On Sep 11, 12:28*pm, "Tom Donaly" wrote:
Roy Lewallen wrote:
[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.

It's always dangerous to do math in public.
73,
* * *Tom Donaly, KA6RUH- Hide quoted text -

- Show quoted text -

Yes! I think it is best to just give the formula and tell the reader
to calculate!