Dear Emanuele and Group:

I have been mentioned. The other responders have well indicated the key
factors of a Cassegrain antenna. They include having the active devices
(transmitting or receiving components) at a convenient location and
potentially a somewhat better antenna temperature than with a typical
parabolic antenna.

At Ohio State, working with John D. Kraus, we actually used a form of a
Cassegrain antenna - the Big Ear. The Big Ear used a (E-W) wide, vertical
parabolic surface to concentrate the radiation from a (E-W) tilting flat
reflector that was north of the parabolic surface into a feed that was at
ground level. The cost effectiveness was outstanding! I could work below
the feed in an underground room accessed through a tunnel under the ground
screen.

You are commended for initiative in wishing to have a radio telescope.
Crafting such an instrument can be an end into itself. However, having an
observational goal seems to be desirable if this is more than a construction
project. I recommend that you read the ample literature that is available
to help you design an observational goal.

Note that you need a large size if you are to achieve resolution, which
might not be an objective. I worked on the receiving system for a system at
NRAO at Green Bank, W.Va. that used a fixed, high gain antenna to observe
(once a day) the strength of one source. The antenna used had a predictable
gain and, with calibration, the system was able to provide flux measurements
(with an attendant uncertainty, of course). Note that the first real radio
telescope by Reber was a parabolic dish - it still exists. (He was a really
bright, innovative guy.)

The presence of antennas used to receive satellite broadcasts that use
offset feeds should be considered. Such antennas can have desirable antenna
temperatures because of the placement of the feed while not having
feed-blockage.

Most important is the avoidance of re-inventing the wheel that can be
effected by serious study of the literature. That is where I recommend that
you start. Kraus' books on antennas and radio-astronomy could be a good
place to start. It would be wonderful to be young and have so much learning
to look forward to! After over 50 years of studying radio related subjects,
I am still excited.

The delay in responding is due to the high noise level in this valuable
news-group. Ignore, as I do, the theological debates. Pay close attention
to those on this group who are knowledgeable.

Enjoy the greatest pleasure - that of learning. Mac N8TT


--
J. McLaughlin; Michigan, USA
Home:
"Richard Clark" wrote in message
...
On Fri, 10 Apr 2009 12:34:34 +0200, Emanuele Colucci
wrote:

Hi to everyone and thanks for answering to my message.

Richard Clark ha scritto:
Hi Emanuele,

This is probably due to the way the question was offered. You want a
complete solution to an unknown problem.

You and Dave were right. I originally posted the message in the homebrew
newsgroup because I was looking for a complete guide to design a cas in
order to build a little radiometer working in the "water hole" (1420 ~
1640 MHz) for an hobbystic aim.
The book I used at university to study antennas - Antennas and Radiowave
Propagation [by Robert E. Collin] - simply doesn't cover in depth this
argument. It just cites the Cassegrain feed system as an alternative to
prime focus paraboloidal reflector antennas, because the cassegrain feed
does't receive the thermal noise from the ground.

Hi Emanuele,

I have that book and I can see your source material. If it is your
choice, that is fine. Other correspondents here can respond to your
desire to eliminate thermal noise from the ground in alternative
designs. You may have to repeat your query to get their attention as
this is not a topic that many have experience with (repetition is for
the occasion when they check in on an irregular basis). An
alternative is to search this group, specifically, at
groups.google.com.

I was looking for a complete resource to study the system, and this is
why I wrote he to find someone with good advices.

Now, the thing I have now understood is that I haven't to be so
synthetic while writing a message.

So, I would like to study (and eventually build) a radiometer who
listens to 1420 ~ 1640 MHz, with a passing bandwidth of 8 MHz, in order
to receive a minumum density flux of 240-260 Jy (a Jansky is 10^-26 W /
[m^2 * Hz]). I need low antenna temperature (but not as cryogenics
ones!).

You can get a lot of practical ideas here (construction materials and
feed issues); and you can use Collin for the math to simply scale the
structure to your frequency band. The design is rather more simple
than building it.

5. Budget (time and/or money);

I can't answer this question yet. Surely, I haven't more than 5-600
euros to spend in the building/buying of the antenna.

That is plenty of money. Do you have any time constraints?

7. Application issues (EME?).

Amateur radioastronomy.

We have a contributor who has been teaching that for 40 years at a
major university, and he studied under the pioneers of that specialty.
Search groups.google.com for posts by "Mac N8TT" or use "J.
McLaughlin; Michigan, USA" as search terms.

Well, that's all by the moment.

Greetings,

Emanuele Colucci


73's
Richard Clark, KB7QHC