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

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

Remove .spam.sux to reply.

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

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.

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

Art Unwin wrote:
On Nov 30, 5:55Â*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. Â*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. Â* -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.

Art wrote:
"I stated "intuitively" since the radiation is axia with respect to the
helix."

A 3 M diameter has about 9.42 M circumference so becomes an effective
axial radiator instead of a radial radiator at the frequency near where
its circumference becomes one wavelength. This frequency is
300/(pi)(diameter) or about 10 MHz. A reflecting surface to make an
axial helix unidirectional needs to be at least as large as the
projection of the helix on the surface of the reflector. Its diameter
should then be about 3 M or more and would be effective only at
frequencies exceeding 10 MHz which is where the helix becomes an
end-fire device anyway.

Best regards, Richard Harrison, KB5WZI

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

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

Remove .spam.sux to reply.

"Art Unwin" wrote in message
...
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

he's sucking you all in now... 'TOP BAND'!!! yes, he means a 3m DISH as a
reflector for his axial mode helix that is probably only inches in diameter
on 160m! this is his dream 'rifle shot' antenna!