Fractenna wrote:
As Dale says, Arecibo uses a spherical reflector to allow a few degrees
of beam steering by pointing the feed antenna at different areas of the
dish. However, this is a very special case: the only practical way to
achieve a 1000ft dish was to build it immovably on the ground, so the
designers then had to find some other way to steer the beam, by moving
the feed antenna at the focus. In this one special case, the optimum
shape for the reflector is not a paraboloid but a sphere (because the
geometry of a sphere is the same in any direction, as seem from the
feedpoint at the centre).


Basically correct. Although the tracking is better than just a few
degrees:-)

Thanks for the correction - I hadn't realised it can steer up to 40 deg.
A Google search for "arecibo feed" produced that information, and much
more.

Nice pictures of the 43MHz feed at:
http://tinyurl.com/64cp9
and
http://tinyurl.com/69sa3
Against the dish itself, it's easy to lose your sense of scale - that
little thing is 90ft tall.


Arecibo was initially designed to be a survellance instrument,
passively listening to Soviet communications through inadvertant
moonbounce. It also was designed, initially, as an ionospheric heating
facility.

Through the huge luck of its overengineering, it was found to be able
to track quite accurately, and the feeds and carriage houses were
designed to accommodate a greater tracking range.

The official history (http://tinyurl.com/46zfd) is less forthcoming
about the original intentions, but there's an interesting "40 years ago"
article at
http://www.elecdesign.com/Articles/A...3700/3700.html



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek