Parabolic reflector
 

Hello!

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

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?

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.


73
Ben / SM0KBW

Parabolic reflector
 

On Jan 16, 10:56*pm, Ben / SM0KBW wrote:
Hello!

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

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?

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.

73
Ben / SM0KBW

If it helps, I recall a long-ago rule for perforated and mesh
satellite dishes. The hole diameter could be up to one-tenth
wavelength and the reflector surface would still appear solid to the
EM waves. I had a 3m mesh dish for 4 Ghz c-band TV. It consisted of
twelve unconnected panels.

I'd say try it.

From my imagination: If you were to set up a normal VSWR test with
your 23 cm gear and move a section of the mesh toward the antenna,
you'd know somewhat whether the mesh is a reflector or is RF-
transparent. Compare the VSWR change to a corresponding test using
solid material. Just a thought from my fertile brain (with no mention
of what kind of fertilizer).

Parabolic reflector
 

John Markham skrev 2012-01-19 06:49:
On Jan 16, 10:56 pm, Ben / wrote:
Hello!

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

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?

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.

73
Ben / SM0KBW

If it helps, I recall a long-ago rule for perforated and mesh
satellite dishes. The hole diameter could be up to one-tenth
wavelength and the reflector surface would still appear solid to the
EM waves. I had a 3m mesh dish for 4 Ghz c-band TV. It consisted of
twelve unconnected panels.


Yes, those rules do trigger something in my brail too ;)

I'd say try it.

From my imagination: If you were to set up a normal VSWR test with
your 23 cm gear and move a section of the mesh toward the antenna,
you'd know somewhat whether the mesh is a reflector or is RF-
transparent. Compare the VSWR change to a corresponding test using
solid material. Just a thought from my fertile brain (with no mention
of what kind of fertilizer).


Of course, you right, empirical is certainly a good way to decide if the
net is good enough. The secret fertilizer you use seems to work ;)

73
Ben / SM0KBW

Parabolic reflector
 

Ben / SM0KBW skrev 2012-01-19 07:34:
John Markham skrev 2012-01-19 06:49:
On Jan 16, 10:56 pm, Ben / wrote:
Hello!

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

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?

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.

73
Ben / SM0KBW

If it helps, I recall a long-ago rule for perforated and mesh
satellite dishes. The hole diameter could be up to one-tenth
wavelength and the reflector surface would still appear solid to the
EM waves. I had a 3m mesh dish for 4 Ghz c-band TV. It consisted of
twelve unconnected panels.


Yes, those rules do trigger something in my brail too ;)


Typo should have been brain not brail!!



I'd say try it.

From my imagination: If you were to set up a normal VSWR test with
your 23 cm gear and move a section of the mesh toward the antenna,
you'd know somewhat whether the mesh is a reflector or is RF-
transparent. Compare the VSWR change to a corresponding test using
solid material. Just a thought from my fertile brain (with no mention
of what kind of fertilizer).


Of course, you right, empirical is certainly a good way to decide if the
net is good enough. The secret fertilizer you use seems to work ;)

73
Ben / SM0KBW

Parabolic reflector
 

On Tue, 17 Jan 2012 07:56:44 +0100, Ben / SM0KBW 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

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

Parabolic reflector
 

Jeff Liebermann wrote:

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

Long ago and far away, I had one of those. It was not made of mesh, it was
made of a piece of sheet metal that had been perforated. The holes have to
be exact, besides the problems Jeff mentioned, they have to be small enough
and properly spaced that the dish still acts as a reflector at the frequencies
desired.

That's why Ku band (10gHz) dishes are usually solid. The mesh would have to be
very fine and precise.

The holes are not there to make it cheaper to produce the dishes, they are
to reduce weight and wind loading. I assume in colder climates they are
also there to reduce build up of rain water or snow.

Here in Jerusalem we have snow storms every 4-5 years. The last one was
enough that the snow caused our satellite dish to malfunction, which was
not welcomed by two kids stuck at home. Due to the rarity of the snowfalls,
the hilly terrain and the warm temperatures, schools just close for the
day of the storm and then reopen the next day after it has all melted.

Geoff.


--
Geoffrey S. Mendelson, N3OWJ/4X1GM
My high blood pressure medicine reduces my midichlorian count. :-(

Parabolic reflector
 

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

Parabolic reflector
 

On Tue, 17 Jan 2012 07:56:44 +0100, Ben / SM0KBW wrote:

Hello!

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

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?

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.


73
Ben / SM0KBW

If these work at 2.5 GHz, I think you'll be able to use about anything as
long as the parabola is correct. http://www.qsl.net/n9zia/wireless/
page04.html

Parabolic reflector
 

On Thu, 19 Jan 2012 07:46:40 -0600, dave wrote:

On Tue, 17 Jan 2012 07:56:44 +0100, Ben / SM0KBW wrote:

Hello!

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

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?

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.


73
Ben / SM0KBW

If these work at 2.5 GHz, I think you'll be able to use about anything
as long as the parabola is correct. http://www.qsl.net/n9zia/wireless/
page04.html

http://www.qsl.net/n9zia/wireless/pi...4dB_dish-1.jpg

Parabolic reflector
 

On Thu, 19 Jan 2012 08:59:04 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

Jeff Liebermann wrote:

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

Long ago and far away, I had one of those. It was not made of mesh, it was
made of a piece of sheet metal that had been perforated. The holes have to
be exact, besides the problems Jeff mentioned, they have to be small enough
and properly spaced that the dish still acts as a reflector at the frequencies
desired.

That's why Ku band (10gHz) dishes are usually solid. The mesh would have to be
very fine and precise.

Ummm... 10GHz is X band and DBS Ku band is 13Ghz. There are a few
wire mess and/or perforated sheet metal dishes available for DBS
satellite TV.
http://i01.i.aliimg.com/photo/v1/475037631/KU_BAND_75CM_SATELLITE_MESH_DISH_ANTENNA.jpg

The holes are not there to make it cheaper to produce the dishes, they are
to reduce weight and wind loading. I assume in colder climates they are
also there to reduce build up of rain water or snow.

Nope. There are black teflon coated dishes for that. Some have
heater wires imbedded in the fiberglass reflector. Also radomes. The
problem with ice and mesh is that the mesh is generally weaker than a
solid dish. Once encrusted with ice, the mesh might rip, bend, or
tear.

Here in Jerusalem we have snow storms every 4-5 years. The last one was
enough that the snow caused our satellite dish to malfunction, which was
not welcomed by two kids stuck at home. Due to the rarity of the snowfalls,
the hilly terrain and the warm temperatures, schools just close for the
day of the storm and then reopen the next day after it has all melted.

Paint it with glossy black paint. I did that to some white fiberglass
antennas. The ice just slides off the antenna once the sun appears.
It's not a total solution, but it helps.


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

Parabolic reflector
 

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

Parabolic reflector
 

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

Parabolic reflector
 

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

Parabolic reflector
 

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

Parabolic reflector
 

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