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[email protected] · September 11th 08, 05:25 PM posted to rec.radio.amateur.antenna

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

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