Why some dishes were made as concentrical rings?

[polgyver]

Going through my saved photos I noticed some pictures showing experimental satellite dishes made from stepped-up rings. I am curious why anybody would attempt to build such devices. The electromagnetic waves, reflected from different rings, most likely would interfere and cause possible cancellations. Such a design can work for solar cookers, hardly for satellite reception. What would be possible advantage for trying such a design? Any opinion from our seasoned Members?
Regards, polgyver

 

[NYDutch]

I'd think the rings would add strength to the dish for holding its shape.

 

[primestar31]

Look up "Fresnal Lense".

Works with optics, AND electromagnetic waves as well.

Check out page 58 (starts on 52 in this pdf): https://ia803203.us.archive.org/3/items/radio_electronics_1985-10/Radio_Electronics_October_1985.pdf

 

[cyberham]

That link downloads the entire 1985 magazine. What an interesting magazine. I want to read the follow-up article on results by the guy who built a large Fresnel lens for a C-band dish.

If no article exists, then polgyver has a task to do. I want a first-hand account of how well a Fresnel lens with modern LNBF and 2-degree satellite spacing works.

 

[Cham]

Used to see those around the area here, but I think they (some?) were made of wood with fiberglass covering, so they would eventually rot away. Not sure who made them or how well they worked, must have been not that great, haven't seen one for years.

 

[polgyver]

Works with optics, AND electromagnetic waves as well.

Yes for optics, when the wavelength of visible light is a very small fraction of width of a ring on a lens. Such design of Fresnel Lens saves huge amount of glass for example in a lighthouse.
I suspect there are losses when the wavelength of, say, 12 GHz, = 2.5 cm (or 1") is only 2 or 3 times smaller than the width of a ring, as the sharp edges of rings do not reflect the rays straight to focal spot, but rather disperse them according to laws of physics.
Also, assuming the ring heights are 1/2 of wavelength, so the amplitudes add (interfere constructively), the four or five consecutive waves add up in a focal spot, mixed in proportions depending on rings' surfaces. Probably it does not interfere with signal modulation.
I believe that, when comparing regular, paraboloid dish with the stepped-up one, the regular dish would provide better signal Quality, Q (%).
The article in Electronics Now is very interesting, thanks for the info.