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Old November 30th 08, 08:02 PM posted to rec.radio.amateur.antenna
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Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG

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Old November 30th 08, 09:45 PM posted to rec.radio.amateur.antenna
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Art Unwin wrote:
Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG


The rule of thumb is things less that .1 wavelengths in size have
negligable effects, so roughly up to about 1 GHz.


--
Jim Pennino

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Old December 1st 08, 12:25 AM posted to rec.radio.amateur.antenna
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Default Reflector mesh surface

On Nov 30, 5:55*pm, wrote:
Richard Clark wrote:
On Sun, 30 Nov 2008 21:45:03 GMT, wrote:


Art Unwin wrote:
Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG


The rule of thumb is things less that .1 wavelengths in size have
negligable effects, so roughly up to about 1 GHz.


Hi Jim,


You neglect to mention that a 3 meter dish would be wholly (no pun
intended) transparent to RF in the top band. *It could be solid metal
to no notice by a radiator (or receiver).


It would be like blocking the sound 3 feet from the stage at a Led
Zepplin concert with a Japanese parasol. * -Huh?-


73's
Richard Clark, KB7QHC


Yeah, I foolishly forgot the source of the question and assumed "top
band" meant highest useable frequency.

Unless I hosed the math, a 3 m dish would be a little over 10 db
isotropic at 2 m, so the combinatation would "work" from 2 m to 23 cm.

--
Jim Pennino

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I ask the question because the radiator is a helix end fed, helix
being approx 0.3 M diameter. Intuitively, I thought that the 3 M dish
would be large enough
and thus needed a check on the perforations. Could you show how the
"combination" changed things so drastically so I may benefit from the
question?
I stated "intuitively" since the radiation is axia with respect to the
helix
Many thanks
Art


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Old December 1st 08, 12:43 AM posted to rec.radio.amateur.antenna
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On Sun, 30 Nov 2008 23:55:02 GMT, wrote:

Richard Clark wrote:
On Sun, 30 Nov 2008 21:45:03 GMT,
wrote:

Art Unwin wrote:
Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG

The rule of thumb is things less that .1 wavelengths in size have
negligable effects, so roughly up to about 1 GHz.


Hi Jim,

You neglect to mention that a 3 meter dish would be wholly (no pun
intended) transparent to RF in the top band. It could be solid metal
to no notice by a radiator (or receiver).

It would be like blocking the sound 3 feet from the stage at a Led
Zepplin concert with a Japanese parasol. -Huh?-

73's
Richard Clark, KB7QHC


Yeah, I foolishly forgot the source of the question and assumed "top
band" meant highest useable frequency.

Unless I hosed the math, a 3 m dish would be a little over 10 db
isotropic at 2 m, so the combinatation would "work" from 2 m to 23 cm.


Well, if you take the question literally, the aperature of a 3 meter
dish is ummm.... 3 meters. If he had said effective aperature, the
answer might be different.

As for top band (160 meters), he might be referring to a chicken wire
ground. This looks marginally applicable:
http://lists.contesting.com/_topband/2002-09/msg00005.html

Maybe he's thinking of burying his 3 meter dish and using it as a
ground radial system?

Incidentally, I've had rather bad luck using twisted galvanized
chicken wire for making dish antennas. I kept getting intermittent
noise crashes and arcing like noise bursts on a 1.7GHz WX satellite
system I once threw together. I eventually found that banging on the
chicken wire would produce the same noise bursts. I didn't bother
trying to find out exactly what was doing the arcing. When I switched
to using aluminum flashing and aluminum duct tape for the reflector,
the noise went away.

--
Jeff Liebermann

150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Old December 1st 08, 01:05 AM posted to rec.radio.amateur.antenna
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Default Reflector mesh surface

Art Unwin wrote:
On Nov 30, 5:55*pm, wrote:
Richard Clark wrote:
On Sun, 30 Nov 2008 21:45:03 GMT, wrote:


Art Unwin wrote:
Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG


The rule of thumb is things less that .1 wavelengths in size have
negligable effects, so roughly up to about 1 GHz.


Hi Jim,


You neglect to mention that a 3 meter dish would be wholly (no pun
intended) transparent to RF in the top band. *It could be solid metal
to no notice by a radiator (or receiver).


It would be like blocking the sound 3 feet from the stage at a Led
Zepplin concert with a Japanese parasol. * -Huh?-


73's
Richard Clark, KB7QHC


Yeah, I foolishly forgot the source of the question and assumed "top
band" meant highest useable frequency.

Unless I hosed the math, a 3 m dish would be a little over 10 db
isotropic at 2 m, so the combinatation would "work" from 2 m to 23 cm.

--
Jim Pennino

Remove .spam.sux to reply.


I ask the question because the radiator is a helix end fed, helix
being approx 0.3 M diameter. Intuitively, I thought that the 3 M dish
would be large enough
and thus needed a check on the perforations. Could you show how the
"combination" changed things so drastically so I may benefit from the
question?
I stated "intuitively" since the radiation is axia with respect to the
helix
Many thanks
Art


A helix doesn't radiate in the axial mode unless the circumference is
between about 0.75 and 1.33 wavelengths.

The would put the minimum frequency for a .3 m diameter helix at about
240 Mhz and the maximum frequency at about 420 Mhz.

Also, the reflector for a helix is a flat surface, not a dish, and is
normally between about 0.8 to 1.1 wavelengths in diameter. If you make
it larger, the sidelobe levels increase.


--
Jim Pennino

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Old December 1st 08, 01:05 AM posted to rec.radio.amateur.antenna
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Jeff Liebermann wrote:

Incidentally, I've had rather bad luck using twisted galvanized
chicken wire for making dish antennas. I kept getting intermittent
noise crashes and arcing like noise bursts on a 1.7GHz WX satellite
system I once threw together. I eventually found that banging on the
chicken wire would produce the same noise bursts. I didn't bother
trying to find out exactly what was doing the arcing. When I switched
to using aluminum flashing and aluminum duct tape for the reflector,
the noise went away.


I've read some reports that such is due to the poor contact of the
twists in chicken wire.

I suppose soldering all the twists would fix it.

--
Jim Pennino

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Old December 1st 08, 01:17 AM posted to rec.radio.amateur.antenna
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On Nov 30, 7:05*pm, wrote:
Art Unwin wrote:
On Nov 30, 5:55*pm, wrote:
Richard Clark wrote:
On Sun, 30 Nov 2008 21:45:03 GMT, wrote:


Art Unwin wrote:
Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG


The rule of thumb is things less that .1 wavelengths in size have
negligable effects, so roughly up to about 1 GHz.


Hi Jim,


You neglect to mention that a 3 meter dish would be wholly (no pun
intended) transparent to RF in the top band. *It could be solid metal
to no notice by a radiator (or receiver).


It would be like blocking the sound 3 feet from the stage at a Led
Zepplin concert with a Japanese parasol. * -Huh?-


73's
Richard Clark, KB7QHC


Yeah, I foolishly forgot the source of the question and assumed "top
band" meant highest useable frequency.


Unless I hosed the math, a 3 m dish would be a little over 10 db
isotropic at 2 m, so the combinatation would "work" from 2 m to 23 cm.


--
Jim Pennino


Remove .spam.sux to reply.


I ask the question because the radiator is a helix end fed, helix
being approx 0.3 M diameter. Intuitively, I thought that the 3 M dish
would be large enough
and thus needed a check on the perforations. Could you show how the
"combination" changed things so drastically so I may benefit from the
question?
I stated "intuitively" since the radiation is axia with respect to the
helix
Many thanks
Art


A helix doesn't radiate in the axial mode unless the circumference is
between about 0.75 and 1.33 wavelengths.

The would put the minimum frequency for a .3 m diameter helix at about
240 Mhz and the maximum frequency at about 420 Mhz.

Also, the reflector for a helix is a flat surface, not a dish, and is
normally between about 0.8 to 1.1 wavelengths in diameter. If you make
it larger, the sidelobe levels increase.

--
Jim Pennino

Remove .spam.sux to reply.


No, No, No. That is not correct. Just as Kraus over estimated the gain
of helix antennas he also made a mistake
in specifying a particular helix angle as well as the impression that
one needs a large diameter helix with reference to wave length.
Ofcourse his work is over 50 years old and is being wittled down with
futher investigation.
I modeled it against perfect ground as the latest papers state that a
wall around the bottom where there is maximum current provides best
gain.
Thus I see the deep and large diameter dish as being an equivalent.
If .1 WL is a rule of thumb then it would seem your first response is
correct.
That is my personal view but I am open to contradiction hopefully with
reasons why
Art
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Old December 1st 08, 01:55 AM posted to rec.radio.amateur.antenna
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Posts: 1,895
Default Reflector mesh surface

Art Unwin wrote:
On Nov 30, 7:05*pm, wrote:
Art Unwin wrote:
On Nov 30, 5:55*pm, wrote:
Richard Clark wrote:
On Sun, 30 Nov 2008 21:45:03 GMT, wrote:


Art Unwin wrote:
Since we are at the low sun cycle I shoose
1" hexigon poultry wire for the mash on my 3 metre dish
At what frequency will there be a noticable effect on aperture
starting with top band?
Many thanks
Art KB9MZ...,XG


The rule of thumb is things less that .1 wavelengths in size have
negligable effects, so roughly up to about 1 GHz.


Hi Jim,


You neglect to mention that a 3 meter dish would be wholly (no pun
intended) transparent to RF in the top band. *It could be solid metal
to no notice by a radiator (or receiver).


It would be like blocking the sound 3 feet from the stage at a Led
Zepplin concert with a Japanese parasol. * -Huh?-


73's
Richard Clark, KB7QHC


Yeah, I foolishly forgot the source of the question and assumed "top
band" meant highest useable frequency.


Unless I hosed the math, a 3 m dish would be a little over 10 db
isotropic at 2 m, so the combinatation would "work" from 2 m to 23 cm.


--
Jim Pennino


Remove .spam.sux to reply.


I ask the question because the radiator is a helix end fed, helix
being approx 0.3 M diameter. Intuitively, I thought that the 3 M dish
would be large enough
and thus needed a check on the perforations. Could you show how the
"combination" changed things so drastically so I may benefit from the
question?
I stated "intuitively" since the radiation is axia with respect to the
helix
Many thanks
Art


A helix doesn't radiate in the axial mode unless the circumference is
between about 0.75 and 1.33 wavelengths.

The would put the minimum frequency for a .3 m diameter helix at about
240 Mhz and the maximum frequency at about 420 Mhz.

Also, the reflector for a helix is a flat surface, not a dish, and is
normally between about 0.8 to 1.1 wavelengths in diameter. If you make
it larger, the sidelobe levels increase.

--
Jim Pennino

Remove .spam.sux to reply.


No, No, No. That is not correct.


Yes, yes, yes, it is all corrct.

Just as Kraus over estimated the gain
of helix antennas he also made a mistake


Nothing was said about gain.

in specifying a particular helix angle


Nothing was said about the helix angle.

as well as the impression that
one needs a large diameter helix with reference to wave length.


A circumferance of 0.75 to 1.33 wavelengths is required for radiation
in the axial mode.

Ofcourse his work is over 50 years old and is being wittled down with
futher investigation.


Nope; correctness has no expiration date.

I modeled it against perfect ground


A pointless exercise.

as the latest papers state that a
wall around the bottom where there is maximum current provides best
gain.


What papers are these; the ones from the outhouse?

A short "wall" around the outer edge of the reflector reduces side lobes.

Thus I see the deep and large diameter dish as being an equivalent.


Most people would call that delusional.

If .1 WL is a rule of thumb then it would seem your first response is
correct.


Of course my first response was correct; things smaller than 0.1 wavelengths
can usually be ignored.


--
Jim Pennino

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