Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Keith Morehouse

Mike gave a good explanation of what our problem was.  The multi-path really became obvious when we tried JT65 and KK6MC was seeing two synch tones.  I moved my antenna 10 degrees west and the result on the paths south end was FOUR synch tones.  Your question about reciprocity is valid.  I don't know why I wasn't being affected - the signal strength explanation put forward by Mike is certainly possible.  When we went to CW, I could detect a slight buzz or beat-note on his signal that was probably multi-path induced.

I have terrible multi-path problems on 2M when working rovers from a couple of commonly used locations to the south of me.  There is one place in particular were signals are very loud but the distortion is so bad on the direct path I need to turn the antenna 20-30 degrees east or west to copy anything on SSB.  On CW, it sounds like a DX pileup on 20M.  The 14,000' peaks of the San Juan range lay just 40 miles south of here.

On 6M, it is very common at this QTH to see multiple FT8 traces on 6M from 'local' stations (50-60 miles away).  The are usually offset by at least 1/2 of the FT8 signal bandwidth (so, maybe 15-20 Hz).  Sometimes, depending on who I'm hearing and where the respective antennas are pointed, they are far enough apart to see two distinct signals.  The result is usually TWO decodes of the same station, reported on two discreet frequencies, not a total failure.  It appears signals offset a smaller amount at much higher frequencies can cause problems.

A good example of multi-path can be viewed if you're near a major airport and can hear a CW beacon on 432 that's out a way - maybe 100 miles.  If the air traffic flow is correct, and the incoming or outbound traffic passes across your path to the beacon, you can watch a spectrum display and see the multiple signals caused by the aircraft reflections.  When I lived in Illinois, I used to monitor a 432 beacon in Michigan, northeast across the lake about 200 miles.  The direct path was right across the traffic flow in and out of O'Hare airport in Chicago.  The patterns of multi-path and Doppler shifted signals caused by the aircraft was amazing.  This is also a good demonstration of a technique called 'passive bi-static radar' that can be used to detect stealth aircraft without having to emit a signal of your own.


Keith Morehouse
via MotoG

On Mon, Sep 16, 2019, 9:40 PM John Klem <klemjf@...> wrote:
I am very curious - what characteristics of these signals led you to conclude multipath was the problem?  What was painfully obvious about it?
Should multipath be non-reciprocal?

Getting an audio recording of this kind of signal for analysis would be really interesting.

John AA5PR

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