Question about Scalar Ring / LNBF placement

[sgs]

Hi guys,

I'm working on some replacement "arms" to hold my scalar ring and LNBF on a 10 foot dish. The old arms are weak and rusting out badly.

Previously when I put the dish up, I simply assembled everything, and adjusted the LNBF in / out of the scalar ring, but I didn't pay attention to the f/D ratio because I didn't measure and calculate it. This time I'd like to do it a little better by measuring things and getting it right. I've measured the dish with a string test (fishing line works well), and come up with a f/D ratio of approximately 0.34 and a focal length of approximately 41 inches.

Knowing that the focal length is ~41 inches, where do I place the scalar ring, and the front lip of the LNBF?

From what I understand, the LNBF should tightened on to the scalar ring at the f/D ratio (0.34) first. From there, the front lip of the LNBF with scalar ring should be placed about 1/4 inch closer than the focal length, so 40.75 inches in my case. Once that's correct, I tighten the new "arms" to hold it all in place, adjusting the arms / scalar ring attachment points to get the focal length correct.

Does this make sense? Am I misunderstanding something?

sgs

 

[FaT Air]

I'd put the scalar on the feed throat set for a F/D of .34 and place the feed at 40 3/4 inches* from the face of the dish.
The F/D numbers on the feed align with the rear, smooth, face of the scalar.
*The focal point should be located approx 1/4 inch inside the throat (for C band)
You may fine tune from there. Make the 'legs' adjustable??
You're right:

the LNBF should tightened on to the scalar ring at the f/D ratio (0.34) first. From there, the front lip of the LNBF with scalar ring should be placed about 1/4 inch closer than the focal length, so 40.75 inches in my case. Once that's correct, I tighten the new "arms" to hold it all in place, adjusting the arms / scalar ring attachment points to get the focal length correct.

(I didn't read the whole post first. whoops!

 

[KE4EST]

Yes you are making sense. You have it figured out.

 

[Titanium]

Here is a PDF LNBF / Scalar install guide that might be helpful.
http://titaniumsatellite.com/downloads/Guides/LNBF Install Guide.pdf

 

[RimaNTSS]

sgs I think if you get another LNB bracket, which is fixed to the feedarms and allows LNBF to slide inside, than your problem will be solved. Than it is possible to fine tune position of the scalar closer/further from the dish. Something like that

 

[sgs]

Thanks for the replies guys.

The F/D numbers on the feed align with the rear, smooth, face of the scalar.

That's what I thought too. However, in the PDF that Brian linked to, it shows the f/D ratio set to the back "lip" of the scalar, not the rear smooth face of the scalar. The picture in the PDF shows the lip at f/D of 36, but if it's the smooth surface of the scalar, then the picture is actually showing a f/D closer to 39 or 40.

So, is the PDF wrong or are we?

sgs

 

[sgs]

sgs I think if you get another LNB bracket, which is fixed to the feedarms and allows LNBF to slide inside, than your problem will be solved. Than it is possible to fine tune position of the scalar closer/further from the dish. Something like that View attachment 110932

That looks nice, it would allow for exact adjustments to be made by adjusting the nuts on the threaded rod.

Might be better if it had 4 support arms instead of three for a 10 foot dish, especially since threaded rod bends pretty easily at 50-60 inches in length.

Is the bracket pictured commercially available or did you build it?

sgs

 

[RimaNTSS]

Is the bracket pictured commercially available or did you build it?

That bracket from picture I posted is not made by me but for me, by friend of mine. Now I also making something similar. And out of the shelf brackets also available on the market, look for star-bracket. There are some examples.

Might be better if it had 4 support arms instead of three for a 10 foot dish, especially since threaded rod bends pretty easily at 50-60 inches in length.

For really big (I mean huge) antennas 4 support arms is an option. In fact, I think, it could be not easy to find > than 10 foot dish with only 3 feedarms. You are right about threaded bars ability to bend, therefore it is better to use pipes with threaded bars adjusted to the ends. That, as a bonus, makes feedarms adjustable in length. Of course, bigger dish - bigger diameter of the pipes and threaded rods used.

 

[FaT Air]

Thanks for the replies guys.



That's what I thought too. However, in the PDF that Brian linked to, it shows the f/D ratio set to the back "lip" of the scalar, not the rear smooth face of the scalar. The picture in the PDF shows the lip at f/D of 36, but if it's the smooth surface of the scalar, then the picture is actually showing a f/D closer to 39 or 40.

View attachment 110943

So, is the PDF wrong or are we?

sgs

Go with the photo. Can't argue, he's right.

 

[Titanium]

Here is a link to an old newsgroup thread archived by Google from many, many, many years ago. You may be very interested in Howard Taylor's comments about the Corotor design and KU performance in Zeno's 9/7/98 post.
https://groups.google.com/forum/m/#!topic/rec.video.satellite.tvro/VBo9upN2tac

Here is a link to the Chaparral Scalar / Feedhorn setting:
http://www.chaparral.net/troubleshooting/feedhorn-troubleshooting-guide/

If the image in the PDF is incorrect, please advise. This was always my starting point for setting the scalar fed ratio and the final optimized C-band placement was usually quite similar. Observations? Suggestions? Comments?

 

[sgs]

Here is a link to an old newsgroup thread archived by Google from many, many, many years ago. You may be very interested in Howard Taylor's comments about the Corotor design and KU performance in Zeno's 9/7/98 post.
https://groups.google.com/forum/m/#!topic/rec.video.satellite.tvro/VBo9upN2tac

Interesting thread. In particular Bert's post on 8/8/98:

Remember focal point should normally be 1/4 of a wavelength inside the
feedhorn for a prime focus feed. 1/4 of a wavelenght for C-band is 3
times longer than 1/4 for Ku, i.e. for C-band 20 mm and for Ku 6 mm (are
you still using inches over there?) Maybe thats why the sliding.

1/4 wavelength is not 1/4 inch for C-band, it's more like 3/4 of an inch! So if that's correct, then the LNBF+scalar should be almost half an inch closer to the face of the dish. Can anyone here confirm the correct placement? I've read 1/4 inch in many places, but I could see how people might start thinking 1/4 inch instead of 1/4 wavelength!!

Here is a link to the Chaparral Scalar / Feedhorn setting: http://www.chaparral.net/troubleshooting/feedhorn-troubleshooting-guide/

Unfortunately that page isn't clear on exactly how to place the scalar, only that it needs to be set correctly:

You simply set the top edge of the scalar ring even with the line that corresponds to your correct f/D setting

The "top edge" could be the flat part of the scalar, but I assume it's the edge with the lip, not the flat back part. Generally that would make sense to me, because we cannot see the markings where the flat back part of the scalar ring is, when installing.

The reason this is confusing to me is because I've got a bunch of different scalar rings, and they do not have the exact same lip length. I suppose it's because the scalars I have are generally cheap garbage.

Does Chaparral even print the f/D ratios on their feeds like we see on a typical LNBF? I have one not in use, with no f/D markings.

If the image in the PDF is incorrect, please advise. This was always my starting point for setting the scalar fed ratio and the final optimized C-band placement was usually quite similar. Observations? Suggestions? Comments?

I've read conflicting information from various sources, which is why I started this thread to verify the correct setup. Unfortunately every LNBF I have didn't come with instructions to indicate where to line up the scalar ring to the LNBF's stamped f/D ratios. Hopefully someone reads this and has instructions on it.

As the thread you linked to points out, it comes down to manually tuning it for your use case and particular dish. However I think it's worthwhile to know exactly how to configure everything in the "ideal case (theory)" as opposed to the "real world (practical)".

If the f/D ratio in my previous post with the picture is set correctly at 0.36 and that's what the dish's f/D ratio is, then everything is good. If the f/D ratio is actually 0.39 or 0.40, then it means the scalar ring is not filtering / choking the entire surface of the dish's reflective area, but a smaller portion of it. Likely this would mean more susceptibility to terrestrial interference and/or less overall gain on weaker signals, correct?

As always, thanks guys

sgs