We've had a LOT of fun (& publicity )here in New Zealand over DIY
antenna brewed up for 2.4GHz USB WiFi.See = www.usbwifi.orcon.net.nz
The parabolic Chinese cookware used gave ~15dB gain at 2.4GHz, but of
course any radiation will be concentrated at a parabola's focal point.
Aha - could suit "log cabin in the woods " cell phone boosting ?
Indeed it does, & 900/1800MHz GSM cell phone reception improvement has
also been verified when the phone is cradled at the FP.
Naturally you'll find it hard to speak at this position,unless perhaps
a Bluetooth headset used, but at least inward calls/text messages can
now be received. See = http://www.usbwifi.orcon.net.nz/cellbear.jpg
& http://www.usbwifi.orcon.net.nz/wokaxim.jpg

This sure beats constructing Yagis,& 12" woks here in NZ are just
~US$5! Stan ( ZL2AJZ )

We've had a LOT of fun (& publicity )here in New Zealand over DIY
antenna brewed up for 2.4GHz USB WiFi.See = www.usbwifi.orcon.net.nz
The parabolic Chinese cookware used gave ~15dB gain at 2.4GHz, but of
course any radiation will be concentrated at a parabola's focal point.
Aha - could suit "log cabin in the woods " cell phone boosting ?
Indeed it does, & 900/1800MHz GSM cell phone reception improvement has
also been verified when the phone is cradled at the FP.
Naturally you'll find it hard to speak at this position,unless perhaps
a Bluetooth headset used, but at least inward calls/text messages can
now be received. See = http://www.usbwifi.orcon.net.nz/cellbear.jpg
& http://www.usbwifi.orcon.net.nz/wokaxim.jpg

This sure beats constructing Yagis,& 12" woks here in NZ are just
~US$5! Stan ( ZL2AJZ )


Hi Stan,

Mine doesn't quite fit a paraboloid. Here's some fun checking the gain:

Gain approx= 10 D(waves)^2

My wok is about 36 cm, or approximately 3 waves. Thus I would expect about a
factor of 90 in gain with 100% aperture efficiency.

You've possibly lost about 2-4 dB. But--you're cookin' anyway:-)

73,
Chip N1IR

Chip - "cooking " is right ! We're "reflecting" on a small front
screen to enhance illumination in fact. Of course mesh parabolas
typically only have 80% of the gain of a solid dish, but extensive
field trials have verified ~15dB for the 300mm (12") mesh scoop at
2.4GHz, & ~10dB for a 170mm at the same freq.At say 1800MHz cellular
freqs, gain only perhaps 2/3rd this =10dB for 12"?

Ref numerous pix & lab. notes = www.usbwifi.orcon.net.nz to support
things

Stan ( ZL2AJZ)

. Of course mesh parabolas
typically only have 80% of the gain of a solid dish,

I would seriously doubt the veracity of this statement and would be
gratified to see valid documented evidence of this. If the mesh openings
are under 0.1 lambda the mesh will appear solid. The problem may be
achieving good surface accuracy with a mesh dish.Those of us using mesh
dishes on 1296 MHz EME are seeing excellent efficiencies with mesh surfaces-
ultimately limited by the ability to properly illuminate the surface. For
EME we tend to underilluminate the surface to take advantage of lower side
lobes and less spill over, which means "seeing" less warm earth noise.

Dale W4OP

. Of course mesh parabolas
typically only have 80% of the gain of a solid dish,

I would seriously doubt the veracity of this statement and would be
gratified to see valid documented evidence of this. If the mesh openings
are under 0.1 lambda the mesh will appear solid. The problem may be
achieving good surface accuracy with a mesh dish.Those of us using mesh
dishes on 1296 MHz EME are seeing excellent efficiencies with mesh surfaces-
ultimately limited by the ability to properly illuminate the surface. For
EME we tend to underilluminate the surface to take advantage of lower side
lobes and less spill over, which means "seeing" less warm earth noise.

Dale W4OP

Hi Dale,

A wok has no focal 'point': it is not a true paraboloid. Its the imperfection
in shape, not mesh:-) Ergo the loss in aperture efficiency.

73,
Chip N1IR

Fractenna wrote:

. Of course mesh parabolas

typically only have 80% of the gain of a solid dish,

I would seriously doubt the veracity of this statement and would be
gratified to see valid documented evidence of this. If the mesh openings
are under 0.1 lambda the mesh will appear solid. The problem may be
achieving good surface accuracy with a mesh dish.Those of us using mesh
dishes on 1296 MHz EME are seeing excellent efficiencies with mesh surfaces-
ultimately limited by the ability to properly illuminate the surface. For
EME we tend to underilluminate the surface to take advantage of lower side
lobes and less spill over, which means "seeing" less warm earth noise.

Dale W4OP


Hi Dale,

A wok has no focal 'point': it is not a true paraboloid. Its the imperfection
in shape, not mesh:-) Ergo the loss in aperture efficiency.

For the purposes of radio signals, does the shape need to be a specific
paraboloid? Certainly at optical wavelengths a parabola is needed to
focus light from infinity, so a parabola is needed. But at radio
Frequency wavelengths a true parabola is not likely needed. In fact,
considering the difference between a sphere and a parabols, I would
suspect that anything out there is not a true parabola unless it is
arrived at by chance.

- Mike KB3EIA -

"Mike Coslo" wrote in message
...
Fractenna wrote:

. Of course mesh parabolas

typically only have 80% of the gain of a solid dish,

I would seriously doubt the veracity of this statement and would be
gratified to see valid documented evidence of this. If the mesh
openings
are under 0.1 lambda the mesh will appear solid. The problem may be
achieving good surface accuracy with a mesh dish.Those of us using mesh
dishes on 1296 MHz EME are seeing excellent efficiencies with mesh
surfaces-
ultimately limited by the ability to properly illuminate the surface.
For
EME we tend to underilluminate the surface to take advantage of lower
side
lobes and less spill over, which means "seeing" less warm earth noise.

Dale W4OP


Hi Dale,

A wok has no focal 'point': it is not a true paraboloid. Its the
imperfection
in shape, not mesh:-) Ergo the loss in aperture efficiency.

For the purposes of radio signals, does the shape need to be a specific
paraboloid? Certainly at optical wavelengths a parabola is needed to
focus light from infinity, so a parabola is needed. But at radio
Frequency wavelengths a true parabola is not likely needed. In fact,
considering the difference between a sphere and a parabols, I would
suspect that anything out there is not a true parabola unless it is
arrived at by chance.

- Mike KB3EIA -


Hi MIke,
I am not certain I understand your answer- or Chip's.
The original poster made the statement that mesh parabolas are only 80% as
efficient as a solid parabola. That was what I was answering.

I can tell you that my 14' 1296 dish is within 5mm of being a true parabola-
this is well within the 0.1 lambda RMS error.

For my purposes- EME- an imperfect parabola (this includes sperical
surfaces) would lead to not only a degradation in gain, but much higher
sidelobes and therefore worse G/T. It is not a difficult exercise to measure
Tsys and therefore know just how well your dish is playing.
Spherical antennas certainly have their place- particularly when the surface
is fixed and the beam is then steered by moving the feedhorn- i.e. Arecibo.
But from memory, they take 2nd place to the parabola when one considers
dB/reflector area
Dale W4OP

Let's try to summarise, and sort out some of the confusion:

* As Dale says, a mesh reflector is almost as efficient as a solid
surface of the same shape, even for hole sizes as large as 0.1wl.

* That was a side-track anyway, since the OP doesn't have a mesh wok.

* Any vaguely bowl-shaped reflector - including a wok - will make a big
improvement in cellphone performance, if you locate the cellphone at
the best possible place, line the whole thing up correctly, and manage
to keep your head out of the way.

* A paraboloid is the best shape for a reflector, because only a
paraboloid can focus all the incoming rays to a single point; and all
the incoming energy from a plane wavefront will arrive in-phase at the
focal point. This applies equally to light and radio waves. Optical
ray-tracing theory breaks down if the reflector is only a few
wavelengths in diameter, but a paraboloid is still the best practical
shape to aim for.


As Dale says, Arecibo uses a spherical reflector to allow a few degrees
of beam steering by pointing the feed antenna at different areas of the
dish. However, this is a very special case: the only practical way to
achieve a 1000ft dish was to build it immovably on the ground, so the
designers then had to find some other way to steer the beam, by moving
the feed antenna at the focus. In this one special case, the optimum
shape for the reflector is not a paraboloid but a sphere (because the
geometry of a sphere is the same in any direction, as seem from the
feedpoint at the centre).

The problem of course is that a sphere doesn't *have* a focal point - it
has a smeared-out focal line with phase variations along it. For many
years, long slotted-waveguide feeds were used to collect the available
energy from along this focal line, and to compensate for the phase
variations. By doing this, the Arecibo designers were able to achieve
similar efficiencies to a paraboloid of the same size, and with some
degree of steerability too. The limitation was that a different feed was
required for every frequency of operation, and this obviously restricted
the range of research.

With computer-aided design replacing 1950s slide-rules, Arecibo now has
a sub-reflector system of a very cunning shape that compensates for the
phase variations. Being a reflector it is not frequency-sensitive, and
it brings everything into focus at a conventional single-point feedhorn
which can be changed in relative comfort - that is, if anyone can feel
comfortable suspended out on cables, 500ft in mid-air...

Because Puerto Rico is quite close to the Equator, the Moon passes
overhead from time to time, and this allowed the Arecibo observatory to
do pioneering work on radar mapping of the Moon's surface. It also
turned out that the old 400MHz line feed would work reasonably well on
432MHz, and back in the old days there were occasional empty time-slots
when the dish would appear on EME. Sometime in the mid-1980s I had the
good fortune to work KP4I on what seems to have been the last
opportunity. He was LOUD.


(That's an outsider's view of Arecibo. Chip, please correct any
inaccuracies :-)




--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Let's try to summarise, and sort out some of the confusion:

* As Dale says, a mesh reflector is almost as efficient as a solid
surface of the same shape, even for hole sizes as large as 0.1wl.

* That was a side-track anyway, since the OP doesn't have a mesh wok.

* Any vaguely bowl-shaped reflector - including a wok - will make a big
improvement in cellphone performance, if you locate the cellphone at
the best possible place, line the whole thing up correctly, and manage
to keep your head out of the way.

* A paraboloid is the best shape for a reflector, because only a
paraboloid can focus all the incoming rays to a single point; and all
the incoming energy from a plane wavefront will arrive in-phase at the
focal point. This applies equally to light and radio waves. Optical
ray-tracing theory breaks down if the reflector is only a few
wavelengths in diameter, but a paraboloid is still the best practical
shape to aim for.


As Dale says, Arecibo uses a spherical reflector to allow a few degrees
of beam steering by pointing the feed antenna at different areas of the
dish. However, this is a very special case: the only practical way to
achieve a 1000ft dish was to build it immovably on the ground, so the
designers then had to find some other way to steer the beam, by moving
the feed antenna at the focus. In this one special case, the optimum
shape for the reflector is not a paraboloid but a sphere (because the
geometry of a sphere is the same in any direction, as seem from the
feedpoint at the centre).


Basically correct. Although the tracking is better than just a few degrees:-)

Arecibo was initially designed to be a survellance instrument, passively
listening to Soviet communications through inadvertant moonbounce. It also was
designed, initially, as an ionospheric heating facility.

Through the huge luck of its overengineering, it was found to be able to track
quite accurately, and the feeds and carriage houses were designed to
accommodate a greater tracking range.

Today the site no longer has chicken wire (and hasn't for 30 years); and the
feeds are Gregorians that accommodate very large bandwidths and spectral
ranges. Most of the recent upgrades were funded by the NSF, and also the Seti
Institute.

73,
Chip N1IR

For the purposes of radio signals, does the shape need to be a specific
paraboloid? Certainly at optical wavelengths a parabola is needed to
focus light from infinity, so a parabola is needed. But at radio
Frequency wavelengths a true parabola is not likely needed. In fact,
considering the difference between a sphere and a parabols, I would
suspect that anything out there is not a true parabola unless it is
arrived at by chance.

- Mike KB3EIA -


No; but optimum aperture efficiency is attained with a point feed, as you get
with a true paraboloid. Spheres work too--if you can make a good line feed.

If you are willing to throw a few dB away, and don't care about the sidelobes
and shape of the main beam to any great degree, then all kinds of imperfections
from paraboloids work well.

Of course, a pringle can also works at 2.4...so there are many options.

73,
Chip N1IR