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

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

Dear Emanuele:

Do well on your exams. It is that time of the year here as well.

It occurred to me that I had quite forgotten that two dishes exist
associated with Reber. One is huge and the other is the 9.5 meter dish he
built himself. Both are at NRAO. Reber was a true scientist and a great
friend of my mentor, Kraus. Here is a story about him:
http://www.nrao.edu/whatisra/hist_reber.shtml

The maps he published just before the war, were studied by Dutch astronomers
who were sequestered during the war. After the war, the Dutch moved the
field forward. Reber's interests were many. You will note papers on beans,
for which he performed truly original work much of it while we were both at
NRAO. After dinner, when one learned not to leave the meat platter too
close to him, he would go off to inspect his research garden. He was a
collector - not quite the right word, but close - of early radios. I have
seen him get them working, construct a box that would just hold the radio,
and then store the boxed radio for a future museum. In his testing of old
(pre war) radios he discovered that one company had discovered stagger
tuning. His note on that subject caused a lot of noise.

His observation - known to everyone in the radio field - that the
ionosphere was most thin near VK7, resulted in his measuring radiation from
space at low frequencies well before satellites. His genius of seeing in
common knowledge new possibilities is his hallmark and one that I wish for
all who would push knowledge forward. He constructed all of his own
measuring instruments, which were strikingly beautiful. I spoke to radio
amateurs in VK7 while he was still alive and they told of how beloved he was
by the people of that island.

By the way, my first name is James, not John. I use "Mac" as it seems
more suited to this newsgroup than "Professor."

It is just possible that the sort of offset-feed antenna for receiving
broadcast signals is not common in Italy. If that is the case, a search
will uncover surplus dishes in North America. They tend to be in the one
meter size, which might be too small unless used in an array. I am told
that surplus, conventional dishes show up from time-to-time as
agencies/people move or up-grade. I like the idea of operating remotely.

Study, learn, plan, do, observe and you will learn even more. Consider
getting an amateur radio license.

Warm regards, James / Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"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

"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.