Maybe ths link can help you:

http://en.wikipedia.org/wiki/Parabolic_antenna

Brgds

Harding

On 10 Apr., 15:12, Bruce wrote:
A new one.

I'm now residing ashore as I sold my sailboat. The place I rented is
nearly a mile for the marina and I can sometimes log onto the marina's
free Wi-Fi net using the laptop's built in antenna.. I am using a
Wi-Fi transceiver that has some level of amplification, or at least it
is sold as a "high powered" Wi-Fi.

I have built a parabolic antenna which (according to the rather crude
signal strength meters included with Linux) seems to *approximately
doubles the signal strength and allows me to access two additional, of
the five or six different circuits at the Marina.

Question: I am now building a larger parabolic antenna system. The
original was a ten inch dish and the new one will be a 17 inch dish.
Are there any formula to estimate the additional gain of the 17 " over
the 10"? In addition the first effort was screen and the second will
be solid aluminum. Again any estimates of gain increase - decrease .

I realize that there are many slip between the cup and the lip and
nothing is certain about antenna systems so am looking for estimates,
best guesses, text book "rule of thumb" basically any information
about parabola size or material and gain.

I do have the basic formula to calculate the focus point and some
recommendations for F.L. versus diameter.
Cheers,

Bruce
(bruceinbangkokatgmaildotcom)