On Thu, 19 Jan 2012 11:13:07 +0100, Ben / SM0KBW 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...

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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

On Thu, 19 Jan 2012 21:22:26 +0100, Ben / SM0KBW wrote:

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.

I don't think it will be too horrible as long as the antenna parts and
pieces can be adjusted for optimum performance. At 1.2Ghz, that
shouldn't be much of a problem. If you were building at 10GHz, I
would be seriously worried.

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).

I will certainly consider inflatable antennas, funny idea!

Not funny. I've tried to build an inflatable tower for Field Day for
the last few years. Each year, something goes wrong, usually by
running out of time. With the proper materials and design, inflatable
antennas are fairly simple to construct out of vinyl and glue.

Incidentally, China has a storage space problem. So, for the annual
parades and events, all the decorations and props are inflatable. When
not in use, they're fairly small. Something like these arches:
http://www.china-inflatable.com/products/inflatable_arches_1.htm
Bosch Aerospace used to build inflatable towers for the military. It's
now LTA Projects. The videos are fun to watch:
http://ltaprojects.com
Yes, they have a ham version:
http://ltaprojects.com/ham.html

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

WELDED galvanized wire mesh, not ordinary chicken wire. Most chicken
wire is not welded.
http://www.wirefenceonline.com/welded/gaw



--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

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

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

73
Ben / SM0KBW

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