Plastic Dish

[playyymaster]

I have a real R&D project in mind::
I have access to 52" diameter plastic sphere cutouts. The cutouts have a diameter equal to 34 inches.
Imagine a 52" ball and cut a 34" diameter circle out of it.
Will a plastic dish work for KU? or does it need a special coating.
I was reading about the plywood dish and this is a much smoother curvature.
Also, how do I calculate the focal point?

Thanks

 

[Zero]

I do not know if it will work, somebody experienced may pitch in.. I guess focal point will be about 13" as per http://www.physicsclassroom.com/Class/refln/U13L3a.html. Good luck!

 

[Katrinasucks]

Sorry but there would be no focal point. I have a link I'll post later to illustrate this.

The guy puts little specks of mirror on various spots on the dish then points it to the sun. All the dots from all the little mirrors hit the LNBF. This is a great way to visualize how a dish works.

If the parabola were spherical you would not have a focal point, you'd have an inverted disco ball.
Have you thought about bird baths? ;-)

Edit: Promised link. Worth a few thousands words.

 

[PSB]

The surface would have to be painted with metalic paint, or foil to reflect the signal. Primestar, Globecast and DirecPC dish have a metal mesh inside the plastic dish.

 

[Mike500]

The original 18 inch DirecTV dishes made of composite polymers were made by Channel Master for RCA, Toshiba and Panasonic. They were made in 1995 and 1996. They had a metallic composite layer on the surface. I can attest that they maintained their shape better than the steel ones.

I currently have one of the composite Panasonic dish pans on my Dish 500. The smaller pan results in a drop of less than 10 percent signal strength.

Metal pans, when stamped have a tendency to "spring back" when they are released from the stamping die. Having examined a large number of dish pans over the years, some have as much as a one inch wide convex perimeter. Most have at least half an inch. So, the effective surface of a metal dish is less than the actual size of the dish pan.

On that account, plastic dishes, if properly made, perform better.

 

[ken2400]

Foil it and if that works find some cold galvinzing spray paint.

Good luck and keep us posted.

I have a real R&D project in mind::
I have access to 52" diameter plastic sphere cutouts. The cutouts have a diameter equal to 34 inches.
Imagine a 52" ball and cut a 34" diameter circle out of it.
Will a plastic dish work for KU? or does it need a special coating.
I was reading about the plywood dish and this is a much smoother curvature.
Also, how do I calculate the focal point?
Thanks

 

[dirtyshame]

Back in the early days (mid seventies) of satellite TVRO (receive only), a few hobbyists actually made a reflector that was as you describe, like a section cut from a large ball, it was called a Spherical reflector. The birds in those days only put out 5 watts per TP and the LNA's were home made and had noise figures of about 200 degrees Kelvin. It look a very large spherical to do the job. The focal point was found by using a string. You kept lengthening the string until you found a place where the string would just touch anywhere on the reflective (foil) surface plus or minus one-sixteenth of an inch. Once you found that that is where you placed your feedhorn. The home made reflectors were much too large and too fragile to rotate, so you had to move the feedhorn up and down, left and right to find the satellite. Lots of work involved, lots of failures, but lots of fun too!

Old John

 

[Tron]

One thing to remember about metallizing or laying foil on a sphere, you must be careful that the surface is not very reflective. Avoid glossy paint or anything that would reflect high amounts of sunlight to your LNB. You can cook your LNB very quickly this way...

 

[Katrinasucks]

The focal point was found by using a string. You kept lengthening the string until you found a place where the string would just touch anywhere on the reflective (foil) surface plus or minus one-sixteenth of an inch.

Then by mathematical definition, that would be the radius of the original sphere (26") off the center of the "dish."

Once you found that that is where you placed your feedhorn. The home made reflectors were much too large and too fragile to rotate, so you had to move the feedhorn up and down, left and right to find the satellite.

I guess that is one way to do it... Since there is no true focal point, perhaps you could put multiple LNBF's and get a poorman's semi-spherical toroidal dish? With a 36" diameter and the lack of a true focal point, I'd think DBS signals would be the only thing you could come close to getting.

 

[caddata]

As mentioned before, spherical dishes were common in the early days of TVRO. There were plans available that utilized redwood frames with an aluminum window screen reflecting surface. The dishes were large (over 300 sq. ft.) and still had "sparklies" in the final picture.

All spherical reflectors suffer from a technical term called "Spherical Apparition" which relates to the focal point. The focal point is the center of the sphere and is not as precisely focused as the focal point of a parabolic reflector. The reflected signal of a spherical dish results in signals that are out of phase with each other. The out of phase signals cancel out signals that are in phase. For all practical reasons, the spherical dish must be 4 or 5 times the size of a parabolic to have the same gain.

 

[sharris]

absolutely it will work. Just do some quick parabola calcs (sugegst a google on parabola and you will get lots of hits for calculators). Someone either on this board or another used an old round snow sled, and made it work, so give' er. Let us know how it works when you try it out.

 

[Azel]

The 1000 ft. Reflector at Arecibo, Puerto Rico is spherical (so they only need to move the feed). To correct for the spreading of the focal point they now use 2 specially shaped mirrors. Way too much work for satellite viewing, but here's a link; http://www.ecs.umass.edu/ece/allerton/papers1999/22/