Jeff Liebermann skrev 2012-01-19 08:54:
On Tue, 17 Jan 2012 07:56:44 +0100, Ben / wrote:

I'm planning to build a parabolic reflector using aluminium net.

Approximate diameter? How much gain to you need (or expect)?

The kind of net i found is just a weave of aluminium threads, no
connnection between the threads. Is this going to reduce the performance
of the reflector?

It won't reduce the gain, but you're going to have another form of
entertainment. Because the wires in the mesh are not electrically
connected (welded), you run the risk of creating small diodes at each
crossover as the aluminum oxidizes. Similarly, as the wind blows
through the mesh, the wires will scrape against each other, creating
small sparks. I found this problem the hard way when I built a 1.7GHz
WEFAX dish using aluminum window screen material. I spent a few
months trying to isolate the highly intermittent noise sources. I
eventually smeared spray glue on the window screen mesh, and plastered
it with aluminum foil, which eliminated all the noise sources.

Note the C-Band DBS dishes may have holes in the mesh, but the mesh
itself is solid sheet metal.


The net is built of 0.4 mm thread and the hole size is 1.5 * 1.5 mm.
I'm planning to use the reflector on the 23 cm band.

A 1.2GHz dish will probably be big. You're going to have some
structural issues to deal with. If you build a deep dish, getting the
wire mesh to make the required bends is going to be difficult, unless
you build it in wedges. If you build a flat dish, the mesh is easier
to handle, but the support for the feed might be ridiculously long.

I suggest you read the details of dish design, especially the part on
matching the f/D
(focal length / Diameter) of the dish, to the illumination angle of
the feed, which has a big effect on the dish efficiency:
http://www.w1ghz.org/antbook/contents.htm

Mesh dish kits:
http://www.rfhamdesign.com/products/parabolicdishkit/dishconstruction/index.html


I'm interested to do some experiment with the stress dish concept:
http://www.nitehawk.com/432_MHz_EME/port_dish.pdf

The antenna is to be used at my country cottage, I don't want to convert
it to a modern communication centre In the winter there are storms
and it can snow heavily at occasions. The stress dish would be perfect
to do some experiment with in the summer period and then put it aside
for the winter.

4m in diameter is the plan, I expect a gain just below 30dB, if I'm lucky.


The noise would be a real problem as the ultimate goal is to use the
antenna in EME works. Maybe a traditional galvanized chicken net is a
better choice? There are nets with 13*13 mm, around 1/17 lambda witch
should be OK.


Ben / SM0KBW