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Eric's Weekly Post #3 (Warning, science enclosed. Do not fold or bend.)
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ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
1. When will we search for helical/circular polarizations? That's a very good point. A crafted database search for simultaneous signals for each linear polarisation could infer circular polarisation without the need for clever tricks with the splitters (and all the extra data). Much more efficient to let the database do some of the work! Keep searchin', Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
1mp0£173 Send message Joined: 3 Apr 99 Posts: 8423 Credit: 356,897 RAC: 0 |
1. When will we search for helical/circular polarizations? ... a bigger question is: does polarization carry any additional information? There are practical reasons for choosing a polarization -- for example, circular polarization is really good if you are trying to send (or receive) signals from a spinning satellite (big expensive satellites are stabilized, but inexpensive small satellites are not). ... and generally speaking, for terrestrial paths, horizontal polarization is quieter. Vertical polarization is better for mobile-to-mobile and base-to-mobile for purely practical reasons. It generally doesn't change once a system is designed and installed. I don't see how anything like that would tell us any more about ET. Maybe a real ET researcher might have a comment.... |
ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
... a bigger question is: does polarization carry any additional information? And for that reason, if I were to radiate a beacon signal, I would use circular polarisation so that the signal is kept out of any natural sourced noise and to allow reception by a spacecraft antenna at any orientation. If we took the "Star Trek" view and aligned everything to the galactic plane then linear polarisations could be used. Then again, are there any features of space that rotate/twist the polarisation plane? (I don't know of any other than star or planet magnetic fields, but...) ... and generally speaking, for terrestrial paths, horizontal polarization is quieter. Vertical polarization is better for mobile-to-mobile and base-to-mobile for purely practical reasons. I wonder how you could have an circular polarised isotropic radiator?... :-( Or could you have a 'phased array' for circular polarisation. Phew, that would be an interesting headache! Or?... ...I don't see how anything like that would tell us any more about ET. Maybe a real ET researcher might have a comment.... It would say that this 'ere signal is deliberately artifical and we are trying to maximise how easily it can be received by you... That extra 3dB over a misaligned linear polarised signal equates to a greatly increased volume of search space! (If we could recieve it with a dedicated circular polarise antenna. Or can the two linear polarised antenna signals be phased/added for zero loss?) Keep searchin', Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
W-K 666 Send message Joined: 18 May 99 Posts: 19060 Credit: 40,757,560 RAC: 67 |
... a bigger question is: does polarization carry any additional information? If you use two receive antenna where the received signals are anti-phase then combining techniques can give a theoretical 6dB improvement in signal to noise ratio. Andy |
ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
If you use two receive antenna where the received signals are anti-phase then combining techniques can give a theoretical 6dB improvement in signal to noise ratio. Indeed yes for signals that are deliberately transmitted that way with the same antenna arrangement. That would certainly prove whatever is recieved to be a non-natural transmission! (However, would that not actually be some sort of circular polarisation in any case?) ... Now, is there any mileage to be had by pre-processing the recorded signal data before the splitters to remove "common mode" noise betwen crossed polarisations and across all polarisations in a similar way to what is being done to remove the instrument interference at the moment?... Regards, Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
1mp0£173 Send message Joined: 3 Apr 99 Posts: 8423 Credit: 356,897 RAC: 0 |
... a bigger question is: does polarization carry any additional information? Martin, There are a couple of ways to get circularly polarized signals, but it's usually done with crossed dipoles (or crossed yagis) fed 90 degrees out of phase. But my point is that we have three choices: 1) Use linearly polarized antennas at 90 degrees, and accept that circularly polarized signals will be 3db down. 2) Use circularly polarized antennas (left-hand and right-hand) and accept that linear-polarized signals will always be 3db down. 3) Use four antennas (horizontal, vertical, left and right) and pick up a lousy 3db on some signals (and record four times the data, where at least two are probably redundant). My gut feeling is that #3 is a lot more complexity with very little gain in data collected. ... but that's me. -- Lynn |
1mp0£173 Send message Joined: 3 Apr 99 Posts: 8423 Credit: 356,897 RAC: 0 |
Well, maybe closer to 3db than 6db, but there are so many ways to get more antenna gain. ... and for the subject at hand, we're talking about a second dish, not just a second antenna at the feedpoint. I don't know if we have another nearby parabolic valley or the money to build another Arecibo. Probably easier to just make that dish bigger. |
ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
Eric, How's the interference search and fix progressing? I've seen a comment somewhere saying that the interference is an instrumentation problem rather than external interference pickup by the antenna. Yet another wild guess: Have you got an impedance missmatch causing reflections on a (long) signal cable?... Keep searchin', Regards, Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
Graeme Stretton Send message Joined: 2 Nov 01 Posts: 392 Credit: 349,012 RAC: 0 |
The antenna, then, at the focus, is only one, 1/2 or 1/4 wavelength long? Umm, errr ... A loading coil is not a variable resistor. It is an inductor which makes the antenna "appear" longer than it really is. Ahhh, cross polarisation won't stop you watching any TV just involves about 20dB loss os signal. And in the real world not even that much. Who says? 40+ years of experiance ;P |
1mp0£173 Send message Joined: 3 Apr 99 Posts: 8423 Credit: 356,897 RAC: 0 |
In a perfect world, cross polarization is an infinite signal loss. In the real world, there is always some signal that bounced off or bent around something and is no longer horizontally polarized, or for that matter, vertically polarized -- which is why 20db or so is a much more practical number. |
Eric Korpela Send message Joined: 3 Apr 99 Posts: 1382 Credit: 54,506,847 RAC: 60 |
We tracked it down to a radar system. I'll try to post more info this week. Eric Eric, @SETIEric@qoto.org (Mastodon) |
Misfit Send message Joined: 21 Jun 01 Posts: 21804 Credit: 2,815,091 RAC: 0 |
We tracked it down to a radar system. I'll try to post more info this week. Do you know which radar system? me@rescam.org |
Eric Korpela Send message Joined: 3 Apr 99 Posts: 1382 Credit: 54,506,847 RAC: 60 |
We tracked it down to a radar system. I'll try to post more info this week. Punta Salinas, 6.2 km west of old San Juan. It's operated by the Puerto Rico Air National Guard. Fortunately the observatory has a system that can tell you when the radar pulses happen, so we can blank those parts of the data stream. http://www.naic.edu/~phil/rfi/rdr/puntaSalinas/puntaSalinas.html Eric @SETIEric@qoto.org (Mastodon) |
Michael Send message Joined: 21 Aug 99 Posts: 4608 Credit: 7,427,891 RAC: 18 |
Like I said over a month ago... Have you figured out what the pulse repitition interval and the pulse duration is? |
Clyde C. Phillips, III Send message Joined: 2 Aug 00 Posts: 1851 Credit: 5,955,047 RAC: 0 |
The Punta Salinas website table gives the duration of the pulses, 41.2 and 409.6 microseconds. I didnt check the frequencies of of the occurrences of the pulses; they might be there, too. The frequencies of the signals themselves are just below those of Seti, from about 1.2 to 1.4 GHz. |
Neil Walker Send message Joined: 23 May 99 Posts: 288 Credit: 18,101,056 RAC: 0 |
the British Services TV in Germany used vertical polarisation (probably still do) to prevent the civilian population from watching British TV!!!!! He he. I've been a little slow spotting this one. ;) The intention was to reduce interference with German domestic broadcasts. The organisation which spends a great deal of effort and money on The BBC World Service has no interest in preventing people receiving it's broadcasts. ;) Be lucky Neil |
ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
...The frequencies of the signals themselves are just below those of Seti, from about 1.2 to 1.4 GHz. Supposedly, the signals are blanked for Arecibo. At 25kW, it doesn't need much band spread or much of a side lobe or even just a geographic reflection to make a real mess of trying to observe the universe! Anyone nearby with camouflage paint, a hacksaw, some wire cutters, and?... :-O So how does that problem get fixed other than suffering degraded observations? Cheers, Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
We tracked it down to a radar system... Got yer camouflage paint and fatigues ready?... ;-O Cheers, Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
Michael Send message Joined: 21 Aug 99 Posts: 4608 Credit: 7,427,891 RAC: 18 |
We tracked it down to a radar system... AN/FPS-117 air traffic control and air search. |
ML1 Send message Joined: 25 Nov 01 Posts: 20283 Credit: 7,508,002 RAC: 20 |
AN/FPS-117 Well, we already know where Arecibo is. We don't need to re-find it!... And we certainly don't want any UFOs to be positively identified either!! Hopefully, discussions are in progress before the HARMs go in?... Cheers, Martin See new freedom: Mageia Linux Take a look for yourself: Linux Format The Future is what We all make IT (GPLv3) |
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