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Author Topic: Persistent dopplered multipath  (Read 230 times)
N3OX
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« on: February 21, 2007, 06:42:08 PM »

I went to check out 2m after AG4DG's post.

I'm listening to the WA1ZMS beacon on 144.285. It's about 150 miles distant to my southeast. (FM07 to me in FM18)  It's 4200ft AMSL so I can hear it most of the time.

Right now, however, I'm getting two signals out of it at two different frequencies, and it's directionally dependent.  

I've heard airplane scattered beacon signals where I get the direct signal and a swooping doppler shifted version that changes heading, especially if I point at the W3VD beacon to the north.  Also to the north is BWI and its constant stream of RF reflectors taking off...

This is different.  I've been listening to it for maybe ten minutes and I'm still getting it.  It's a very persistent shift of a few tens of Hz.

It does change and shift a bit, but it doesn't appear and disappear like airplane scatter.  In fact, I think I just heard an airplane go by... this is really something else.

It's pretty neat.  I'm thinking it's scattering off a wind shear or something.  That's the only thing I can think of that would be giving me a persistent, doppler shifted copy of this beacon's signal...

What do you think?

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB4QAA
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« Reply #1 on: February 21, 2007, 07:59:27 PM »

Not a wind shear, but a temperature inversion or change in gradient aloft.   I have seen it many times on radars, but never thought about it for ham radio.  

This phenomenon can be a real problem on air search radars, setting up large pattern returns.  and when the layer suddenly shifts it appears as contacts racing at humongous speeds.  I'm sure that it what has been attributed to UFO's.   Radar DOES lie.

73, bill
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N3OX
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« Reply #2 on: February 21, 2007, 08:13:59 PM »

Are you talking about this?

http://www.wonderquest.com/radar-ghost.htm

That seems to be pretty radar-specific in that it happens due to overlapping radar pulses...

That's pretty neat though too.. thanks!
 
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Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K8GU
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« Reply #3 on: February 22, 2007, 07:47:18 AM »

You didn't happen to make note of the time and headings or record the signal, did you?  One of the nice things about this beacon is that it's synchronized to GPS so you can be pretty sure that the signal frequency is stable.

I like the wind shear (or some sort of persistent turbulence) theory.  Of course, Bill is right that temperature gradients will affect the refractive index.  However, the atmosphere is not dispersive at radio frequencies such that different frequency components arrive along different paths (per Fermat's Principle).  So, the scattering/refracting volume is most likely moving.

Unfortunately, though, all we can do is speculate without designing an experiment to test a theory.  I'd be curious about more details if you have them.
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KB4QAA
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« Reply #4 on: February 22, 2007, 09:08:04 AM »

I have seen that particular example of false radar returns. The one I gave is somewhat different, but related.   The atmosphere can refract or bend RF signals.

One other possiblity, though less likely is that you could have some object like a building or terrain that is reflecting the signal and providing multipath signals. Though I would expect that you would notice this from time to time with other signals.

bill
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N3OX
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« Reply #5 on: February 22, 2007, 12:46:38 PM »

"So, the scattering/refracting volume is most likely moving."

I think that's right.  I emailed WA1ZMS and he mentioned that he'd seen doppler 1296MHz signals, likely from wind-driven movement of temperature fluctuations. If this is what it is, you're both right... it's the temperature gradients that do the refracting and the wind that causes the scattering volume to move.

I listened for a while and fired up Spectrum Lab on my computer... I was getting about 10Hz of shift when I was getting steady constant shift.

If I do my calculations right f'=f*(1/(1-v/c))and v must be just under 50MPH for 10Hz of shift on 144.28500 MHz.

I also heard what I usually take to be airplane scatter signals (they start above the beacon frequency and move to below it).  The shift was maybe +/- 80Hz when I acquired and lost those signals which is in the range of several hundred miles per hour with respect to me.

I have pretty bad heading resolution (RS TV rotor and not especially careful alignment to true north), and the heading determination was made problematic by the fact that my rotor stop is to the south, and it seemed that I had to swing past south to peak the signal...

At any rate, the main beacon signal was strongest to the SE, abt 225 degrees, and the doppler shifted signal was more or less strongest straight south but I couldn't really peak it effectively.

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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