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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. |
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.
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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 |
"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 |
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 |
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 |
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 |
Chip:
Amen. An objective of most who teach in the area is to nurture students who are able to discern when it is analog time and when it is digital time. The allure of digital has to be mollified. Occasionally one has a student who's eyes light up when he or she is exposed to the art of analog after seeing the science of same. Many more students are appalled. It has been one of the pleasures of my life to collaborate with a wonderful colleague (and extra class radio amateur) as a catalyst and teller of stories while he puts down some of the accumulated analog wisdom reinforced by his analysis, insight, and exposition. No doubt some was acquired in this group. Look for his encyclopedic book from CRC to be available within about a month. 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Fractenna" wrote in message ... 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 |
One of the concepts that I have had trouble getting 'younger' engineers to
understand is that a digital signal IS analog! Just because you are running at "only 10 MHz" doesn't mean that you can have 4 inch pigtales. The actual signal bandwidth would extend up to 50 MHzor even more, and you have to treat it like a VHF/UHF RF signal or the bits are going to get lost. Digital is Digital only in the purely logical sense (pun very much intended, by the way). As for digital signal processing is concerned, just because you have digitized the signal doesn't mean that you have eliminated all of the analog problems. I can think of only three ways that DSP is better...1)no cross talk in signal paths, and 2) lossless signal duplication (think of an analog power splitter), and 3) you can do things easily that previously were just a glimmer in an analog engineers eye (such as multiple simultaneous beams from an antenna array?) -- Jim N8EE to email directly, send to my call sign at arrl dot net |
On Fri, 24 Sep 2004 14:42:54 -0400, "JLB"
wrote: As for digital signal processing is concerned, just because you have digitized the signal doesn't mean that you have eliminated all of the analog problems. Hi Guys, This reminds me of the problems I had teaching the digital types Shannon's law for BER. When I designed the black box for the 757/767, all of my digitized readings (taken from 600 leads) was passed over to a specialized tape recorder (25 hour capacity). Problem was that this digital signal was fed into the recording head without any bias. Many may be unaware of the advances in audio tape recording BW that came by the addition of bias, and more, that it reduced the head's tendency to erase its own stored signal. In essence, with no bias, you were recording data with two strikes against you. This was not the hallmark design for a data sensitive product. Worse yet, was this digital mentality had recorder specifications that allowed for a S+N/N of 2. What were the comments I heard in response? "This is not HiFi, it's digital data, on/off." Within weeks I was drawn into their simulator lab to view a simulated cockpit of an KLM aircraft that was being used to display flight recorder data that was rather -um- noisy (much too much for KLM's engineers to make sense of it). I watched that plane hit the ground several times as they struggled to recover digital bits lost in analog noise. Several (many) years later I was called to consult on the TWA flight 800 data - more noise that lead to hints of missile strikes. The panel's best spin on the topic was that it was old data printed through the new data (even though the difference in time would not correlate to the two data sets overlapping). We reported it as exploding gas tanks, Tehran reported it was revenge for our shooting down one of their civil aircraft during the first Gulf War. Like the first attack on the Twin Towers, government and the media shrugged off correlations. 73's Richard Clark, KB7QHC |
"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 |
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 And your variation of gain, excluding mismatch, is...? 73, Chip N1IR |
"Fractenna" wrote in message ... 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 And your variation of gain, excluding mismatch, is...? 73, Chip N1IR And that should be complex gain, since in the stated application you are concerned about phase and magnitude. Also, don't forget about the mutual coupling between elements. What happens to it over a 3000 to one bandwidth? Remember we are talking about an array of antennas, not a single isolated one! -- Jim N8EE to email directly, send to my call sign at arrl dot net |
Ed's one meter vertical over a large conductive ground plane has an
effective height of close to 0.5 meters up to something like 20 MHz. With an amplifier at the base of the vertical that has high input impedance and some tailored feedback, one can have a system that can be used to measure field strength with very little frequency dependence. I use a miniaturized version of such an antenna as a probe in a TEM cell. I have seen the use of resistively loaded (short) dipoles connected to resistively loaded transmission lines used by the NBS (as it was then called) to measure field strength with minimum disturbance to the field. These are all receiving antennas with essentially uniform performance over very large frequency spans. My feeling is that to have a small variation in transmitting gain over more than something like five to one will require an adaptive antenna system. (I am assuming antennas with an "average" gain that is close to one - no resistive loading.) 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Fractenna" wrote in message ... 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 And your variation of gain, excluding mismatch, is...? 73, Chip N1IR |
Ed's one meter vertical over a large conductive ground plane has an
effective height of close to 0.5 meters up to something like 20 MHz. With an amplifier at the base of the vertical that has high input impedance and some tailored feedback, one can have a system that can be used to measure field strength with very little frequency dependence. I use a miniaturized version of such an antenna as a probe in a TEM cell. I have seen the use of resistively loaded (short) dipoles connected to resistively loaded transmission lines used by the NBS (as it was then called) to measure field strength with minimum disturbance to the field. These are all receiving antennas with essentially uniform performance over very large frequency spans. My feeling is that to have a small variation in transmitting gain over more than something like five to one will require an adaptive antenna system. (I am assuming antennas with an "average" gain that is close to one - no resistive loading.) 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Fractenna" wrote in message ... 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 And your variation of gain, excluding mismatch, is...? 73, Chip N1IR The array is RX only, and G/T is important for each individual element, over a broad range. You need a truly compact element that is wideband. Wideband means about the same gains and impedances across a very wide range. Mediocre broadband RX antennas have been around since WWII. The discussion on TX, albeit interesting, does not apply. Also, mismatch produces dramatic signal drops without an ATU in conventional designs, such as inverted V's. The assumption has been 'no ATU' in this project, because the costs are prohibitive. The one mitigating issue is that the sky temperature --and this system is designed to synthesize a measurement sky tempertaure with high angular resolution--increases with lower frequency (below about 1GHz) because the emission is non-thermal. Thus the signal to noise gets better in VHF. 73, Chip N1IR |
"Fractenna" wrote in message ... 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 And your variation of gain, excluding mismatch, is...? 73, Chip N1IR Maybe I gave you a trick answer, because the antenna I cited is a receive-only device. It is concerned only with measuring the strength of the electric field, and derives no phase information. I'm sure its efficiency is horrible, but its bandwidth is huge and its gain is about +/- 0.5 dB. FYI, here's an example of this style of antenna: http://www.ets-lindgren.com/productp...ttype=Antennas Even if it did not face into an active pre-amplifier, it would still be a poor emitter anywhere below a few MHz. Electrically short is something that hasn't been finessed yet. Ed wb6wsn |
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