"Emanuele Colucci" wrote in message
...
Thank you John for this message.


J. Mc Laughlin ha scritto:
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.

Yep, I'm still documenting. Perhaps, there are alternatives to a Cas
antenna (Yes, I admit, because of exams I haven't crafted it yet, but for
me this is a very long term project), like a little array of offset
parabolic antennas.


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).

This is interesting!
How much time did it take to make a complete radio stellar map?

Note that the first real radio telescope by Reber was a parabolic dish -
it still exists. (He was a really bright, innovative guy.)

Reber's parabolic dish is giant My site is situated in countryside, and
it could be able to host dishes of max 3 meters of diameter: the data
would be transmitted over the internet to my home (near Naples, ITA).


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.

Great. I'll watch for these antennas too!


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.

Thanks again for reporting your experiences and the advices!

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.

Jerry KD6JDJ

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.

Thanks to James for the advices and to Jerry and Jim for the news.

Jerry: I'd like to know more about your solution, if you agree.

Jim Lux ha scritto:
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.

Where I can find these articles?


Best wishes,

Emanuele Colucci

"Emanuele Colucci" wrote in message
...
Thanks to James for the advices and to Jerry and Jim for the news.

Jerry: I'd like to know more about your solution, if you agree.

Jim Lux ha scritto:
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.

Where I can find these articles?


Best wishes,

Emanuele Colucci


Hi Emanuele

I submit that, if you intend to actually build an antenna for L-Band and
are restricting its diameter to 3 meters, you need not consider a cassegrain
feed. The reflector at the focus is too small to allow the primary feed to
illuminate it with a high percentage of the available source at the rear of
the dish.

For one example, let the cassegrain reflector be 1/2 meter diameter and be
spaced 1meter from the apex of the dish. Wouldnt that suggest that the
primary beam, from behind the dish will have a beamwidth of about 30 degrees
(at -3dB)??
If the cassegrain reflector is made bigger, that blocks more of the
parabolic dish apperature. If the cassegrain reflector is made smaller,
that increases the need for a high gain primary feed.

It is my contention that, if you want to research Cassegrain antennas,
that is a worthy project. If you want to build a functing L-Band telescope
with 3 meter dishes, dont include cassegrain feeds in the project.

Jerry KD6JDJ

Emanuele Colucci wrote:
Thanks to James for the advices and to Jerry and Jim for the news.

Jerry: I'd like to know more about your solution, if you agree.

Jim Lux ha scritto:
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.

Where I can find these articles?


Best wishes,

Emanuele Colucci



You might want to take a look at W1GHZ's online book
http://www.w1ghz.org/antbook/conf/Mu...r_antennas.pdf

He gives a procedure, and refers to a variety of sources, among them,
the ones I was thinking of:
The articles I was thinking of were by Christophe Granet at CSIRO in
Australia, and published in Tom Milligan's "Antenna Designer's Notebook"
column in the A&P Magazine

April 1998, "Designing Axially Symmetric Cassegrain or Gregorian
Dual-Reflector Antennas from Combinations of Prescribed Geometric
Parameters"

June 98, Minimum blockage
Dec 99, Displaced axis dual reflector antenans (4 types)
Dec 2001, Dragonian Dual-Reflector
June2002, Offset Cassegrain or Gregorian
Dec 2003, Designing ..offset.. Part 2, Feed-horn Blockage Conditions.

There's some FORTRAN available too, from Tom Milligan.