On Sep 11, 9:18*am, Art Unwin wrote:
On Sep 11, 2:53*am, Jon Kåre Hellan wrote:





writes:
Roy Lewallen wrote:
wrote:

Of course.

Everyone knows the gain of a parabola is directly proportional to
the size in wavelengths, or:

G=10*log k(pi*D/L)^2

Where G= gain in DB over an isotropic, k ~ .55 for most real parabolas,
D is the diameter, and L is the wavelength (wavelength and diameter
in the same units.

So a 2,000 foot parabola on 20m would have just about 58db gain.

Hm. I get 47.

Roy Lewallen, W7EL

Hmm, when I use 14 Mhz and 6 decimal places I get 37; must have fat
fingered it the first time.

Going a different way, I also get about 37.

Aperture of a dish is the area, pi*r^2. r is about 14.2 wl, so area
is about 635 sq. wl.

Aperture of a dipole is 1/4 * 1/2 wl = 1/8 sq. wl.

That makes gain 635/(1/8) = 635*8, i.e about 5100 or just over 37
dBd. This assumes 100 % illumination of the dish, which we won't
achieve. So make it 35 dBd or so, i.e. 37 dBi. Using the o.p.'s
formula, I get 36.5 dBi. *

It's odd that pi is squared in the formula. The squared part must be
to account for the area of the dish, which is pi*r^2. Obviously, this
can has been compensated for by the choice of 'k'.

Whoaaa guys............!

*Let us think a bit more regarding the basics presented instead of
parrotting
dish's as used in the present state of the art.
Isn't a dish built around phase change of a half wave dipole in inter
magnetic coupling?
If I have a flash light that is focussed does this wavelength aproach
still apply?
*I thought it would be a question of action and reaction. Trow a ball
against the wall and it bounces back
in a reflective manner to the angle of velocity.
A dish as presently used changes the phase of a given signal to
reverse it's direction.
In physics we can also talk about mechanical force that rebound and
rebound has nothing to do
with wavelength! * * *If we consider radiation as being the projection
of particles instead of wavelike oscillation
then surely the size of the reflector is solely based on what can be
collected from the
*emmitter such that it rebounds to a point or a focussed form ?
*I ask the question as I know nothing about the reflective phenomina
of dish's tho I have visited
*the one in P.R. where the dish is formed with the knoweledge that the
radiation spreads out
according to the emmiter used and thus when it reaches the reflector
the unit strength is weaker which the
dish attempts to reverse by refocussing. But then I could be totally
in error thus the question to the experts
Best regards
Art Unwin KB9MZ
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I think you cannot use the particle analogy with HF when dish size is
not greater than wavelength. For a small dish at HF, the waves will
simply bend around the dish and act as if it wasn't there. At much
higher frequencies, particle concepts become more accurate.