[HamWAN PSDR] HamWAN over tideflats
Bart Kus
me at bartk.us
Tue May 6 23:08:14 PDT 2014
Matthew,
The wavelength here is 5cm. Which means the 1/2 wavelength (180 degree
phase shift) is 2.5cm. I think the tide decreases the water level by
far more than 2.5cm. He'd start seeing constructive interference again
every 5cm. But he didn't. He saw a smooth gradual reduction the whole
way through (right?).
My current theory on this is that the mud that was exposed is far less
reflective to 6GHz than the water itself. I originally proposed the
multipath idea too, but that was when I thought I heard him say there
were fast 10dB swings in signal strength, which would be consistent with
small (water) waves. But he said the whole thing was very smooth, so I
don't think it's multipath phase interference anymore.
I actually have gear that can measure the reflectivity, absorption and
transmission of 6GHz through arbitrary materials, if Tom ever feels like
doing more tests on this phenomenon.
--Bart
On 5/6/2014 9:19 PM, Steve wrote:
> If you are ever in West Seattle, in the Junction area, look for the
> old microwave tower. You will see two antennas. One is horizontal and
> one is vertical polarized. This helps a lot over water. When I worked
> at the TV station we did this also with the microwave links.
> Very common, and it can even occur in fog.
>
> Steve
>
> On Tuesday, May 6, 2014, Matthew Lawson <kc7eqo at gmail.com
> <mailto:kc7eqo at gmail.com>> wrote:
>
> Its called tidal multipath. you are getting a reflected signal
> that is causing cancellation or attenuation from the AP (source).
> take for example a sign wave and mix it with another sign wave at
> the same frequency but change the phase a little or a lot you will
> get addition or subtraction, depending on the phase. when the
> waves are 180 degrees*out of phase* they will cancel each other,
> but as you shift phase they will attenuate or add to each other.
> when both waves are *in phase* with each other you will have a
> stronger signal. I am not a very good typist or writer, so I hope
> this makes sense. I have in the past plotted (graphed) RSL vs
> tidal height for microwave links over water. Its pretty neat to
> see the correlation of tide versus RSL.
>
> Another great example of multipath was when TV was analog you
> would get ghosting. This is a great visual of multipath. As you
> receive a direct signal at a given point in time you are also
> receiving the same signal at a slightly delayed point in time.
> Thus a ghost image would appear due to the time delay. You also
> used to be able see the picture flutter due to Doppler shift mixed
> with multipath as an airplane would fly over at the right path.
>
> Hope this helps in understanding on what going on.
>
> 73
> Matthew Lawson
> KC7EQO
> 442.100 Blyn Mt Repeater
>
>
> On Tue, May 6, 2014 at 1:41 PM, Benjamin Krueger
> <ben.krueger at gmail.com
> <javascript:_e(%7B%7D,'cvml','ben.krueger at gmail.com');>> wrote:
>
> It would also be interesting to see two of these test setups,
> some reasonable distance apart, using one as the control and
> doing adjustments on the other.
>
> Tom, fwiw, I have all the same gear you do. We could set mine
> up for this kind of thing.
>
>
> On Tue, May 6, 2014 at 11:08 AM, Mike Culver
> <mculver at extencia.com
> <javascript:_e(%7B%7D,'cvml','mculver at extencia.com');>> wrote:
>
> So signal strength is clearly correlated to the moon :)
> ________________________________________
> From: PSDR <psdr-bounces at hamwan.org
> <javascript:_e(%7B%7D,'cvml','psdr-bounces at hamwan.org');>>
> on behalf of Tom Hayward <esarfl at gmail.com
> <javascript:_e(%7B%7D,'cvml','esarfl at gmail.com');>>
> Sent: Tuesday, May 6, 2014 10:59 AM
> To: Puget Sound Data Ring
> Subject: Re: [HamWAN PSDR] HamWAN over tideflats
>
> On Tue, May 6, 2014 at 10:53 AM, Bart Kus <me at bartk.us
> <javascript:_e(%7B%7D,'cvml','me at bartk.us');>> wrote:
> >
> > That's a cool observation. I would caution against
> equating correlation with causation though.
>
> Agreed.
>
> > "The effect of tides on signal level" may indeed be an
> effect of temperature changes (ducting?) or something else.
>
> Temperature didn't change much all day.
>
> > If I recall correctly, you were seeing very large
> (10dB?) high frequency (1s period?) signal swings.
>
> Nope. I never saw more than 1 dB change during a 10 second
> period
> (never really watched it for longer periods than that).
> The signal
> very smoothly increased as the tide came in from the
> minimum to the
> maximum I shared earlier. As it rose, I never saw it dip
> again by more
> than 1 dB.
>
> > It'd be fun to understand that phenomenon as well as the
> slower one you're reporting here. Perhaps you were just
> being slowly moved through an interference node with the
> water level, and an antenna mounted +/- 2ft from your
> antenna's elevation might have reported the opposite power
> behavior? I also wonder if there is any polarization
> rotation happening here.
>
> Maybe I should try the same location with a MIMO modem.
>
> Tom KD7LXL
>
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>
> --
> Benjamin
>
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