archive: Re: SETI Formula to calculate a dish's focal point

Re: SETI Formula to calculate a dish's focal point

Dr. H. Paul Shuch ( (no email) )
Sun, 20 Sep 1998 19:06:47 -0400

At 05:06 PM 9/20/98 -0500, Ray Shank wrote:

>Diameter squared divided by 16 times Depth. (Dia * Dia) / (16 * Depth)

The formula I used in <parabola.xls> on the website is:
F = (radius)^2 / (4 * depth)
which ought to come out exactly the same.

>Is the focal point where I put the opening of the feed horn, or the
>position of the dipole within?

Neither. The focal point falls at a point just inside the mouth of the
feedhorn, by a patricular amount which is a function of dish diameter,
focal length *and* feedhorn diameter. The spreadsheet <feedhorn.xls> in
Software section of the website calculates the exact point for you.
For my dish, for example, the focal point must fall inside the mouth of
the feedhorn by exactly 3.7 cm.

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