CBand dish refurb question

[iammike]

A friend of a friend gave me an old cband dish last winter. The good news is its an 8.5ft spun aluminum dish, which I think is a good thing. The bad news is the guy didn't have all the parts. It's a pretty old dish with a design I haven't seen before. The LNBF sits in a cone that is mounted to the center of the dish with the lnbf pointing away from the dish. The signal from the dish bounces off of a reflector that is about 18" wide and back to the lnbf. Sort of like the toroidal KU dishes sold today. I've got the dish, the cone, the reflector and the polar mount but am missing the motor and whatever mounting hardware is needed for the reflector. The manufacturer was Commander Satellite systems, but I haven't been able to find anything about them on the Internet. Does anybody have one of these that could send me some pictures and/or give me some advice? I think if I can figure out the proper way for attaching the reflector I can probably get it working pretty well. I've also experimented with taking the cone off the dish and holding the lnbf in front of the dish in the approximate focal point. I can get some analog signals that way, so I figure if I can't make the reflector work I might be able to rig up some kind of lnbf holder and use the dish as a normal prime focus dish. If I go this route, does anybody have any thoughts on best materials for building the lnbf support arms?

 

[Shawn95GT]

feed supports

A friend of a friend gave me an old cband dish last winter. The good news is its an 8.5ft spun aluminum dish, which I think is a good thing. The bad news is the guy didn't have all the parts. It's a pretty old dish with a design I haven't seen before. The LNBF sits in a cone that is mounted to the center of the dish with the lnbf pointing away from the dish. The signal from the dish bounces off of a reflector that is about 18" wide and back to the lnbf. Sort of like the toroidal KU dishes sold today. I've got the dish, the cone, the reflector and the polar mount but am missing the motor and whatever mounting hardware is needed for the reflector. The manufacturer was Commander Satellite systems, but I haven't been able to find anything about them on the Internet. Does anybody have one of these that could send me some pictures and/or give me some advice? I think if I can figure out the proper way for attaching the reflector I can probably get it working pretty well. I've also experimented with taking the cone off the dish and holding the lnbf in front of the dish in the approximate focal point. I can get some analog signals that way, so I figure if I can't make the reflector work I might be able to rig up some kind of lnbf holder and use the dish as a normal prime focus dish. If I go this route, does anybody have any thoughts on best materials for building the lnbf support arms?

I can't help with the reflector deal but I used the formulas from geo-orbit.org , I calculated the F/D and correct focal length for my dish. I then drew up a scale drawing of the dish to model the feed arms. I figured the feed supports are like a tri-angle that sits on the face of the dish.

The trick is that you don't want a triangle, you want a tri-angle with the top lopped off with a 7-8" flat section on the top. I used 8".

There is probably a mathmatical way to do it but I centered an 8" section (to scale) on a scaled drawing of the cross-section of my dish. I then measured the focal length desired and marked it on the drawing. Connect the lips of the dish to the end of the 8" section and you've got a length for your feed supports. I scaled them up and they came out dead on. Actually the first try they were way too long but that was due to an error I made scaling the arms up .

I used 1/2" conduit for my feed arms and it is VERY steady. For your larger dish you might concider 3/4" or even 1". I pinched the ends in my bench vice and bent the angles close while it was still in the vice. The length measurement was bend to bend.

It sounds more complicated than it really was. I'll have to scan my sketches - hehe.

 

[iammike]

Hey Shawn, Thanks for the quick reply. I like your conduit idea, that one hadn't occured to me. Drawing it out to scale on paper is a good one too. Back in my college days, I could have probably figured it out mathematically but I did a core dump on all that math the day after graduation. I think I'm going to try to make the dish work as originally designed, if anybody still knows what the design was, but if it doesn't work out your method sounds like a good direction.

 

[drhydro]

that sounds like a cassegrain design, i would love to see pics of it set up

 

[iammike]

that sounds like a cassegrain design, i would love to see pics of it set up

So would I.

Thanks for the tip on the design type though. Now I have something to Google.

 

[ACD]

Well, if all else fails, you can follow the route of Red Green and make a huge WOK out of it

 

[pro96]

Shawn, great job getting that feed, finally into that 5ft Winegard.
When will it be up and running??

Keep those great pictures coming pal

 

[iammike]

Well, if all else fails, you can follow the route of Red Green and make a huge WOK out of it

Yes, yes, very funny. And before someone else posts it, the giant bird bath suggestion was already made by a coworker.

I actually played around with the dish a little tonight. I propped it up against my deck, and set up the lnbf on a little stand in front of it. Not exactly a standard set up, but I was able to get a reasonable analog signal on G3C. Didn't try it with the reflector though. That's the next experiment.

 

[iammike]

Thought I'd post an update. I'm definitely into mad scientist mode now. I tried over the weekend to set up the dish in the normal prime focus mode. Per Shawn's suggestion, I picked up some conduit and made some arms to hold the lnbf. The dish is still on the ground, but I played with this configuration by making myself into a human az/el mount. Took a tv and my analog receiver out into the yard and picked up a few channels. According to the forumla on the geo-orbit site, and my own observations, I've concluded the focal distance is about 28 inches. Since the dish is 18.5 inches deep and about 7ft 7in. wide, this reduces the effective diameter of the dish to something in the 4 or 5 foot range if used as a normal prime focus dish. After playing with it for a while, I came to the conclusion that to get the best results I'm going to have to set the dish up in cassegrain mode as it was intended.

Tonight, I tried testing the dish in cassegrain mode. I hooked up an lnbf to the cone and mounted the cone on the dish. Back in human mount mode again, I held the reflector in front of the cone and played around with the dish until I started to get a signal. What's really amazing is how close the reflector has to be to the lnbf. It appears that the optimal distance is around 2 or 3 inches. Since the cone is about 21 inches tall, I guess this makes sense if the focal distance is around 28 inches.

Now, since I'm still missing parts, I guess I'm going to have to manufacture some small arms to hold the reflector. I'm thinking of hooking 3 or 4 arms to the lnbf cone to hold it. Since there's only a few inches of separation, I'm thinking that I can get away with something pretty small. Probably some support arms in the 1 foot long range. My only concern is that they'll be passing through the now focused beam from the satellite, and I'm going to really need to minimize their width or I'll block too much of the beam. Anybody have any thoughts on what I could use. I'm sure I could make something out of some metal flats, but they'll definitely block signal. I wonder if something like Lexan would allow the signal to pass through?

I'm attaching a couple pictures. The edges of the dish are bent up because the former owner moved it around in his yard by rolling it.

 

[Shawn95GT]

That is one very intersting looking dish!

Good luck with it!