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#11
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I am not certain I understand your answer- or Chip's.
The original poster made the statement that mesh parabolas are only 80% as efficient as a solid parabola. That was what I was answering. The wok's inefficiencies stem from the fact that it is not a true paraboloid. 73, Chip N1IR |
#12
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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 |
#13
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The official history (http://tinyurl.com/46zfd) is less forthcoming
about the original intentions Correct. 73, Chip N1IR |
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