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.