screws then turn the thumb screws to tilt the scalar ring or move the scalar ring closer or further to the dish by turning the thumb screws an equal distance.

Depending on where I have the scalar ring distance set from the plate the feed horn face could be close or far from the face of the scalar ring if I keep the feed horn face at 36.8". The scalar ring is there for a reason, does it matter how much the feed horn sticks out from the scalar ring?

Also I noticed that when I tighten the set screw on the feed horn it pushes the feed horn off center to the right like the feed horn was tapered. Any suggestions as to how I can keep the feed horn perpendicular when I tighten the set screw?

 

[FaT Air]

does it matter how much the feed horn sticks out from the scalar ring?

The length of feed that sticks out sets the F/D of the feed. The purpose and function of the scalar is mainly to cut off interference, noise, from reaching the feed, by making the feed "see" only the dish, and not past the dish. Calculate the focal length, set the feed at that distance, fine tune it in/ out for best performance. Measure the optimized focal length, calculate new F/D, and set the scalar to that. Feed moving about when trying to lock it down -- I had a feed that, no mater how careful, would move the feed when you tried to tighten the screw to hold it in place. I resorted to sharpening a point on the screw. That and some thin rubber on the opposite side got it to not "move around" when locking it into place. Didn't have to tighten the screw that snug either.

 

[Anole]

I folded up a little aluminum bracket/shim, and inserted it between the LNBF and the scalar, but captured by the screw.
Then, I filed down the screw point so it was flat.
It was pretty easy to get the LNBF snug but movable in the scalar, then when I got it to the right place, give the screw a quarter turn to secure it.

Not so great on the LNBFs which have a tapered body as it is on the ones which have a straight-walled cylinder.
But still better'n nuttin.

 

[Anole]

Actually, page three of this thread has a more through discussion with several suggestions for solving the wiggle/wobble problem:
NOW IT'S HERE. NOW IT'S GONE. 70% QUALITY TO 5% QUALITY...?

 

[SatScanner]

Thanks guys for the responses. I'll see what I can do.

I'm about to give up on my 10' dish for Ku band. My positioner is apparently of the low count type, the feed holder and set screw is too crude for the fine adjustment required, and I don't think my dish has the tight focus needed to effectively capture the Ku band frequencies properly. Last night I was able to make more adjustments to the feed and could get a minimal 30% on Ku on AMC9 and minimal 50% on AMC21 (PBS HD). After getting some more locks on Ku most of the C band signals would not lock. It seems I can only get either C band or Ku.

 

[FaT Air]

SatScanner - here's what I did to "fix" the same symptom. I "Lip mounted" the Ku. the 30 inch easily outdoes the Ku performance I had on the BUD. A 36 or 39 would work great.

 

[SatScanner]

Very interesting, I would not have thought of that.

I am primarily interested in receiving Galaxy 19 97W and AMC 21 125W.

G19 11842 (3ABN) seems to be a low power transponder, the best I have been able to get with a 36" dish is 50% quality at best. If I were to buy a 48" dish would I be able to get 100% quality? What is anyone's experience with adding 1' to a dishes diameter and signal gathering capability?

I haven't tried to receive PBS HD (AMC 21) with a 36" dish, is it possible to receive 100% quality with a 36" dish?

Thanks again.

 

[gabshere]

if it was me i would mount the c plate ( the plate that hold the scaler) below the feed arms and probably get a few washers to use as spacers ( this might involve some longer bolts) and try to get the scaler centered and level while maintaining a equal amount of thread ( on both sides) on the plastic/ nylon adjustment screws.

have you tried to hand hold the lnbf and move it closer to the dish and out more from the dish while on KU transponder.

 

[SatScanner]

if it was me i would mount the c plate ( the plate that hold the scaler) below the feed arms and probably get a few washers to use as spacers ( this might involve some longer bolts) and try to get the scaler centered and level while maintaining a equal amount of thread ( on both sides) on the plastic/ nylon adjustment screws.

have you tried to hand hold the lnbf and move it closer to the dish and out more from the dish while on KU transponder.

Yes, it is very tedious, the signal does get weaker or stronger when I move it in and out. I turned the plastic thumb screws about half way from fully in or out of the "C" plate and measured the focal distance at about 3' for the feed. Where I positioned the scalar plate and after moving the feed horn at it's focal length, the feed is almost flush with the scalar plate.

 

[FaT Air]

To peak Ku, you have the throat of the feed almost flush with the scalar? If so, Then you have to measure how much you have to move the scalar away from the dish to align the scalar onto the correct F/D mark on the feed. Make the required modifications, and the feed should be at the correct (peaked) location (focal length). Getting the scalar onto the correct position should improve C band operation. If that requires the feed support legs be attached to the face of the scalar, I don't think there will be much, if any, degradation. (many threads about holes cut into the C band scalars to mount Ku feeds, also some scalars almost cut in half, to mount 2 C band feeds close together)

 

[rv1pop]

I am tired and having trouble following all the thread, but the Adventist channels do not peak at the same locations as the other channels. Whether that is a good quality depends completely on your receiver / meter. I have one STB that shows 98 on hope channel and another that shows 45 on there - same 1 M dish, same bullet lnbf and same Q readings if I swap he cables. (TBN is 99 / 55 on this setups). Strong pictures off of both. Just different meter readings. As to PBS HD, there possibly will be a problem on a 36". I do not have HD.

 

[Anole]

That C-shaped plate is very reminiscent of the Paraclipse described by Stogie5150.
I know he and ACradar had some discussions about how to float the scalar closer or away from the dish.
And I'm pretty sure he was using a 741 or C1 at the time, and was getting Ku.
If you'll search for that thread (started by Stogie5150), I'm sure there will be some revealing tidbits to absorb. :up

 

[SatScanner]

That C-shaped plate is very reminiscent of the Paraclipse described by Stogie5150.
I know he and ACradar had some discussions about how to float the scalar closer or away from the dish.
And I'm pretty sure he was using a 741 or C1 at the time, and was getting Ku.
If you'll search for that thread (started by Stogie5150), I'm sure there will be some revealing tidbits to absorb. :up

Thanks, I'll go look for it now.