Radio astronomers build huge antenna farm 350km across in Netherlands
 

See article at
http://skyandtelescope.com/news/article_1334_1.asp

It is to observe the sky from 10 to 250MHz, what they
call "low frequency". 15 thousand antennas in an
array 350 kilometers across.

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

Pardon the stupid question, but where is the best place to locate low
pass filters? Closer to the antenna or the receiver?

I am finally setting up my base station and I need to know the best
location for my low pass filter.

Thanks,

Tod
N7JQW

On Thu, 16 Sep 2004 09:32:19 -0700, Tod Glenn
wrote:

Pardon the stupid question, but where is the best place to locate low
pass filters? Closer to the antenna or the receiver?

Hi Todd,

You want high pass filters for a receiver with the roll-off frequency
set at the lowest end of your listening range. This is usually the AM
band's top end to keep their power out of your receiver's front end.

Low pass filters are for transmitter outputs to reduce spurs and
harmonics (and should be as close to the source as possible).

73's
Richard Clark, KB7QHC

Between the audio output and the woofer.

"Tod Glenn" wrote in message
...
Pardon the stupid question, but where is the best place to locate low
pass filters? Closer to the antenna or the receiver?

The best place is at the output of the transmitter or linear amplifier.

In article ,
(K9SQG) wrote:

The best place is at the output of the transmitter or linear amplifier.

Thanks all who answered.

Tod

On Sat, 18 Sep 2004 19:24:31 -0700, Tod Glenn
wrote:

In article ,
(K9SQG) wrote:

The best place is at the output of the transmitter or linear amplifier.

Thanks all who answered.

Tod

When I've used a low-pass filter, I've simply attached it to the
output of the transceiver with a double male coax connector, so the
unwanted higher harmonics go directly into the filter, and then the
hopefully clean signal moves on to the antenna feedline.

bob
k5qwg

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