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#1
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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. |
#2
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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 |
#3
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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 |
#4
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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 |
#5
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"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 |
#6
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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 |
#7
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"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 |
#8
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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 |
#9
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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 |
#10
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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? |
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