I was unable to retreive any of the documents on the LOFAR website, so I
can't comment on the details.

I do see that the Sky and Telescope article mentioned the work at Ohio
State. This one is working at S band (3 GHz +/-) and is currently detecting
TVRO satellites and the solar emissions. I'm not sure what type of antenna
they are using, however.

I built the first prototype of the OSU system some 17 years ago, by the way,
as my Master's thesis, so I think I am qualified to comment on this. The
bandwidth of the LOFAR system is huge, percentage wise. There are a number
of problems that have to be overcome to get this to work in addition to the
RFI problem. I was able to ignore most of these problems in the prototype
because I used a very narrow bandwidth (just a few kHz). Unfortunately, my
thesis is not available on-line, but there is some information on this and
the current desgin at www.bigear.org.


Are they perhaps using circular polarization? There is an advantage to this
as most of the 'noise like' signals are randomly polarized.

As far as the VHF signal interference is concerned, it can be shown that
most VHF signals arrive at elevation angles of 15 degrees or less, so
perhaps they designed the antenna elements to have nulls at this angle.

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net
"Fractenna" wrote in message
...
See article at
http://skyandtelescope.com/news/article_1334_1.asp

My educated opinions on this matter are as follows--thisreport is very
sugar
coated: The Dutch decision broke up the original consortium and, in my
opinion,
severely degraded the success as originally outlined. The astronomical
community is not happy: this is the first time that an international
astronomy
community has worked against itself.

This is NOT 'LOFAR' as defined, but a highly compromised derivative
version.
Holland is a very poor site location for these frequencies, because of the
high
population areas and extant HF/VHF use. Also, the cross polarization
inverted
V element is a poor antenna for the relevant passband.

A good link on the original plan is:

http://www.lofar.org

73,
Chip N1IR

I do see that the Sky and Telescope article mentioned the work at Ohio
State. This one is working at S band (3 GHz +/-) and is currently detecting
TVRO satellites and the solar emissions. I'm not sure what type of antenna
they are using, however.

I built the first prototype of the OSU system some 17 years ago, by the way,
as my Master's thesis, so I think I am qualified to comment on this. The
bandwidth of the LOFAR system is huge, percentage wise. There are a number
of problems that have to be overcome to get this to work in addition to the
RFI problem. I was able to ignore most of these problems in the prototype
because I used a very narrow bandwidth (just a few kHz). Unfortunately, my
thesis is not available on-line, but there is some information on this and
the current desgin at www.bigear.org.


Are they perhaps using circular polarization? There is an advantage to this
as most of the 'noise like' signals are randomly polarized.

As far as the VHF signal interference is concerned, it can be shown that
most VHF signals arrive at elevation angles of 15 degrees or less, so
perhaps they designed the antenna elements to have nulls at this angle.

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net
"Fractenna" wrote in message
...
See article at
http://skyandtelescope.com/news/article_1334_1.asp

My educated opinions on this matter are as follows--thisreport is very
sugar
coated: The Dutch decision broke up the original consortium and, in my
opinion,
severely degraded the success as originally outlined. The astronomical
community is not happy: this is the first time that an international
astronomy
community has worked against itself.

This is NOT 'LOFAR' as defined, but a highly compromised derivative
version.
Holland is a very poor site location for these frequencies, because of the
high
population areas and extant HF/VHF use. Also, the cross polarization
inverted
V element is a poor antenna for the relevant passband.

A good link on the original plan is:

http://www.lofar.org

73,
Chip N1IR

Hi Jim,

I am confused: are you saying that my comments contain errors? If so, what is
incorrect?:-)

Yes; OSU masters students in antennas are very good. I have one working for me
right now.

73,
Chip N1IR

"Fractenna" wrote in message
...

I am confused: are you saying that my comments contain errors? If so, what
is
incorrect?:-)

Yes; OSU masters students in antennas are very good. I have one working
for me
right now.

73,
Chip N1IR


No, Chip.

I do not see any errors in your comments.

What I was saying was that building such a system as originally described is
a daunting task. There are many problems to overcome, one of which is
getting an antenna to work over a 25: 1 bandwidth with reasonably constant
performance. Another is that the pattern of the array will change
tremendously over the same bandwidth, but this can be "fixed" by using only
part of the array at higher frequencies.

Maybe that is why it was "broke up"? Also keep in mind the reputation of
news outlets as to technical accuracy.


My Maser's wasn't so much in antennas (although I did do a bunch of research
to identify the problems and propose solutions), but more at the systems
level to show how digital signal processing can be used to solve previously
"impossible" problems. I recall that at the time the experts in Radio
Astronomy thought the idea wouldn't work at all.
--
Jim
N8EE--

to email directly, send to my call sign at arrl dot net

What I was saying was that building such a system as originally described is
a daunting task. There are many problems to overcome, one of which is
getting an antenna to work over a 25: 1 bandwidth with reasonably constant
performance.

That's do-able.

Another is that the pattern of the array will change
tremendously over the same bandwidth, but this can be "fixed" by using only
part of the array at higher frequencies.


Yes. Actually not an element problem, but a problem with fixed height above
ground; ground characertistics; mutual coupling; and element spacing--I am sure
you know this; others might not.

Maybe that is why it was "broke up"? Also keep in mind the reputation of
news outlets as to technical accuracy.


My Maser's wasn't so much in antennas (although I did do a bunch of research
to identify the problems and propose solutions), but more at the systems
level to show how digital signal processing can be used to solve previously
"impossible" problems. I recall that at the time the experts in Radio
Astronomy thought the idea wouldn't work at all.

They were wrong; you and Bernard Steinberg (at Valley Forge/UPenn) showed
otherwise:-) BTW, I read your thesis about 10 years ago.Nice work. Others
might also like to know that a synopsis isup on the NAAPO site.

I think the Euro folks are too jazzed by the computational technology end and
have lost sight of the overall goal. That is my personal opinion.

73,
Chip N1IR

Some more "Trivia" for you and other interested parties....

The actual idea was conceived by Dr. Robert S. Dixon W8ERD, who at the time
was Assitant Director of the Ohio State RadioObservatory (Big Ear) and
Director of the Academic Computer Center. I was working there as a Graduate
Research Assistant (official title) and Chief Engineer (unofficial title)
and had not yet decided on a thesis topic. He was wondering if it would be
possible to digitize the signals at each antenna element in an array and
then, through digital signal processing, form all possible beams in all
directions simultaneously. And the rest is history, as the saying goes.

Also, I had previously read the book Imperial Earth by Arthur C. Clark,
where he describes a space based array that could look in all directions
simultaneously. He called it "Argus", and I thought that we should use the
same name for our system. My prototype, however, was called a Radio Camera
since the basic idea was to form a picture of the RF environment, and the
name Argus is now being used for the present system at OSU (not to be
confused with the system that uses satellite TV dishes at many locations).

You'll get a laugh out of this... My original system used a PDP-11/40 with
a whopping 10 MB of hard disk. I had the whole thing to myself as exclusive
user. Even at that it was too slow (even with a 10 kHz signal bandwidth),
so I collected the raw data and processed it off-line. I used weather radio
stations at 162.55 MHz as my sources since they all run the same power and
same antennas. Any differences in signal would be due to propagation and
distance.

The basic idea of Argus is to be able to look in all directions at the same
time, thereby increasing the chance of detecting transient signals. Big Ear
had detetected some transients (the most famous being the WOW! signal)
mainly from the direction of the galactic poles, but considering that it
would take Big Ear several years to survey the entire sky the detection of
such transient signals is pure luck. An Argus system, however, can survey
the entire sky (by this I mean what is visible from a given location---you
would need one in space to see everything) in one day, theoreticaly.

--
Jim
N8EE

You'll get a laugh out of this... My original system used a PDP-11/40 with
a whopping 10 MB of hard disk.

I still have my PDP10 programming manual. Jeez, I --AM-- OLD!:-)

Yes; Bob had a great idea.

I suspect it will take 30 years (from now) to do this properly, get all the
birdies out; get a nice patch of land in Arizona or Utah; build the
system;find ET's; and so on.

Pioneering doesn't seem like that to contemporaries. We all need lasik!

73,
Chip N1IR

Chip wrote:
"I still have my PDP 10 programming manual."

As I recall from circa 1970, the Digital Equipment tiny-biny was the
programmable data processor, PDP 8. I remember Schlumberger having in
its Houston Headquarters a PDP 10, The PDP 10 was a rather large number
cruncher which could be used for scientific purposes as well as for
business applications. I think they ran both FORTRAN and COBOL.

My employer tried Raytheon 704`s for minicomputers, then switched to
Digital Equipment VAX machines. They worked well.

Best regards, Richard Harrison, KB5WZI

Even more trivia:
My work at the Big Ear under W8JK years before N8EE's work involved the
first digital recording of radio astronomy signals including time
information. (Slave labor, AKA graduate students, were used previously to
digitize the strip charts used.) The recording medium was punched paper
tape and the (very off-line) processor was an IBM 1620 (or something like
it - it was a true decimal machine intended originally for accounting - some
arithmetic operations involved table look up!). It took several orders of
magnitude improvement in computational power and in ancillary equipment to
arrive at what Jim was able to do. I delight in that progress.
Pertinent to this group, is the admonition that one still needs to
understand the analog part of any such information gathering system even
while digital power increases. At least within my remaining lifetime, the
antenna(s), transmission line(s), and "first stage" will remain the province
of analog engineering. This group will have plenty to discuss before it is
supplanted with an A to D converter!
73 Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"JLB" wrote in message
...
Some more "Trivia" for you and other interested parties....

The actual idea was conceived by Dr. Robert S. Dixon W8ERD, who at the
time
was Assitant Director of the Ohio State RadioObservatory (Big Ear) and
Director of the Academic Computer Center. I was working there as a
Graduate
Research Assistant (official title) and Chief Engineer (unofficial title)
and had not yet decided on a thesis topic. He was wondering if it would
be
possible to digitize the signals at each antenna element in an array and
then, through digital signal processing, form all possible beams in all
directions simultaneously. And the rest is history, as the saying goes.

snip
--
Jim
N8EE

while digital power increases. At least within my remaining lifetime, the
antenna(s), transmission line(s), and "first stage" will remain the province
of analog engineering. This group will have plenty to discuss before it is
supplanted with an A to D converter!
73 Mac N8TT

Mac,

These days, good analog RF folks are worth their weight in platinum. I like to
maintain a library of older RF books just to keep the younger guys on their
toes:-)

73,
Chip N1IR


73,
Chip N1IR

"JLB" wrote in message
...



"Fractenna" wrote in message
...

I am confused: are you saying that my comments contain errors? If so,
what
is
incorrect?:-)

Yes; OSU masters students in antennas are very good. I have one working
for me
right now.

73,
Chip N1IR


No, Chip.

I do not see any errors in your comments.

What I was saying was that building such a system as originally described
is
a daunting task. There are many problems to overcome, one of which is
getting an antenna to work over a 25: 1 bandwidth with reasonably constant
performance.


I regularly use an active 41" monopole to accurately measure electric field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1, and
that is 25-year old technology.

--
Ed
WB6WSN