Spun aluminum, worth a try?

[upnatm]

We took this one down today, it's still sitting on the trailer.

It's an 8' spun aluminum, 19" deep and about 3/32" thick.

Only the glue is left, where a name plate once was on the back.

The owners knew nothing about it, other than it was there when they
bought the place 10 yrs. ago, and no spare parts laying around anywhere.

It has a few dings and dents, but none too serious. I'll try to smooth them out.


Would this be a Cassegrain type dish with the secondary reflector missing? The
center looks like the scalar and LNB would have faced the sat.

How much C/Ku fun will I have with this one?

Thanks for looking.

 

[Anole]

Some noticeable edge and surface dings show in the pictures
Still, I'd think it would do fine on C-band.
Of course, all things are relative... if you've got a good 12'er, this might not do as well! -

Unless you want to try to restore the original feed, I'd calculate the focal distance, and outfit it with four legs to hold a scalar.
The legs could be made from conduit, or maybe anodized aluminum tubing, according to your budget.
Changing a button-hook feed (single pole up the middle of the dish) to four legs is a common mod.
Someone did a fine job of it just recently... and if I can recall who it was, I'll post. - :up

 

[crackt]

ive had a few spuns in the last 2 years.... i just want to say your dish is yummy. it will perform like a 10 foot mesh, or close to it. what a beautiful dish.

crackt out,.

 

[jsattv]

Upnatm, that is one beautiful dish. Do you know if it has any LNB's inside the Feedhorn (in the middle).

 

[FaT Air]

Yep, it's a casse

OH,OH, found this: C-band dish sold as "The Eliminator" out of Apopka, FL.

http://www.flickr.com/photos/ajmexico/3121410158/in/set-72157613646383797/

 

[Anole]

new info:

Cassegrain-feed 8 foot C-band dish sold as "The Eliminator" out of Apopka, FL. ...
The big dish is a spherical (not parabolic) shape.
Inside the cap is a weird-shaped secondary reflector that sends the signal into the feedhorn which faces the sky, not the dish.

Doesn't look like your dish has the sub-reflector.
Not sure my suggestion about adding quad scalar-support legs is a good idea, if the dish is spherical instead of parabolic.

 

[upnatm]

Hi All,

Thanks for the interesting responses. Does anyone know who the manufacturer
is? I'd like to bug them for parts and/or details.

I spent an hour or so last night, tapping and tweaking some of the dents and dings,
The large dents popped back with the heel of my fist, and the smaller dings are flattening out
with the gentle persuasion of a ball-peen hammer and a block of wood.

Anole,
Yes, the sub is missing. If the pan is spherical, I'll have to restore the original feed type, and
I'll need formulas for that design. If I'm lucky, I may find a prime-focus pizza pan that I can use the backside of.

I just found this on the web; http://www.w1ghz.org/antbook/conf/Multiple_reflector_antennas.pdf
It looks like a good place to start.

jsattv,
No LNBs, but I did find this box (1st pic) when I lifted the dish off the trailer. it must've been sitting in the feed base
for years before it fell out. No big deal though, modern stuff is far better anyways.
2nd pic shows who made the thing. Google says they make electronics, not dishes.

FaT Air,
Awesome find on the picture! (3rd) It's gotta be the same make and model! Too bad we can't get
more details from it!. Damn!, I wonder who made these things??

Thanks again, gotta run, be back later.

 

[caddata]

.....if the dish is spherical instead of parabolic.

Information in picture caption may be bogus. I doubt the manufacturer would have spun a spherical reflector because the efficiency of the dish would have been reduced to that of a six foot parabolic.

Microwave signals reflected near the periphery of a spherical reflector arrive at the focal point out of phase and cancel out similar waves reflected more from the center. The loss of canceled waves makes an eight foot dish work like a six foot. The phenomenon is called "spherical aberration" (Google that for a better explanation), and is not a big problem with light reflectors but a killer with microwave reflectors.

The caption made reference to the sub-reflector as "a weird-shaped secondary reflector". That "weird shape" was most probably a hyperbola, the inverse of a parabola. A sheetmetal hyperbola would be difficult to fabricate without the use of expensive metal forming equipment (an English wheel). I've successfully substituted a conical reflector in the past.

 

[upnatm]

Spherical or Parabolic?

Time to brush up on my maths and physics, and put Google to work.

It seems my first task is to determine the geometry of this dish.

I believe this can be done by establishing the focal point and/or center of curvature, then taking
measurements from that point to various circumferences around the pan.

So, if it is an 8' spherical dish (1st pic), the center of curvature will be 4' from the vertex, along the principal
axis, and at all points along the reflecting surface . A simple enough task, I hope.

A parabolic dish (2nd pic) on the other hand, will have incremental distances to the focal point, along the
curve of the reflecting surface. The distance increasing, the further you get from the vertex.

Telling the two apart, I learned that on Sesame Street!

If the pan is parabolic, then I'll remove the old LNB holder (center tube) and rig the thing up with regular support arms for the scalar ring.

But if the pan is in fact spherical, I'll need the secondary reflector! That's where they come into play; correcting
the spherical aberration, and giving a coherent signal to the focal point. Can everybody say "Hubble"?

Two types of secondary reflectors can be used. Convex or concave. (3rd pic, top half)
Using a convex secondary, is the "Cassegrain" style, whereas the concave application is the "Gregorian".

"Gregorian" is used mainly for optics, I think it's the interference produced (F2) between the secondary and
the focus, that makes it unsuitable for microwave use. If anything, it'll be Cassegrain.

caddata,
Could you please elaborate on your "conical" substitution?

Heh-Heh, Maybe I could get away with using the Wok from the kitchen!

Enough yammering, time to get upnatm! Be back later.

 

[caddata]

......please elaborate on your "conical" substitution.....

Formulas that I used called for my sub (virtual) reflector to be 6" O.D. with a .4375" rise in the center. I didn't even try to plot the profile as a hyperbola, I just fabricated it from a piece of .015" aluminum sheetmetal laid out (developed) using an exercise in an old drafting textbook.

My homebrew satellite receiver kit came with documentation on various microwave feedhorns. The LNA, with a homebrew pyramidal feedhorn, was rotated with a Channelmaster T.V. antenna rotator for H/V selection. The dish I experimented with was an old Bell Telephone terrestrial microwave dish. I adapted it to a Cassagrain feed to keep from having to mount the LNA/rotator out in front for a prime focus arrangement. Later traded the dish for a 10' fiberglass.

This exercise took place in 1981 and I amassed several notebooks of diagrams and information on antenna design. Those notebooks were lost during a household move and I haven't been the same since.

 

[upnatm]

Spherical!

Well, that didn't take long!

The pan is spherical, but with a 5' center of curvature, not 4' as I was guessing.

It now looks like the focal point is at the hole in the center tube where the scalar
ring fits so nicely. Which is 18" to the back of the scalar, about 19-1/2" after the scalar
thickness and the LNB projection is taken into account.

Now I need to figure out the proper size and shape for the secondary. It's diameter is simple, just match the LNB tube dia.

I'm off to study some more formulas and plot the actual curve onto a cardboard. If have any
luck at all, perhaps there is a pot-lid around somewhere that will do the trick. Or maybe even
make one out of fiberglass.

I still hope the old wok will work!

 

[Anole]

step back and . . .

Why not contact the owner of the other dish in Florida?
Invite him to join our forum.
Maybe he'd be willing to give you some better pictures of the pieces you need, and the proper measurements and spacing.
Better yet, maybe he'd take an impression (mold) of his sub-reflector for you...! -

And the truth is, once you know what you are looking for, it's always possible you'll run across some old installer with a few sub-reflectors on a shelf in his barn.
I once was resigned to the fact that there were no Birdviews on the west coast...
... 'till I started looking for one....
.... and found many! -
After all . . . it only takes ONE ! -

 

[upnatm]

Hey, good idea Anole!

I just spent like 4 hours trying to find the right formulas and equations on the web. I fell asleep at least once, lol !

So far I figure that if the primary is spherical, then the sub reflector most likely is too. That's still only a guess though.

Parabolic reflectors are different in that they use hyperbolic curves for their secondaries. Those formulas were easy to find.

I just hope it's as simple as finding what size of sphere to take a section from.

If anyone is familiar with the formulas I need, please post them.

I'm off to take a cyber-trip to Fla. to see what I can see.

Thanks.

 

[iammike]

Hey, just found your PM. I just got rid of my old dish, literally in the last week. I picked up a 10' Unimesh a couple years ago, and my Cassy has been laying on the back of my old lot. I just sold the old house and the new owners closed on Monday. I only brought the 10 footer with me when I moved. Wish I'd have seen this thread sooner. I could have sent you my secondary reflector. The secondary reflector is circular with a little raised up place in the middle. Think about a post in the middle of a circular tent. I'll see if I have any pics laying around that may help you out.

 

[iammike]

I found this pic. It will probably help some. I'll look some more tomorrow to see if I can find a better view of the business side of the reflector.

 

[iammike]

Here's a couple of good ones of the secondary reflector.

 

[upnatm]

So THAT's what it looks like!

Hello Iammike, great to hear from you! The pics say a thousand words!

Now I know what the secondary looks like.

How well did this dish perform for you? Is it really worth my while to continue?

I must say, reproducing a piece like this, will take some doing! I'd pretty much need to have one in my hands, in order to copy the dimensions.
Did you ever locate the manufacturer of the dish?, in order to get replacement parts or specifications?
Is there any chance at all, the new owner might let you retrieve the part?

I never did find any useful formulas on the net. Anything I did find, dealt with the primary reflectors being parabolic, requiring an hyperbolic secondary.

Anole, I followed your suggestion and contacted the fellow in Fla., the best he could offer was some pictures too.
Now I understand his description of "weird-shaped secondary reflector".

Oh-well, it looks as though this project is going to be on the back burner for a bit, as I was just given permission to pluck a 10' mesh, (pic)
from out near one of the power-dams in the area. A bit if moss and lichens growing on it, but still in very usable condition!

Oh, by the way, the jack that came with the 8' solid dish still works!

If anything else comes up, please post here.

Thanks all, for everything so far! :up

 

[iammike]

I'd try to get the reflector for you, but it went in the trash when I cleaned out the shed at the old place. Performance was pretty decent on the old dish, but it was finicky. I never did find a manufacturer. I think they are long gone. That's why I had the duct tape and home made holders for the reflector. I'd go with the new dish if I were you. It looks a lot like my Unimesh, which works far better than the Cassegrain ever did.

Here's a pic of my Unimesh for comparison