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'.

writes:

On Sep 10, 10:29Â*pm, Art Unwin wrote:
No. The shoebox size antenna would approximate an isotropic if it did
radiate. It would still have to be placed at the focal point of a very
large parabola due to the size of the wave length. Such an antenna, I
believe, on the island of Puerto Rico (the SETI antenna) although it
is currently used primarily as a receiving antenna. That parabola is
positioned to have a very high radiation angle and might not be be
that good for terrestrial DX.

I believe that hams once were allowed to use Arecibo for EME on 80m.

[Slaps self upside the head] 47 dB for a 2000 meter dish, 37 dB for a
2000 foot dish. And that's why I didn't choose bridge design for a
career. . .

Roy Lewallen, W7EL

wrote:
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.

Working backward from 47 I get a wavelength of 21 feet.

wrote:
If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.

Is there any kind of material from which to build
a 14.2 MHz Maser? :-)
--
73, Cecil http://www.w5dxp.com

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

Arecibo? :-)
--
73, Cecil http://www.w5dxp.com

On Sep 10, 11:56*pm, wrote:
On Sep 10, 10:29*pm, Art Unwin wrote:





On Sep 10, 9:23*pm, Art Unwin wrote:

On Sep 10, 8:45*pm, wrote:

Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?

Money.

If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.

--
Jim Pennino

Remove .spam.sux to reply.

Then are you saying it is the antenna size that is the main factor?.
So my antenna which is physically small
can be focussed on a dish which would provide straight line radiation
or a radiation beam?
Working on a single element on the ground with a optimizer instead of
a half sphere I got a
straight vertical line at the sides which suggested a gun barrel
radiation with a perfect earth as the reflector.
Gain was around 8db vertical which is why the question regarding
focussing! If it was properly focussed the gain should be more.
2000 foot dish seems somewhat odd, probably based on a "straight"
wavelength and not a small volume in equilibriumas the directer
right?
Art

Let me ask the question another way. Whether it is believed or not,
if a 80 Metre antenna was compressed to the size of a couple of shoe
boxes
would the dish be reduced in size accordingly?
Regagards
Art- Hide quoted text -

- Show quoted text -

No. The shoebox size antenna would approximate an isotropic if it did
radiate. It would still have to be placed at the focal point of a very
large parabola due to the size of the wave length. Such an antenna, I
believe, on the island of Puerto Rico (the SETI antenna) although it
is currently used primarily as a receiving antenna. That parabola is
positioned to have a very high radiation angle and might not be be
that good for terrestrial DX.- Hide quoted text -

- Show quoted text -

Is it possible to ploink threads based on the person who starts them?

Jimmie

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
..

On Thu, 11 Sep 2008 05:20:58 -0700 (PDT), JIMMIE
wrote:

Is it possible to ploink threads based on the person who starts them?

Hi Jimmie,

If you used Forte's Agent, yes. It would be thread wide and ignore
all contributors, or you could simply kill-file (what it is called)
one contributor. Other reader's offer some variant of this capacity.

73's
Richard Clark, KB7QHC

On Thu, 11 Sep 2008 06:18:14 -0700 (PDT), Art Unwin
wrote:

If I have a flash light that is focussed does this wavelength aproach
still apply?

The reflector (or magnifier lens, take your pick) is on order of at
least 1 centimeter. The light wavelength is on order of 500
nanometers.

Ratio = 20,000:1

Beam is generally no narrower than 15 degrees. At a distance of, say,
6 feet, that beam would cover a diameter of 18 inches. Nothing like a
Lazer (sic) if that is the goal.

73's
Richard Clark, KB7QHC

Jon K??re 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'.

The k is generally called the "efficiency factor" which is supposed to
account for diversions from the theoretical optimum.

From what I've read, it appears most real, well constructed and fed
parabolas wind up with a k of around .55, which is why I used that number.


--
Jim Pennino

Remove .spam.sux to reply.