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<div class="moz-cite-prefix">Matthew,<br>
<br>
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?).<br>
<br>
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.<br>
<br>
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.<br>
<br>
--Bart<br>
<br>
<br>
On 5/6/2014 9:19 PM, Steve wrote:<br>
</div>
<blockquote
cite="mid:CANYEi67jwr+b=qw+zgY5qR0rfpZ+R6PHOs0OaaBUu3PgFaDS_A@mail.gmail.com"
type="cite">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.
<div>
Very common, and it can even occur in fog.</div>
<div><br>
</div>
<div>Steve<br>
<br>
On Tuesday, May 6, 2014, Matthew Lawson <<a
moz-do-not-send="true" href="mailto:kc7eqo@gmail.com">kc7eqo@gmail.com</a>>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>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<b> out of phase</b> they will cancel each
other, but as you shift phase they will attenuate
or add to each other. when both waves are <b>in
phase</b> 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. <br>
<br>
</div>
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. <br>
<br>
</div>
Hope this helps in understanding on what going on. <br>
<br>
73<br>
</div>
Matthew Lawson<br>
</div>
KC7EQO<br>
</div>
442.100 Blyn Mt Repeater<br>
</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Tue, May 6, 2014 at 1:41 PM,
Benjamin Krueger <span dir="ltr"><<a
moz-do-not-send="true"
href="javascript:_e(%7B%7D,'cvml','ben.krueger@gmail.com');"
target="_blank">ben.krueger@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">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.<br>
<br>
Tom, fwiw, I have all the same gear you do. We could
set mine up for this kind of thing.</div>
<div class="gmail_extra">
<div>
<div><br>
<br>
<div class="gmail_quote">On Tue, May 6, 2014 at
11:08 AM, Mike Culver <span dir="ltr"><<a
moz-do-not-send="true"
href="javascript:_e(%7B%7D,'cvml','mculver@extencia.com');"
target="_blank">mculver@extencia.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0
0 0 .8ex;border-left:1px #ccc
solid;padding-left:1ex">So signal strength is
clearly correlated to the moon :)<br>
________________________________________<br>
From: PSDR <<a moz-do-not-send="true"
href="javascript:_e(%7B%7D,'cvml','psdr-bounces@hamwan.org');"
target="_blank">psdr-bounces@hamwan.org</a>>
on behalf of Tom Hayward <<a
moz-do-not-send="true"
href="javascript:_e(%7B%7D,'cvml','esarfl@gmail.com');"
target="_blank">esarfl@gmail.com</a>><br>
Sent: Tuesday, May 6, 2014 10:59 AM<br>
To: Puget Sound Data Ring<br>
Subject: Re: [HamWAN PSDR] HamWAN over
tideflats<br>
<div>
<div><br>
On Tue, May 6, 2014 at 10:53 AM, Bart Kus
<<a moz-do-not-send="true"
href="javascript:_e(%7B%7D,'cvml','me@bartk.us');"
target="_blank">me@bartk.us</a>>
wrote:<br>
><br>
> That's a cool observation. I would
caution against equating correlation with
causation though.<br>
<br>
Agreed.<br>
<br>
> "The effect of tides on signal level"
may indeed be an effect of temperature
changes (ducting?) or something else.<br>
<br>
Temperature didn't change much all day.<br>
<br>
> If I recall correctly, you were
seeing very large (10dB?) high frequency
(1s period?) signal swings.<br>
<br>
Nope. I never saw more than 1 dB change
during a 10 second period<br>
(never really watched it for longer
periods than that). The signal<br>
very smoothly increased as the tide came
in from the minimum to the<br>
maximum I shared earlier. As it rose, I
never saw it dip again by more<br>
than 1 dB.<br>
<br>
> 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.<br>
<br>
Maybe I should try the same location with
a MIMO modem.<br>
<br>
Tom KD7LXL<br>
<br>
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<br>
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<span><font color="#888888">-- <br>
<div dir="ltr">Benjamin<br>
</div>
</font></span></div>
<br>
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