Nigel M wrote:
In rec.radio.amateur.antenna, Richard Clark wrote:

They all look the same: Parabolas.

A flat one is a smaller section of a pointy one, see:

http://mathworld.wolfram.com/Parabola.html

I can envisage that one that ends with its extremities at between 45 and
90 deg to each other would "seem" to envelop the signal better.
--
Nigel M
"Occam's razor is not always sharp"

Nope.

The maximum theoretical gain of any parabola is determined by the diameter
in wavelengths.

The things than subtract from the maximum gain are the surface accuracy,
both in terms of how close the curve is to a true parabola and any surface
"bumps", and the illumination of the feed.

Irregularities in the curve and "bumps" less than about 1/8 wavelength
have little effect, bigger than that and they can have significant effect.

The pattern of the feed (the feed itself is an antenna and has it's own
pattern) determines the illumination.

The feed is mounted at the focal point of the parabola.

A "flat" parabola has a longer focal point than a more "curved" one, to
use your terms.

If the pattern of the feed is such that it just exactly and perfectly
illuminates the whole reflector, you get maximum gain.

If, as in the real world, the pattern of the feed spills out beyond the
edge of the parabola or doesn't fill the whole parabola, you get less
then the maximum theoretical gain.

The ARRL antenna book has a pile of equations, tables, and graphs showing
these relationships and how to make working parabolic antennas.

--
Jim Pennino

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