How much does the scalar ring affect signal quality?

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wagonman76

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Nov 11, 2006
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I have my dish up and tuned in quite well. Still using the existing wire run into my camper, as I won't be running new wire until the snow is gone and the ground thaws. June perhaps.

My setup uses the Chaparral Corotor. When I installed it I lathe bored the existing Ma-Com scalar for a very close fit. It is centered within 1/16". Skew has been peaked.

I made a spreadsheet for the F/D, etc. calcs and my Ku quality peaks right where it says it should. I get a quality of 99 on many satellites. The Geosat receiver has a much better quality meter than the Traxis for peaking the quality.

However my C quality isn't as great. At the end of the fine adjustments I was able to get Decades to come in well, but still at a C quality in the 40s. Some channels show a C quality of 5 but still come in just fine.

2 things I can think of to improve C quality. 1 is a newer LNB, kind of pricey as I'd get a good Norsat if I'm buying anything.

2 is to adjust the scalar, which would require some fabrication but would be free. According to the calculations, my scalar is about 3/8" too close to the dish, and that's where the buttonhook puts it. It is a fairly deep dish, .30. I'm not going by the numbers on the side of the feedhorn, but hard measurements. From what I've read I'm supposed to be going by the inside rear surface of the scalars. This puts the face of the feed about flush with the front of the scalars.

I'd have to cut the buttonhook pipe and make an adjustable sleeve to change the length.

Just wondering if the scalars would likely affect quality that much. Thanks.
 

Titanium

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May 23, 2013
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The numbers on the side of the feedhorn are used for the Focal Diameter Ratio setting. This scale is where the scalar ring is set and is not for determining the Focal Length. Now measure from the center of the reflector surface to approximately 1/4" inside the feedhorn opening. This should be approximately the Focal Length. On deeper FD reflectors the sweet spot of signal convergence is closer to the opening of the feedhorn throat and on higher FD reflectors the point is typically set deeper inside the feedhorn throat. If the scalar is set to the correct FD scale marking on the side of the feedhorn and the Focal Length measurement is not as calculated, then the scalar/feedhorn combination is the wrong distance from the reflector.

Scalars can greatly affect the Signal Quality as it shapes the area of illumination on the reflector surface and contains the look from over illuminating the edges and being exposed to terrestrial noise. The scalar also plays an important role of attenuating side lobes by rejecting and deflecting off axis adjacent satellite signals and noise.
 

armadillo_115

SatelliteGuys Pro
Jun 10, 2015
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Virginia
Be aware that Decades,Heroes, and Movies on 101W are acting up again on some receivers.(Ali chipsets in particular IIRC) I could not get a stable lock on them this morning with my 9000i. SQ jumping from 98% down to 0%.

So I would NOTuse those particular channels to determine how well things are set-up right now.
 
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. Raine

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Aug 6, 2013
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It sounds like you are using a Geosat Pro 3500, the Q scale on that receiver is grossly inaccurate. On mine, some TP's will read 5 and come in, switch to a different receiver and I get a reading in the 80s.

The 3500 isn't a good receiver to judge quality readings on.
 

wagonman76

Well-Known SatelliteGuys Member
Nov 11, 2006
29
8
Thanks all.

I did read that thread earlier, out of many. Good read, but being a mechanical engineer I don't have as firm of grasp on radio wave theory. I'd love to be able to lay it out in AutoCAD like I can do with the focal point, but pure geometry doesn't seem to cut it here.

Tuning the feedhorn and scalar is like putting the cart before the horse, the way it is set up. The feedhorn pretty much needs to be set first. My calculated focal point I did put 1/4" inside the feedhorn throat. And that works out perfectly for Ku. So in order to adjust the scalar to get the proper relation to the focal point, I need to structurally modify the buttonhook.

I'm not going to rely on the markings on the side of the feedhorn, because there are different brands of dishes and scalars and feedhorns, and how they setscrew in place. Measuring my dish, my F/D works out to .30, so it looks like I should rework the buttonhook to get the inside rear face of the rings (so I read somewhere) about 1.32" from the feedhorn throat. Right now (where the buttonhook places it) is about .37" too short, and the front of the scalar is only .06" from the throat, so I can't even put the plastic cap on.

120.5" curved surface diameter
25" depth (buttonhook plate in center is flush with dish surface, not raised)
36.3" focal distance works out perfect
.30 f/D = 1.32" from throat to inside rear of scalar (please correct me if this is not right)
Rings .88" deep so 1.32" - .88" = .44" feed throat to front face of rings

I tried the .44" measurement first, which made the scalar-to-feed throat relationship proper, but had lacking Ku quality. The focal point was much more important.

I was wondering if that work would be worth it, but it sounds like I'm only utilizing a fraction of the dish and modifying the buttonhook to properly set the scalar is something I should do when it gets nicer out. I'll build some good adjustability into the modification. Definitely before spending any money on a new LNB.

That's good to know about the Geosat receiver Q scale, maybe it's not that big of a deal. My old Traxis would just max out at 80 over a large adjustment range before it dropped. The Geosat seems to be extremely sensitive and gives me an actual quality all the way up to 99, but it requires me to make much finer adjustments to get there. But these are the only two FTA receivers I've worked with.

Good to know about the Decades channel mux. I won't be pulling out what's left of my hair wondering why they might not come in sometimes.
 

Titanium

AI6US
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May 23, 2013
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I am not following the extra measurements and calculations between the scalar and the feedhorn settings. Not sure it is worth putting too much time into this if the scalar cannot be placed into the correct position to provide the approx. calculated focal length measurement. Probably best to initially use the matched feedhorn/scalar from a single model rather than a mixture of mismatched components to determine if the buttonhook requires modification.

With a .30 FD ratio, the feedhorn will be extended almost to the maximum distance in front of the scalar (on my sample scalar/feedhorn units the throat to the inside rear of the scalar measurement is approx. 2 1/4" for reference).
 

wagonman76

Well-Known SatelliteGuys Member
Nov 11, 2006
29
8
Thanks, that helps a lot. 2 1/4" is a big difference. If I just reattach the scalar to the back side of the buttonhook flange, and maintain my focal distance without modifying anything, I'll end up in the ballpark of 2 1/4". I'll do that when it gets a little warmer out and see how much of a difference it makes.

I'm pretty sure I have the Chaparral scalar somewhere. I had just used the Ma-Com scalar because it was matched to the dish. I'll have to see if there is any difference.
 
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