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Question about parabolic reflectors on offset TX/RX antennas

 
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misterbishop69
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Joined: 29 Mar 2011
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Location: Southwestern United States

PostPosted: Tue Mar 29, 2011 10:40 am    Post subject: Question about parabolic reflectors on offset TX/RX antennas Reply with quote

Hi antenna theory discussion board!

We have two satellite antennas where I work; both are 2.4 meter offset parabolic antennas. One is a Ku band antenna and the other is a C Band antenna.

They both appear to be made by different manufacturers, although I am not sure who makes either one of them.

My question is specific to their reflectors:

Both reflectors seem to be made out of fiberglass. The Ku Band antenna's reflector surface appears to be smoother than the C Band antenna's reflector surface; it looks almost as if it has been coated in some sort of clear epoxy-like substance. The surface of the C Band antenna's reflector seems to be just made of normal fiberglass, or I suppose what you would call Glass Fiber Reinforced Polyester (GFRP).

My question, is that for Ku and C Band parabolic reflectors to function properly (i.e. to reflect Ku and C Band signals efficiently), do they need to be coated in some kind of reflective material?

Thanks for any help that you can offer to this newbie!
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Wed Mar 30, 2011 7:34 am    Post subject: Reply with quote

Hi Mr. Bishop 69,

Two things:

1) The reflector must be reflective (i.e., metallic) to have the parabolic reflector work properly
2) I believe it is made out of a fiberglass-type material for mechanical reasons. That is, a big dish is lighter, cheaper, easier to construct and more weather-resistant when made of fiberglass than a metal dish.

The actual metal in the reflector needs only to be paper thin (or foil thin). I'm not sure exactly how they do it, but i would guess they either implant some conductive material or use a conductive coating somewhere in there.

In regards to the smoothness, the Ku-band (10-18 GHz) antenna will have a higher "smoothness" requirement than the C-band (5 GHz) antenna. This is because higher frequencies have shorter wavelengths - the wavelength at 15 GHz is 1/3 of that at 5 GHz. Hence, if there is a "bump" in the smoothness of the dish, it will look 3 times larger at 15 GHz than at 5 GHz. The dish's required smoothness depends on its highest operating frequency.
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misterbishop69
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Joined: 29 Mar 2011
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Location: Southwestern United States

PostPosted: Wed Mar 30, 2011 12:38 pm    Post subject: Yarp! Reply with quote

Reading more on the internet, sounds like you are right. Fiberglass itself is not reflective for any RF (or if it is, the actual figures are so small that it is practically nil).

What they must be doing is that they build a parabolic reflector out of fiberglass to proper mathematical spec, and then they coat the reflective side of it in some kind of very thin foil-like metal, probably aluminum capacitor foil such as this below:

http://www.republicfoil.com/capacitor_foil_product.html

Afterwards, they finally coat the recently "foiled" reflective side of the reflector with something like white, non-transparent epoxy.

What do you think? Theory sound solid?

Cheers!
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bigSteve
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Joined: 14 Mar 2009
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PostPosted: Thu Mar 31, 2011 5:21 am    Post subject: Reply with quote

Yes, I believe that is what they do. They could also use some sort of metallic paint. I don't know anyone in that industry so I don't know for sure.
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misterbishop69
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PostPosted: Thu Mar 31, 2011 7:36 am    Post subject: Reply with quote

It looks like some Andrew [Channel Master] reflectors still us a metal wire mesh screen embedded in the fiberglass. With C, X and Ku band, metal wire mesh screen reportedly works just fine so long as the holes in the mesh aren't larger than 1/10th the wavelength of the highest freq of the band it is intended for.

For instance, Ku Band's highest freq = 18 GHz.
18GHz = wavelength of 1.67cm
1/10th of 1.67cm = 0.167cm OR 1.67mm

Theoretically, any metal wire mesh screen (aluminum, steel, copper, etc) with holes SMALLER than 1.67mm each would reflect C, X and Ku Band just fine if it was used to line the face of a reflector made out of any non-reflective material.

Cool!
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