Prime focus Ku performance

[Shawn95GT]

It feels wierd asking this in the C-band forum, but for any of those who have both, is it possible that my 5' Winegard mesh could be out-performed by my little 26x36 Primestar dish on Ku?

The 5' mesh has a fine mesh (couldn't fit a zip-tie through the holes) and I tweaked the focal length last night as it was about 1/4" too long.

I still can't see some stuff I hit EASILY on my smaller Ku dish.

The C-band side is working great. I'm actually suprised just how well it's working out. The Ku side is working, it just isn't as good as I'd like. I think the best Signal quality I've seen has been in the low 60's on Ku. Most hover around 30 which makes viewing them marginal at best . I can't see 11720V on G10R at all!

 

[caddata]

How is your Ku feed attached? Maybe you could post pictures. Bet there is something that needs tweaking.

 

[Shawn95GT]

How is your Ku feed attached? Maybe you could post pictures. Bet there is something that needs tweaking.

It's a Calamp C/Ku feedhorn:



Would it help to give a pic of it on the dish?

Shawn

 

[Shawn95GT]

More pics

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[caddata]

Shawn:

I'm as perplexed as you as to the reason for the poor performance. Your feed mount looks rigid and well constructed. Have you double checked your focal length and verified that the feed is centered? I had a similar problem with my old BUD system back in Arkansas and discovered that the focal length had been not been calculated correctly. I'm not sure about the Calcomp (very good products) but my Corotor II+ instructions required the focal point to be approx. 0.250" inside the throat of the feed. There was also a template included to help set the skew correctly.

The formula to determine the focal length is f (focal length) = DxD (diameter squared) / (divided by) 16d (16 times the dish depth). Draw a string tight or place a 2"x4" across the rim of the dish and measure to the deepest point of the dish on it's axis, perpendicular to the string or 2"x4".

My first homemade dish (16 ft. in dia.) used a Radio Shack over-the-air antenna rotator at the focal point to rotate the feed for vert. and horiz. The feedhorn was fabricated from copper sheetmetal. This was in 1981, which was before servo feeds or switching lnb's were available at reasonable prices.

 

[Shawn95GT]

Interesting read on the focal length.

I set the focal length to the back flat of the scaler ring. I set the depth of the feedhorn in the scaler ring as-per the F/D ratio of the dish. I just recently re-calculated the focal length. I'm going to re-measure the depth as I came up with 11" even back when I measured and that just seems too perfect.

I'll have to see if I can find any documentation on the CalAmp feedhorn. The focal point is definately WAY more than 1/4" into the feedhorn.

Of course I got the dish / feedhorn 2nd hand and I'm pretty much on my own for instructions. This is half the fun (and pain) of this hobby.

I think for the next step I'm going to take the dish off of the mount and get the feedhorn PERFECTLY centered. I think it's off by 1/4" or so. I also want to re-bend the feed supports that I got with the dish to fit the five footer. The ones I got with the dish were for an 8' dish. If I don't destroy them in the process I'll shave some weight off the dish and I'll have a better means to adjust the focal length since I'll have the slots to work with up at the scaler ring.

Thanks for the input!

Shawn

 

[drhydro]

Shawn,
As i was playing with my feedhorn during the summer, i noticed if the feedhorn is off even a 1/4 of an inch from center i saw a large (30-40%) drop off in signal quality.

 

[caddata]

Shawn:

Read your reply to my post again today and realized that you measured your dish depth at 11" (missed it the other day when you posted). If the 11" is correct, and the dish measures 60" across, then your focal length would be 20.454". That factors to a .341 F/D. Over the years, most American made dishes are in that range, with .375 being what one might call the standard. I notice that my DMS C-Band LNBF Scaler ring will adjust down to .340. As a side note, the first Chaparral Corotor feedhorns wouldn't go that low, so you had to by their famous "Gold Ring". It was no more than a gold anodized aluminum ring that pressed onto the throat of the feed to give about .375" extra length to the body of the feedhorn.

Hope you get your "five-footer" tweaked out. I know it will perform once you do.

I'm in the process of fabricating a light weight aluminum dish (it will be a prime focus "six-footer") that will weigh less that 18 lbs (much lighter than my 36" steel offset dish). If it works as well as I think it will, I'm going to post a "how-to" documented with photos and text that will allow someone to build their own. It's fabricated from material that's readily available at Lowe's or Home Depot. Getting lots of flak from the family because various parts always seem to be on the dinning table at meal time.

Keep us posted on your progress

 

[Shawn95GT]

It's 63 1/8" across the face. I wonder if this isn't actually a 5.5' dish - hehe. 63" is a lot closer to 160cm than 60" is though. Maybe 5' was 'close enough' for a name.

I figured 22 5/8" if I remember correctly. I'll check the numbers when I get home.

Shawn

 

[Anole]

oops, meant to send a PM

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