On Dec 30, 12:38*am, "Sal M. Onella"
wrote:
"Dave" wrote in message

news
"Sal M. Onella" wrote in message
...

"Dave" wrote in message
.. .

"Art Unwin" wrote in message

...





Well the idea of 1 inch mesh was a bum idea. I should have kept to my
own thinking.
When transmitters have holes in the casing of 1 inch diameter will be
the time I will use such large holes.
Will now have to take it of and replace with aluminum window mesh.
*The present mesh has no idication of working in any sence of the word
Art

let me give you a hint... its not the mesh that is the problem.

The theoretical gain of a dish is expressed as (9.87 times D-squared) /
(wavelength-squared), where D is the dish diameter. *If you have a 3
meter
dish and you're working 10m, I calculate the gain as less than unity.
Ref:
http://en.wikipedia.org/wiki/Parabolic_antenna

Even then, the efficiency of the dish is assumed to be 100% -- which it
never is. *One limitation is the effectiveness of illuminating the
entire
surface of the dish uniformly. *How can you do that at HF? *You need a
compact illuminator at the focal point of the dish but HF doesn't lend
itself to such gyrations.

I think you should not consider a dish for HF. *It works only for
wavelengths that are small, compared to the dish size. *Don't fight the
math.

what do you get for gain when you use it on 160m like art is doing?

OK, a 3 meter dish at 160m:

Numerator is 9.87 times 3-squared *= *88.83
Denominator is 160-squared *= *25600

The quotient is the nominal power gain *= *0.00347

In technical terms, this equals a fart in a windstorm.

---------------------------------------------------------------------------

BTW, to validate the formula for a practical dish, plug in the values for
the same 3-meter dish, but use C-band TVRO freqs around 4 GHz. Take the log
of the quotient, multiply by 10 and you get close the customary 40dB gain
associated with those backyard beauties. *(I just got rid of mine this year;
the new owner wants to try EME at 1.2 GHz.)

"Sal"
(KD6VKW)

Sal
Are you using formula based on phasing i.e. has a focal point?
If so that is not applicable to CP (circular polarisation) antennas
One is a broardside radiator and the other is a axial or end fire
radiator.
BIG BIG difference. If you study the use of reflectors with helix
antennas you will see that all
reflectors used are straight sided whether as a flat plate, cupped or
as in one instance
conical for the length of the antenna. The parabala is based on inter
inductive coupling
of a dipole so the parabola reflects at maximum current amplitude.,
The helix reflector is not based on
coupling but the true mechanical impact of particles which is vastly
different. It is wonderful when
you use formulas but it is always best to initial perform the
calculation from first principles
to ensure that the formula is applicable where you intend to use it
I have removed my dish and have replaced it with a sheath an aproach
that has already been used for CP
radiators.

Art