Jeff Liebermann skrev 2012-01-19 18:41:
On Thu, 19 Jan 2012 11:13:07 +0100, Ben / wrote:

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

That works for a few days. The problem with reflectors under tension
is that they don't stay in any stable position. The aluminum mesh is
the worst because it moves with temperature. If you look carefully at
various mesh reflectors available, they're usually galvanized steel.
However, at 1.2Ghz, the tolerances are much less than at X band or Ku
band, so it will probably work.

The antenna is to be used at my country cottage, I don't want to convert
it to a modern communication centre

The uglier the antenna, the better it works. Nice looking antennas
just don't seem to function as well as ugly kludges. Might as well
resign yourself to building an eyesore.



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.

My I suggest an infaltable dish antenna? (I'm not joking).
http://www.gatr.com
http://www.qsl.net/pe1rah/RAHdish.htm
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1203415
If you want it out of the way when the neigbhors arrive to protest,
and inflatable antenna structure might be the answer.

Another design I tried (and discarded) for the WEFAX antenna might be
worth raising from the dead. Instead of a dish, it's a parbolic
"strip" reflector. It was in the shape of a parabola in one axis (as
enforced by several sheets of plywood cut into a parabola like shape),
but was less than 1 meter wide. The idea was to obtain the same
surface area as a large antenna, but with a very different aspect
ratio. That makes it much easier to build, ship, and assemble. I'll
see if I can excavate some photos.

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

That's a reasonable size without becoming monsterous. No clue if
you'll get 30dB. I could do the calcs, but I'm busy right now.
http://www.antenna-theory.com/antennas/reflectors/dish3.php
Use 50% of the overall efficiency.

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.

I'm not sure, but I think anything under 1/10 wavelength should work.
Welded galvanized steel mesh should be fine.

Nicely built 4.5 meter dish:
http://www.keplerian.com/dish/4.5m_dish.html

Gotta run...


The stability is one of things I want to study - and you're probably
right in that long term stabilty will be a problem. I've done some
calculation and used 45% efficiency and the result was around 30 dB.

Yes I've thought about single curved parabolic reflectors as you
describe, but it would give polarization in one plane only and the
standard is circular polarization for EME at 23cm.

I will certainly consider inflatable antennas, funny idea!

Galvanized Chicken net is easier to obtain, cheaper and as you wrote,
it was what people used in earlier designs.


73
Ben / SM0KBW