Dave Platt wrote:
In article , wrote:

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.

In the example being discussed, the feed antenna appears to be an
omnidirectional vertical attached to an 802.11 access point.

In this particular case, because the feed has an omnidirectional
pattern, it seems to me that there would be a definite advantage to
using a relatively "deep" and thus somewhat "pointy" parabolic
section, in which the focal point lies a fair distance back from the
forward-most edges of the actual reflector.

This would tend to increase the portion of the feed antenna's omni
pattern which actually illuminates the reflector and is focused in the
forward direction.

Using a more shallow parabolic section, and getting high gain out of
it, would require a modification to the feed antenna so that it
illuminated the reflector more efficiently, with less spillover. I've
seen some designs for 802.11 which use a fairly shallow dish (with the
focal point well forward of the edges of the reflector), illuminated
by a feed antenna which is either a two-element dipole+reflector or a
"backfire" patch antenna.

I don't know whether it's feasible to make such a non-omni-feed design
as inexpensively as the "cardboard, tin foil, standard access point"
designs posted on the Net.

In the case of an omni vertical, it would be easier to construct a
corner or trough reflector with a ground plane and it would probably
work better since the bottom half of the parabola (or full corner
reflector) most likely won't be illuminated anyway.

--
Jim Pennino

Remove -spam-sux to reply.