Question about signal polarization

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Message 1634312 - Posted: 28 Jan 2015, 23:22:21 UTC

Hello everyone,

I have a few questions about signal polarization as it applies to SETI research in particular, although I suppose it's really more of a basic radio question. I tried a search here but got lots of returns and couldn't effectively sort through them.

Anyway, let's assume:

...an intentional ETI signal is discovered, and the polarization periodically flip-flops. (As I understand it, at the long distances dealt with, detecting polarization might be a bit tricky, but for this let's say it's easy to detect.)

...the science team isn't concerned about the signal polarity and therefore isn't checking it. To them, the signal is just one of undetermined and unexamined polarity.

...another science team looks at the signal but they're experiencing some sort of technical glitch and can only detect the signal when the polarity is the one they're currently able to see.

So the questions:

What sort of glitch, if any, would let them detect only one of the two polarities? What equipment would be the source of the problem?

Is this even a scenario that makes any sense in the real world, or is it something that wouldn't/couldn't happen because of the nature of radio signals?

Just so you know, I'm getting in way over my head. I usually can't even find the TV remote. ; )

Dave
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Profile Gary CharpentierCrowdfunding Project Donor
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Message 1634383 - Posted: 29 Jan 2015, 2:20:14 UTC - in response to Message 1634312.  

Hello everyone,

I have a few questions about signal polarization as it applies to SETI research in particular, although I suppose it's really more of a basic radio question. I tried a search here but got lots of returns and couldn't effectively sort through them.

Anyway, let's assume:

...an intentional ETI signal is discovered, and the polarization periodically flip-flops. (As I understand it, at the long distances dealt with, detecting polarization might be a bit tricky, but for this let's say it's easy to detect.)

...the science team isn't concerned about the signal polarity and therefore isn't checking it. To them, the signal is just one of undetermined and unexamined polarity.

...another science team looks at the signal but they're experiencing some sort of technical glitch and can only detect the signal when the polarity is the one they're currently able to see.

So the questions:

What sort of glitch, if any, would let them detect only one of the two polarities? What equipment would be the source of the problem?

Is this even a scenario that makes any sense in the real world, or is it something that wouldn't/couldn't happen because of the nature of radio signals?

Just so you know, I'm getting in way over my head. I usually can't even find the TV remote. ; )

Dave

For the purposes of Seti there are 2 types of polarization, linear and circular, which has two flavors. Polarization is a property that is determined by the physical construction of the antenna. As such there isn't a way for it to fail. Circular polarization antennas will receive linear polarization signals, but at a small loss. Linear antennas will receive circular signals, also with a small loss. As two the flavors for circular they are either left or right. Left antennas are terrible at receiving right signals and vice versa. That is why every Seti project collects in pairs. As to linear, here on earth, we have two flavors commonly used, vertical and horizontal, but which way is vertical in space? That is why linear is generally ignored in favor of circular.
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Message 1634391 - Posted: 29 Jan 2015, 3:24:04 UTC - in response to Message 1634383.  

Hi Gary,

Thanks for the quick answer! That's exactly the information I needed. I'm working on a story and want it to be accurate, so this helped a lot.

Dave
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Message 1636795 - Posted: 3 Feb 2015, 0:03:29 UTC - in response to Message 1634391.  
Last modified: 3 Feb 2015, 0:09:18 UTC

Side note, polarization is also affected by magnetic (or is it gravitational?)fields, it can change alignment over distance.
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Message 1636825 - Posted: 3 Feb 2015, 1:31:43 UTC - in response to Message 1636795.  

It sounds like polarization, in the long run, isn't a very useful characteristic for radio astronomers.

In semi-related news, nice that Kepler - K2 - is in service again!
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Message 1636850 - Posted: 3 Feb 2015, 2:56:01 UTC - in response to Message 1636825.  
Last modified: 3 Feb 2015, 2:57:54 UTC

I'm not an astronomer, but I would think everything in the universe is unpolarised. Take the sun for example everything it spews out (light, radiation, etc.) is unpolarised, meaning it is all mangled with horizontal, vertical and left and right polarizations over a very wide range of frequencies. So we use things like polarized sun glasses to filter out, say, horizontal light and UV to pass to our eye. We didn't block it all but we still have a good representation of what the original sunlight was - even though we blocked 99% of it.

So we do the opposite in communications. We can send horizontal and vertical signals at the same frequency to a receiver that selects the desired polarity to decode the signal. Same with left and right polarity.

In communications, rotating the receiver 90 degrees gives you some other signal (or nothing) but in astronomy I would think it would be the same since the signal is unpolarised.
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Message 1636870 - Posted: 3 Feb 2015, 4:13:07 UTC - in response to Message 1636850.  

But I would assume that a natural signal would be more likely to be un-polarized than an intentional signal, not to say that an intentional one would be polarized. I wonder if there are even natural sources of polarized radio signals - I would guess that some phenomenon could possibly do that, maybe nature's unlikely version of a million monkeys typing randomly for a million years managing to output a Shakespearean play by chance. ; )
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Message 1636872 - Posted: 3 Feb 2015, 4:39:16 UTC - in response to Message 1636870.  
Last modified: 3 Feb 2015, 4:47:57 UTC

But I would assume that a natural signal would be more likely to be un-polarized than an intentional signal, not to say that an intentional one would be polarized. I wonder if there are even natural sources of polarized radio signals - I would guess that some phenomenon could possibly do that, maybe nature's unlikely version of a million monkeys typing randomly for a million years managing to output a Shakespearean play by chance. ; )


Yes natural would be unpolarised, I think, but taking a polarized sample of it is still the same as the original signal (just lower power) like the sun glasses.

EDIT: Anyone looking at the skies looking for anything in the radio/microwave band are gonna see some man made signals from satellites we put up there. That why everyone has to use RFI filters to get just what they want. I guess there could even be laser communication between ground and satellite now a days ...

A simplistic way would to be to look at all polarities, if a signal is present in all 4, its unpolarised. If only one, then likely man made, polarized.
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Message 1636885 - Posted: 3 Feb 2015, 6:00:27 UTC - in response to Message 1636850.  

I'm not an astronomer, but I would think everything in the universe is unpolarised.

You would be wrong. A magnetic field can impart polarization. They are everywhere in space.
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Message 1636887 - Posted: 3 Feb 2015, 6:08:51 UTC - in response to Message 1636885.  

Thank You for correcting me on that Gary.

I just know that earthly terrestrial (over ground) signals can have their polarity 'rolled' a little when you try to stretch distances too far. Would be fine if it stayed in one position, but it change with the earth's moods.
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Message 1636893 - Posted: 3 Feb 2015, 6:30:48 UTC - in response to Message 1636885.  

A magnetic field can impart polarization. They are everywhere in space.


Now that I think of it that makes perfect sense the magnetic fields can polarize waves/particles - since that is how they make magnets!

Now I'm getting a headache ... so that also means that since magnetic fields can align particles, it can also bend their trajectory. So what we see as distant stars in comparison to originating signal location are not necessarily the same. Interesting.
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Message boards : SETI@home Science : Question about signal polarization


 
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