Having used a system like this for several years, I can offer some comments
from experience.
> How do you distinguish between a 180 degree phase shift,
> and polarity reversal? Unless you're timing logic edges,
> or counting cycles, they look the same to me.
They are indeed the same. 180 degrees is 180 degrees.
> The Topband discussion had to do with feeding each ear with separate
> signal sources...or the same source, phased by 180 degrees.
> True diversity reception (separate rx's, separate ant's) will
> give you some immunity from fades. I don't think it does much for
> minimum detectable signal.
To the contrary, if the receivers have similar delays in filters and have
perfectly synchronized oscillators it DOES improve readability but only if
the antennas have substantial spacing. The minimum distance I can
sucessfully use is about 1500-2000 feet on 160 meters. I'd guess the copy
improvement at "3 to 6dB", with 6 being the extreme. In other words a
station 20% readable would be about 40% to 80% copy when using wide spaced
similar pattern antennas. Of course these are loose numbers that are just my
impression so they may be off a bit. A barely readable signal (occasional
letters and numbers) becomes readable enough to work (i.e. you can actually
hear the exchange and callsign) without resorting to more modern means like
a cluster "I heard something beeping" QSO.
I never have seen an improvement using a dipole in one ear and a vertical in
the other for fading or copy on 160, but that might be because I don't spend
much time listening on transmitting antennas. They just aren't that good in
the first place.
I don't find *substantial* improvement when using my Beverages that are
close to my vertical arrays in stereo.
I never see a reduction in fading with distance. When the signal drops on
one array 2500 feet away from another, it drops on both arrays. The enhanced
copy by spatial separation does make fading less bothersome because it
extends the depth of copy, but looking at levels from each channel they fade
in step at the same rate and depth.
I strongly suspect the deep fades we have on 160 for long distance signals
are due to a lack of signal rather than polarization rotation. Carl K9LA
might shed some light on this. But whatever the predictions, I know the
fades occur at the same time with distance and polarization any time I have
listened on 160 with antennas 2500 feet apart or of different types but
similar wave angles.
> Where split headphones make a lot of sense is with split operation.
> You can listen to the dx in your left ear, and the pileup in your
> right. Or, your run freq in the left, and pounce stn in your right,
> if you contest. Spatial separation of the sources helps the brain
> keep them apart. (The MP does this with its 'dual receive' fn. The
> PRO doesn't...it mixes the two signals in one audio path.)
I find that it actually has negative impact on copy when noise (or other
trash like a pileup) is in one ear and a desired weak signal in the other.
Maybe that is just how my brain works. Bill Fisher W4AN was good at
listening two directions at once, and didn't seem distracted. Of course he
lost the stereo enhancement that he also noted was substantial ONLY if the
receivers were phase locked and antennas were very wide spaced.
What we would do is use the rebuilt R4C's in stereo lock for weak signals
and split directions, and the FT1000D mixed in at adjustable level mono as a
spotting or strong signal "run" receiver.
73 Tom
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