Anyone tried to make BIG dish at home?

[franksanderdo]

Hi and Hello,

well, some test further I can say:
1. The "Focus" of the Dish is about 9 cm closer to the Dish than the original support suggests ;-)
2. To talk about Focus is some how exaggerating. it is more a 10cm2 area.
3. With a quickly produced template I learned the shape of the Dish is some what round but far away from parapolic :-)
4. Lessons learned and preparing to build my own one.

Keep trying guys
Frank

 

[dante2]

Congratulations! Great job!
Yet, I have a small observation: I think is not sure that this form is a parabola, also can be a catenary.
_http://en.wikipedia.org/wiki/Catenary]Catenary - Wikipedia, the free encyclopedia
_http://www.carondelet.pvt.k12.ca.us/Family/Math/03210/page5.htm]Parabolas in Suspension Bridges! Oh, my!
_http://personal.strath.ac.uk/j.wood/Biomimetics/Engineering%20Areas/structures/STRUCTURES.htm]STRUCTURES
I also want to build a large dish at home, so I will continue to investigate this phenomenon.

 

[JanH]

"Therefore, the cables of a suspension bridge is a parabola, because the weight of the deck is equally distributed on the curve."

"When the structure is being built and the main cables are attached to the towers, the curve is a catenary."

I can not see difference between these two definitions and also can not imagine other curve similar parabolic with one focal point. Main cables still have weight equally distributed
on the curve with or without bridge. Probably bridge have not weight equally distributed.
It is shortcut.

I think same princip used discussed membrane parabolic.

What the point of the catenary usage?

 

[JanH]

Hi and Hello,

well, some test further I can say:
1. The "Focus" of the Dish is about 9 cm closer to the Dish than the original support suggests ;-)
2. To talk about Focus is some how exaggerating. it is more a 10cm2 area.
3. With a quickly produced template I learned the shape of the Dish is some what round but far away from parapolic :-)
4. Lessons learned and preparing to build my own one.

Keep trying guys
Frank

Anything new Frank?

 

[PaulR]

I can not see difference between these two definitions and also can not imagine other curve similar parabolic with one focal point.

It's a bit mathematical but have a read of this _http://whistleralley.com/hanging/hanging.htm

 

[Satcom1]

This example defines ray trace between the parabola and catenary. LNBF focus can be assumed better with a parabola.

However, I have found that offset dishes tend to be catenary while prime focus are parabolic.:-gnome

 

[Channel Hopper]

From memory a catenary curve (apart from being two dimensional) has no single focal point that can be derived within the equation.

 

[PaulR]

Agreed with you CH. This is why it's OK to use a catenary for a cooker as it doesn't matter if the focal point is diffused.

Satcom1 - where did you find that offset dishes are a catenary? I was under the impression that they were an angled slice through a parabolic.

 

[Channel Hopper]

Agreed with you CH. This is why it's OK to use a catenary for a cooker as it doesn't matter if the focal point is diffused.

Satcom1 - where did you find that offset dishes are a catenary? I was under the impression that they were an angled slice through a parabolic.

Two earlier posts on the page mentioned caternary in their answers.

For solar cookers it is important that there is no distinct focal point otherwise hot spots occur, leading to burnt food and reduced life of the cooker.

 

[JanH]

Looks like to me that catenary is similar parabolic with some difference what make less efficiency (for sat. antenne). Vacuum or presure antenne mentioned above should be catenary not a parabolic curve. It I figure out from the material above if it is right.
Cooker for shure do not need small focus point more likely with size of cup -

 

[Satcom1]

Agreed with you CH. This is why it's OK to use a catenary for a cooker as it doesn't matter if the focal point is diffused.

Satcom1 - where did you find that offset dishes are a catenary? I was under the impression that they were an angled slice through a parabolic.

Yes, I believe that an offset starts out as an offset section of a larger true parabola. However, measurements of Winegard dishes have their horizontal edges tipped in from a perfect parabola. I understand that their may be an attempt to improve issues with cross polarization that all offsets have. As you know no design is perfect and there are always trade offs in the design. Where focus may be sacrificed for improvement of signal to noise.

 

[Channel Hopper]

Yes, I believe that an offset starts out as an offset section of a larger true parabola. However, measurements of Winegard dishes have their horizontal edges tipped in from a perfect parabola. I understand that their may be an attempt to improve issues with cross polarization that all offsets have. As you know no design is perfect and there are always trade offs in the design. Where focus may be sacrificed for improvement of signal to noise.

There is every chance someone was matching the reflector design to a combination of feed systems, over a number of bands.
Or it could just be a failure in the original design blueprints.

 

[JanH]

Offset dish designer should solve problem that feedhorn can see whole parabolic not only a cated part. Missing parabolic but "visible" section make noise and decrase effectivness as well as make problems with feedhorn/dish impedance.
It is why feedhorn is not pointed to centre of virtual parabolic but somwhere to centre of offset disch. Offset dish should not be parabolic or catenary but special curve based of parabolic.

 

[PaulR]

Offset dish should not be parabolic or catenary but special curve based of parabolic.

As I said!

they were an angled slice through a parabolic.

 

[JanH]

Nice, we are two :-)

Who is against the idea?

 

[PaulR]

Satcom1 has posted that he has found that some off-set dishes are not.

However, I have found that offset dishes tend to be catenary while prime focus are parabolic.:-gnome

 

[fta-elite]

hello everyone

I new in this forum, I read the whole thread and it´s very interesting, I made my own home dish and I want to know your opinion.


Best regards

 

[franksanderdo]

Hi fta-elite,

this looks like an interesting approach.
Especially for C Band I believe it is very good.
Can you give some description how you made it?
Which Materials have been used?
Do I read the drawing right that the dish is +- 2m dia?

Well done!!

See you around
Frank

 

[PaulR]

What a great bit of work there. It won't work on Ku band but for C band - fantastic.

 

[Vipersan]

Very Impressive work m8 ..
I do love it when ingenuity Is displayed accompanied by obvious talent..
This board abounds with such creative types..
certainly count yourself amongst them.

As has been said already ..
I doubt the construct is accurate enough for KU fringe work ..
..but for C band (a passion of mine) ...I suspect it will perform well.
rgds
VS