Art Unwin wrote:
On Nov 30, 7:05Â*pm, wrote:
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.

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

Remove .spam.sux to reply.

On Nov 30, 7:55*pm, wrote:
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.

I am aware of that. One person once said the world was round!


If one was persuasive enough at the beginninghundred years or so later
if one said
the world was flat he would be called delusional.
If one was very persuasive in the first place he would be rediculed as
it was so obvious to all in the first place.
I hope you get to live another hundred years so that you can meld into
the new crowd of lemmings
Regards
Art





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

Remove .spam.sux to reply.

Art Unwin wrote:

I am aware of that. One person once said the world was round!


Actually most learned people knew the world was round since before
Christ. The circumference was measured to something better than 10%
accuracy using only sticks and shadows.


If one was persuasive enough at the beginninghundred years or so later
if one said
the world was flat he would be called delusional.
If one was very persuasive in the first place he would be rediculed as
it was so obvious to all in the first place.
I hope you get to live another hundred years so that you can meld into
the new crowd of lemmings
Regards
Art

And delusional you are. No doubt about it.

tom
K0TAR

Tom Ring wrote:

...
Actually most learned people knew the world was round since before
Christ. The circumference was measured to something better than 10%
accuracy using only sticks and shadows.

tom
K0TAR

Actually, "sticks and shadows" is nice, for emphasis ... "wells and
shadows" would be a bit more accurate:

http://www.madsci.org/posts/archives...4968.Sh.r.html

Regards,
JS