Hi Guy,
> >w8ji wrote:
> >>"Space diversity" does nothing but establish a new pattern,
> >>unless there is some voting system that automatically selects
> >>the optimum phasing or antenna. During times of slow QSB, the
> >>operator might do that manually.
>
> That assumes a broad front largely invariant signal being received. If it
is
> possible on H.F. to have multipath effects, then on *receive only* one
can
> have unequally excited antennas. This does not behave the same as a
> transmitted pattern where each antenna is, by design, equally excited.
I'll save space and not reprint most of your well thought out post.
The theory presented assumes:
1.) There is no phase shift.
2.) Fading is by some unknown phenomena that simply "shuts off" the signal
at one antenna like an attenuator pad or switch.
Unfortunately neither is the case. Any theory that overlooks the reason for
fading "goes out the window".
> Presume that one antenna could be in a null, and the other antenna
hearing
> mostly full strength. Signal level X on Antenna A is already down 3db
> compared to the combined feedline signal of both antennas equally
excited.
> Now the signal arrives at the feedline junction (assume a plain "T"
> connector for worst case).
That fails to consider the reason for the fade. The reason is the same
signal arrives 180 degrees out of phase from two directions or angles. As
the amplitude and phase delay of the multiple path signals change, points
are reached where antenna terminal voltage can go to zero. But we must
remember these changes in voltage are always accompanied with a change in
phase.
That phase shift is what causes phase distortion in the audio. The phase
and level is frequency selective due to the nature of the path causing the
phase shift. I can build a "filter" that uses just such effects out of a
long coaxial delay line, and use it to attenuate a specific frequency in a
narrow frequency band by the same effect...it just doesn't shift around
like the ionosphere.
> This phenomenon is very straightforward if one is talking about diversity
> microwave horns. We use separate receivers to prevent the unexcited
antenna
> from becoming an energy sink or from having out of phase signals from the
> two horns cancel. I wonder if appropriate one-way combining devices
exist
> on H.F. that can handle transmit power, and change that 10 dB down to 3
or 4
> dB down. (That's a whole S-unit receive on fades).
Not many people realize an S unit is really not six dB!!!! I wish our
technical journals would face that fact. They even have it wrong on the
question pool for our licenses, claiming it is 6 dB!
> >5. Remote station always reports the combination as a tremendous
> > improvement even though when pressed for peak strength
> > comparisons, there is usually a less than 1 s-unit reported
> > difference. W7DD uses this technique to beat up on N7DD in
> > shootouts to Europe.
>
> Perhaps the improvement has more to do with the lack of fades at their
end
> due to your use of a stack at your end. That's a completely different
> discussion.
Yep. Same reason a 5/8 wl vertical fell out of favor on the AM BCB. The
more focused .54 wl or so tall verticals have less fading and less
multipath induced audio distortion than higher "gain" .625 wl or so tall
antennas.
The stack has a cleaner pattern, and being so is less subject to multipath
fading and distortion.
73 Tom
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