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Sqaure Parabolic Dish Calculations?

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Posted by Jim richards on June 27, 2008, 1:07 am
 
Is there a difference between the calculations for round and square
shaped parabolic dishes? I have calculations for round shaped dishes
and all models from these calculations work fine, but using the same
calculations for a square shaped dish and no go. Does someone know
what I'm missing?

TIA

Jim


Posted by Jim richards on June 27, 2008, 3:46 am
 

On Thu, 26 Jun 2008 22:23:39 -0500, david.williams@bayman.org (David
Williams) wrote:

Thanks for your response David,

Linear Diam.     20.49
Diameter         20.00
Depth            02.00
Focal Length     12.50

x -10.00  y   2.00
x  -5.00  y   0.50
x   0.00  y   0.00
x   5.00  y   0.50
x  10.00  y   2.00


While pondering your question to my question, it may have sparked to
answer to my original question. If there is no difference in
calculations between a round and square shaped parabola then in
essence a square shaped parabola is just a round parabola with it
edges trimmed square?

That would seem to make sense, since the calculation is prompting for
diameter.

Jim


Posted by Steve on June 27, 2008, 4:18 am
 [snip]

Exactly.

When speaking of a parabola, it is in reference to a parabolic cross
section.

The basic formula for a parabola is:

  y = (x^2) / 4c, where c is the focal length.

There are simpler ways of constructing parabolic curves than plotting
points.

When I build a cylindrical parabolic reflector I used the technique
described in "Direct use of the sun's energy" by Farrington Daniels.  The
technique is to draw a parabolic curve on a piece of plywyood using a
straight edge and a nail, and a rectangle (for my dish I used a carpenter
square).

The straght edge is held along a "base line" that is the bottom of the
curve, and a nail is driven at the focus.  Initially the square is placed
against the base line and the nail.  Then the square is moved such that the
corner of the square is still against the base line and the nail forming an
angular gap between the base line and the straight edge.  A line is drawn
along the base of the square.  The square is moved a little more, and
another line drawn.  As this is repeated the interior of those lines form a
parabolic curve.

Its easier to follow in a diagram, but I don't have a place to post one.
There is probably a diagram already available on the internet somewhere.

Regards,
SteveD



Posted by Jim richards on June 27, 2008, 6:46 am
 On Thu, 26 Jun 2008 21:18:45 -0700, "Steve"


Thanks for the reply Steve,

What can't be done with a framing square? I saw a similar concept that
involved a piece of plywood, a nail, and (what looked like) a
T-Square. My problem is that neither technique works well in Auto-Cad.
Being an old computer guy it's cheaper for me to waste bits than
materials. Since I'm not just talking about this, I plan to build one
so I need blueprints.

I think David pointed to my problem, I was trying apply parabolic
calculations to a square instead of making round parabolic then
trimming it to a square shape. It's 2:45 now and I'm toast, I'll try
David's idea tomorrow.

Thanks again for the help.
Jim



Posted by Duane C. Johnson on June 27, 2008, 12:04 pm
 Hi Jim;


 > What can't be done with a framing square? I saw a similar
 > concept that involved a piece of plywood, a nail, and
 > (what looked like) a T-Square.

Here are some links to the technique you mention:
http://www.redrok.com/main.htm#parabola

 > My problem is that neither technique works well in
 > Auto-Cad. Being an old computer guy it's cheaper for me
 > to waste bits than materials. Since I'm not just talking
 > about this, I plan to build one so I need blueprints.

Here is a program to generate these data points
by Jeremiah Chace:
http://www.redrok.com/main.htm#chace

 > I think David pointed to my problem, I was trying apply
 > parabolic calculations to a square instead of making
 > round parabolic then trimming it to a square shape.

Yes, now you have it.

 > It's 2:45 now and I'm toast, I'll try
 > David's idea tomorrow.

 > Thanks again for the help.
 > Jim

Duane

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