Eric's Weekly Post #3 (Warning, science enclosed. Do not fold or bend.)


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Message 521029 - Posted: 21 Feb 2007, 17:43:01 UTC - in response to Message 520912.

Just a thought Eric...

To illustrate my point, I once experienced the same effect in London, England, while listening to a radio station transmitting on 1.214 MHz. I could receive a weaker, but perfectly clear signal on 310 KHz, a fourth harmonic of the true signal...
Errrr... I didn't know that "sub"-harmonics existed!... [edit] (Note: They don't! However, you can see an interferometric effect whereby the amplitude of a signal gets modulated at the difference ferquency from the modulating/interfering signal. This is different from generating a (higher frequency) harmonic from non-perfectly-sinusoidal signals or from FFT decomposition of one signal. Corrections welcomed!) [/edit]

This example isn't a sub-harmonic, it's a harmonic. 4 times 310khz is 1240khz.

There are all kinds of chirps and birdies, undesirable mixes (like fundamental + if when you wanted fundamental - if, or mixing with a harmonic of the LO, or a whole bunch of other ugly things), or just plain so much signal from a strong nearby source that it just plain comes in.

The signal source could be leaking from another instrument on the telescope.

Maybe someone with more radioastronomy background can comment, but I'd think that multipath distortion is a terrestrial phenomenon -- you need some hills to refract around, or a nice big reflective building to make something other than the direct path viable. I don't think a relatively small nearby object like Jupiter is big enough to cause enough multipath distortion to matter.
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Message 521310 - Posted: 22 Feb 2007, 0:40:11 UTC - in response to Message 520912.
Last modified: 22 Feb 2007, 0:47:19 UTC


Errrr... I didn't know that "sub"-harmonics existed!... [edit] (Note: They don't! However, you can see an interferometric effect whereby the amplitude of a signal gets modulated at the difference ferquency from the modulating/interfering signal. This is different from generating a (higher frequency) harmonic from non-perfectly-sinusoidal signals or from FFT decomposition of one signal. Corrections welcomed!) [/edit]


Essentially, I was receiving the "ghost" signal on 4 times the wavelength of the original signal. I believe that makes it a harmonic. Been a long time since I did radio theory, and my memory's a bit hazy.

More seriously, you may well have had that (strong) signal breaking through on the mixer stage alternate frequency of one of the IF stages in your receiver. Was it a non-superhetrodyn receiver with only one (or no) IF stages even?


It was a long time ago, but the radio used valve technology. No phase-loocked loop back then, so I'd say it heterodyned.

"Ghosting" as commonly seen on TV pictures where you get multipath reflection is from the same frequency signal arriving via multiple paths. You get the same signal but displaced in time. Hence, for an analogue modulated TV picture that is scanned across the screen horizontally, you get multiple identical pictures overlayed but slightly displaced horizontally.


Arecibo is in a mountainous region, so I'm led to believe. Diffraction of a signal would be pretty common. Harmonic wavefronts are caused by a signal passing between objects that are less than one wavelength apart. A rough mental calculation tells me 1.4 GHz is a 0.2 metre wavelength. Unless there are a lot of objects 20 cm apart up there, (trees, maybe?) I'd say refraction is unlikely.


If what you notice are indeed "ghosts" but time delayed, then that would suggest multipath pickup. The "multipath" part could be ANY different route, whether external or just different routes through the pickup path in the affected equipment.


What you're describing is quite interesting, I reckon. Multiple reflections of a single signal by multiple intrusions (objects in the path of a signal) could easily produce time-delayed images of the same signal. The graph Eric posted seems to display no frequency difference, but a definite time difference, indicating reception of the same signal from a different direction. That would, I think, remove the possibility of electronic interference ("tempest" emissions} from a nearby piece of electronics.

I wonder if other Arecibo users are getting the same sort of interference?

It could just be that the interference source produces that repeating pattern and that it "just looks like that".

Who knows?


Not gonna argue with that! lol

Until Eric tracks it down.

It would be interesting to see what more of the WUs show and how consistent/repeatable it is, and so on...

Much better to cure the source rather than to try to remove the symptoms.

(Hey, we could make a reality-TV show out of "What next!"... :-) )


@ Ned:

There are all kinds of chirps and birdies, undesirable mixes (like fundamental + if when you wanted fundamental - if, or mixing with a harmonic of the LO, or a whole bunch of other ugly things), or just plain so much signal from a strong nearby source that it just plain comes in.


Hafta agree... I don't envy Eric's job with this one... it could be a taxi firm radio dx'ing, due to atmospherics, and will disappear as mysteriously as it appeared.

The signal source could be leaking from another instrument on the telescope.


Hmmmmmm.......
I would have thought that would have produced a single set of stripes on Eric's graph, but the fainter stripes indicate there's a time-related thing going on. I may be way off line with that argument, though... the strength of the signal may be rising and falling with time, in a cycle that makes it undetectable for part of it's cycle and a pain at another part. A rotating antenna would fit that scenario. Is there a temporary military radar in operation nearby? A military, mobile radar, such as Rapier (British kit, but you get the idea) would produce such a signal, which will disappear as soon as the military unit goes back to barracks at endex.

Maybe someone with more radioastronomy background can comment, but I'd think that multipath distortion is a terrestrial phenomenon -- you need some hills to refract around, or a nice big reflective building to make something other than the direct path viable. I don't think a relatively small nearby object like Jupiter is big enough to cause enough multipath distortion to matter.


Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.

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Message 521648 - Posted: 22 Feb 2007, 17:59:58 UTC - in response to Message 521310.


Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.


It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)
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Message 521876 - Posted: 23 Feb 2007, 0:29:24 UTC - in response to Message 521029.

This example isn't a sub-harmonic, it's a harmonic. 4 times 310khz is 1240khz.

As written there, true. However, that is not as was originally written. (Fuzzy confusion of cause and effect [affect for the Americans :-b].)

Good comments all round also.

Regards,
Martin

(ps: Humour alert regarding effect and affect!)

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Message 521938 - Posted: 23 Feb 2007, 2:09:14 UTC - in response to Message 521876.

This example isn't a sub-harmonic, it's a harmonic. 4 times 310khz is 1240khz.

As written there, true. However, that is not as was originally written. (Fuzzy confusion of cause and effect [affect for the Americans :-b].)

Good comments all round also.

Regards,
Martin

(ps: Humour alert regarding effect and affect!)

Not all Americans were taught that way, To Me It is cause and effect. Interesting discussion though. I think some don't pay attention to sound a like words and use their brain.
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Message 522088 - Posted: 23 Feb 2007, 10:57:56 UTC - in response to Message 521648.
Last modified: 23 Feb 2007, 11:01:08 UTC


Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.


It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)


That's interesting, but doesn't make your job easier. Most transmitters radiate in ONE polarity. I'm not sure if helical (circular) polarisation would turn up in a linear-polarised receiver. If so, it would be pretty weak. Beginning to look like the local equipment theory may be right.

Incidentally, does the receiver distinguish between Amplitude and Frequency modulation?
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Message 522224 - Posted: 23 Feb 2007, 19:31:56 UTC

What is polarization as it applies to Seti signals and the way Alfa receives them? I understand that with light it allows the waves to move in only one direction. So if polarized glasses are oriented correctly they will reduce glare while one is driving. Apparently glare is polarized in a direction 90 degrees from that of the sunglasses. Maybe glare is polarized horizontally(?).
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Message 522273 - Posted: 23 Feb 2007, 20:53:18 UTC - in response to Message 522224.
Last modified: 23 Feb 2007, 20:54:27 UTC

What is polarization as it applies to Seti signals and the way Alfa receives them? I understand that with light it allows the waves to move in only one direction. So if polarized glasses are oriented correctly they will reduce glare while one is driving. Apparently glare is polarized in a direction 90 degrees from that of the sunglasses. Maybe glare is polarized horizontally(?).

The two liniear polarisations of each ALFA beam can be combined to give both circular ("helical") polarisations. SO we could do the s@h search on FOUR polarisations per beam to give 4 x 7 = 28 streams of data.

I wonder if a bit more programming for the data splitting could add searching the circular polarisations?

Are there any natural phenomena that produce circular polarisation?


For our sun, a very interesting artical is "The Viking Sunstone". That suggests that the light from our sun is indeed preferentially polarised. Anyone know if this is so? Determined by the sun's magnetic field??

Cheers,
Martin

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Message 522274 - Posted: 23 Feb 2007, 20:54:54 UTC - in response to Message 522088.


Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.


It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)


That's interesting, but doesn't make your job easier. Most transmitters radiate in ONE polarity. I'm not sure if helical (circular) polarisation would turn up in a linear-polarised receiver. If so, it would be pretty weak. Beginning to look like the local equipment theory may be right.

Incidentally, does the receiver distinguish between Amplitude and Frequency modulation?

Most transmitters don't radiate -- it's the antenna that does that.
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Message 522279 - Posted: 23 Feb 2007, 21:05:25 UTC

Here's potentially a new and unexpected interference source:

"Alarming microwave"

Anyone nearby got a new car (and car alarm)?...

Or even bluetooth/WiFi devices in general?

And then there could be the old problem of an electrical power switch acting as a spark-gap transmitter...


Unless the source can be RF sniffed, it's down to looking for what is active when the interference is seen.

Eric: Any further clues?? Are we warm, cold, or completely off the planet?

Regards,
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Message 522281 - Posted: 23 Feb 2007, 21:07:06 UTC - in response to Message 522274.

Most transmitters don't radiate -- it's the antenna that does that.

That depends on how bad the SWR might be!

:-b
Martin

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Message 522408 - Posted: 24 Feb 2007, 0:26:35 UTC - in response to Message 522273.

For our sun, a very interesting artical is "The Viking Sunstone". That suggests that the light from our sun is indeed preferentially polarised. Anyone know if this is so? Determined by the sun's magnetic field??

I believe the polarization isn’t present in light as it emanates from the sun, but rather is an atmospheric phenomenon—vaguely similar to the way glare from the surface of a water-body, or from the layer of hot air over sunlit asphalt, gets horizontally polarized.
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Message 522509 - Posted: 24 Feb 2007, 5:11:23 UTC - in response to Message 522273.

snipped...

Are there any natural phenomena that produce circular polarisation?

and a bit more snipped...
Cheers,
Martin

I have been told that in the tropics, on east-west propagation routes the polarisation can be twisted. Ansd as a result cirular polarisation is used in those areas.
I do know that in the 60's Siemens had a UHF LOS system that could be supplied with circular polarised antenna.

And when I worked in the Med area, we suffered severe fading in spring and autumn at dawn and dusk, on SHF LOS links that crossed over the sea. Tests there showed the polarisation was being twisted.

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Message 522594 - Posted: 24 Feb 2007, 10:10:11 UTC - in response to Message 522274.


Hafta agree about the Jupiter thing, but Arecibo does have some hills around, and 1.4 GHz is well within the band used for UHF TV transmissions round the world. I wonder: does the interference show up on BOTH polarisations, or only one? (Horizontal or vertical?) I take it the Alfa receiver doesn't check for helical.


It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)


That's interesting, but doesn't make your job easier. Most transmitters radiate in ONE polarity. I'm not sure if helical (circular) polarisation would turn up in a linear-polarised receiver. If so, it would be pretty weak. Beginning to look like the local equipment theory may be right.

Incidentally, does the receiver distinguish between Amplitude and Frequency modulation?

Most transmitters don't radiate -- it's the antenna that does that.


Sorry Ned... slip of the typing fingers! *embarrassment*

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Message 522597 - Posted: 24 Feb 2007, 10:21:43 UTC - in response to Message 522224.

What is polarization as it applies to Seti signals and the way Alfa receives them? I understand that with light it allows the waves to move in only one direction. So if polarized glasses are oriented correctly they will reduce glare while one is driving. Apparently glare is polarized in a direction 90 degrees from that of the sunglasses. Maybe glare is polarized horizontally(?).


Clyde,

Natural light isn't polarised. Once light passes through a "polaroid" lens, it is polarised. That is, the sine waves that comprise the light beam have been filtered to allow only those which vibrate in one plane to pass through the lens.

If "polaroid" lenses are positioned such that they polarise light at right angles to one another, then no light will reach the observer's eye.

ML1 and WinterKnight

I haven't heard of any natural source of helical polarisation! If I'm right, a helical polarised signal would be proof positive of ET, so long as the other project parameters were all met. (Many years ago, some LOS equipment I used in the Army had helical antennae. I seem to remember Siemens made it.)
What I understood from Eric's post is that some work would have to be done to combine signals received by the vertical and horizontally polarised antennae, to derive a helical signal. Thus helical polarisation wouldn't be detected directly, but by inference. Interesting concept, though. :)


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Message 522726 - Posted: 24 Feb 2007, 17:46:38 UTC

Thanks, Martin and LittleGreenMan. I was just wondering how the radio waves are polarized. Possibly with sunglasses it's microscopic stripes only a micron or so apart. I did have two polarized filters once that passed perhaps 20 percent of the light. When I crossed them the transmission was probably less than one percent.
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Message 523553 - Posted: 26 Feb 2007, 7:24:17 UTC
Last modified: 26 Feb 2007, 7:29:06 UTC

No worries, Clyde

What you got was pretty typical. Polaroid lenses are like the rest of us: imperfect. Therefore, although the theory says they will blank out the light, in practice, a little does get through. Obviously, as always, it depends on quality.

Radio waves are polarised by the orientation of the antenna. A standard whip antenna stands vertically, producing vertically polarised radio waves. The same whip placed horizontally will produce horizontal polarization. TV signals are almost universally horizontally polarised, by the way, which limits their range. Helical polarisation is achieved by a coiled antenna, which can be clockwise or anti-clockwise. Many "outside broadcast" units have Line-of-Sight (LOS) antenna, composed of coiled antenna enclosed in a plastic or glass fibre tube. You may have seen a news van or similar, prowling round sporting one on it's roof, often on a telescopic mast.
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Message 523728 - Posted: 26 Feb 2007, 19:34:30 UTC - in response to Message 523553.

No worries, Clyde

What you got was pretty typical. Polaroid lenses are like the rest of us: imperfect. Therefore, although the theory says they will blank out the light, in practice, a little does get through. Obviously, as always, it depends on quality.

Radio waves are polarised by the orientation of the antenna. A standard whip antenna stands vertically, producing vertically polarised radio waves. The same whip placed horizontally will produce horizontal polarization. TV signals are almost universally horizontally polarised, by the way, which limits their range. Helical polarisation is achieved by a coiled antenna, which can be clockwise or anti-clockwise. Many "outside broadcast" units have Line-of-Sight (LOS) antenna, composed of coiled antenna enclosed in a plastic or glass fibre tube. You may have seen a news van or similar, prowling round sporting one on it's roof, often on a telescopic mast.


Thanks again, LittleGreenMan. I'll be more observant next time I walk or drive and notice all types of antenna I see. Sometimes I see parabolic antennas pointing horizontally, on towers, perhaps at others a mile or so away, but I don't know whether those signals are polarized or not. Maybe they're relays.

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Message 523993 - Posted: 27 Feb 2007, 6:50:08 UTC - in response to Message 523728.

Thanks again, LittleGreenMan. I'll be more observant next time I walk or drive and notice all types of antenna I see. Sometimes I see parabolic antennas pointing horizontally, on towers, perhaps at others a mile or so away, but I don't know whether those signals are polarized or not. Maybe they're relays.


Ya welcome, Clyde.

I left the Royal Signals nearly 19 years ago, and all this talk of radio is taking me waaaay back! :) The parabolic dishes are "collectors" that reflect radio waves inwards towards the "focus" of the parabola. The antenna is postioned at the focus, so as to receive the maximum amount of radio waves.

The antenna is usually referred to as a dipole, and is STILL polarised.
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Message 524650 - Posted: 28 Feb 2007, 19:53:30 UTC

The antenna, then, at the focus, is only one, 1/2 or 1/4 wavelength long?
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