Re: SETI Newbie questions about the Fresnel Zone Plate Antenna.


David Woolley (david@djwhome.demon.co.uk)
Thu, 29 Jul 1999 22:53:59 +0100 (BST)


> After struggling with my physics book for a couple of hours, I still

Not being prepared to download 7MB of document, I presume this is
a flat series of metal rings one quarter wave in front of a metal
plane, with the feed in front of that.

> don't understand how this antenna is supposed to work. Does it act as
> a diffraction grate? Does the incoming signal need to be phased in

That would probably be the easiest way of viewing it, or rather as two
gratings offset by a quarter wave. Equivalently, you could consider
it to be the radio hologram of a point source at the feed point, with
respect to a parallel reference beam along the line from the feed to
the centre of the rings.

You can also treat it as sections from a series of nested parabolas,
all with a common focal point at the the feed. These sections are
then flattened, but they are nested such that you move to the next
one in the nest when one flattened parabola gets too far out of true
(this could be considered as treating a parabola as a diffracting
object, but rules which allow a twelfth wavelength profile error
effectively do that anyway).

> order for it to work? I'm also wondering if it would be possible to

It needs to be coherent across the pattern and dead centre, which means
a distant source will produce a constant phase across the pattern.
The centre of each ring will diffract at the same phase (but slipped
360 degrees for each front ring) into the feed and the rings will step
when the phase error at the feed reaches 90 degrees, so there won't
be destructive interference, of in phase signals (quadrature phase
components will completely destroy each other). Similar things happen
for the exposed hack reflector rings.

> design the feedhorn in such a way as to get the same phasing effect on
> a parabolic scope.

I don't understand. The characteristic of a parabola is that the
distance from a point on a plane perpendicular to the axis of
symmetry, proceeding parallel to the axis until one hits the
parabola and then heading for focus is constant. This means that
an incoming plain wave will reflect so that all parts of it are
in phase at the focus.

The reason for using the zone plate, rather than a paraboloid, is that
it is easier to construct and support, although having a rather large
profile error presumably means it has worse side lobes.



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