[HamWAN PSDR] HamWAN over tideflats

Wed May 7 07:10:48 PDT 2014

Good point Bart, very curious to see what's going on. I didn't pay
attention if lat and long of both source and subscriber were posted. Would
like to see the terrain profile.

73

Matthew Lawson
KC7EQO
442.100 Blyn Mt Repeater
On May 6, 2014 11:08 PM, "Bart Kus" <me at bartk.us> wrote:

>  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> 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>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>wrote:
>>>
>>>> So signal strength is clearly correlated to the moon :)
>>>> ________________________________________
>>>> From: PSDR <psdr-bounces at hamwan.org> on behalf of Tom Hayward <
>>>> 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> 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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>>>
>>
>
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