On Sat, 21 Jan 2012 14:27:10 +0100, Ben / SM0KBW wrote:

Jeff Liebermann skrev 2012-01-20 06:19:
On Thu, 19 Jan 2012 21:22:26 +0100, Ben / wrote:

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.

Nope. As long as the width of the antenna is 1/2 wave or larger, it
will work with CP. I don't think that there's any way such a
reflector can convert circular polarization into elliptical (but I'm
not sure).

Are we talking about the same type of reflector?
I'm thinking about a Parabolic Trough:
http://www.solar-facts.com/light-con...-reflector.php

Nope. Different form factor. I'll see if I can dig out some photos
(a major project). Mine was very different. Visualize taking an
ordinary parabolic dish, and slicing off a chord from opposite sides
of the dish. The result is a large parabolic shape in one axis, and a
much smaller parabolic shape in the other axis. Something like these,
but with a much more radical aspect ratio:
http://cdn3.explainthatstuff.com/solar-cooker-parabolic-reflector.jpg
http://www.greenjoyment.com/images/butterfly-community-solar-cooker.jpg
http://www.dipol.com.pl/pict/a72122+.jpg
As I vaguely recall, one reflector axis was about 15ft, while the
other was about 3ft.

Obviously the gain will be reduced if large parts of the reflector
disappeared. However, if the surface area of the dish were maintained
by adding to the longer axis, most of the lost gain can be recovered.
Note that most of the gain provided by the dish comes from reflections
close to the center axis (near the feed). As the sides of the dish
get steeper, as in a deep dish, the increase in gain per dish area
added becomes less. When the sides of the dish are nearly
perpendicular to the main lobe, no gain is produced at all. Therefore,
a flat dish (large f/D) is a more efficient use of reflector area,
than a deep dish (small f/D).

I thought that this type of reflector just have one polarization plane?

I'm not sure, but I don't think it matters. As long as the continuous
reflective surface is large enough, the reflected signal will not
magically convert from circular to elliptical polarization. It will
change sense (RHCP to LHCP), but not symmetry. The problem is that
I'm having some trouble visualizing how it works and need to think
about this for a while.

WEFAX on 1691MHz is circularly polarized as are most satellites.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558