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#14
June 12th 09, 12:51 AM
posted to rec.radio.amateur.antenna
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Cassegrain Antenna Development
Jerry wrote:
"Emanuele Colucci" wrote in message ... snip I haven't read Kraus book about radio astronomy: my sources are Collin and some italian books by Flavio Falcinelli. But now I'll surely look for this book too! Greetings, Emanuele Colucci Hi Emanuele The probability of your realizing any success with parabolas less then 10 feet diameter using cassegrain systems is extremely low at L-band. I realize that you are able to decide for yourself, which antenna system is best for your project. I do want to alert you to the fact that the beam from the "source radiator", behind the dish, needs to be shaped to be concentrated on the reflector at the focus. There is not enough room in a 3 meter dish to provide access to a primary feed with the narrow beam required for realizing the benefits of a cassegrain feed. I have has some succes with a 1 meter off center fed dish at L-band (1.691 GHz) for redceiving signals from a geosynchronouis satellite. The feed I designed for that dish might be of some use to you. You might also look at the design for the Allen Telescope Array, which is a Gregorian feed, and is offset. All of these are useful to have minimum noise contribution from the surrounding earth.. if the feed over-illuminates the secondary reflector, you are still looking at cold sky, unlike with a prime focus sort of feed, where overillumination looks at the dirt behind the reflector. There's a set of articles in IEEE Antennas and Propagation Magazine (not the transactions) a few years back that has all the equations and design rules for all of the various reflector configurations. As for reflector sources.. 1.5m parabolic reflectors intended for offset feet are readily available in Europe. They're used by British Expatriates in places like the Canary Islands to get BskyB broadcasts (since they're well out of the satellite footprint, they need more gain than the usual 50cm sort of dish on the side of your house). A couple years ago, I remember googling for reflectors of this size for a project and ran across this kind of thing. The reflectors were real cheap (10-15 pounds), but shipping to the US would cost many times that. They seem NOT to be readily available surplus here in the US.. VSAT stations in Alaska might be the only application. |
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