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,

*>-> 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 *

*> *

*>It would help if you showed us what calculations you are doing, *

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

*>and what seems to be wrong when you use them for square dishes, as opposed *

*>to round ones. *

*> *

*>Also, what do you mean by a "square" dish? It has four corners, I *

*>assume, but what defines the line beween adjacent corners? Is it *

*>straight when projected onto a plane perpendicular to the axis of the *

*>paraboloid, or does it follow the shortest path between the corners *

*>along the paraboloidal surface, or what? *

*> *

*> dow *

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]

*> 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?*

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

*> That would seem to make sense, since the calculation is prompting for*

*> diameter.*

*> Jim*

*> *

Posted by *Jim richards* on June 27, 2008, 6:46 am

On Thu, 26 Jun 2008 21:18:45 -0700, "Steve"

*>[snip]*

*>> 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?*

*>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*

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

*>>*

*>> That would seem to make sense, since the calculation is prompting for*

*>> diameter.*

*>>*

*>> 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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>-> 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>>It would help if you showed us what calculations you are doing,