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