[TowerTalk] simultaneous Horizontal and Vertical antennas

[Jim Lux]

At 09:37 AM 4/4/2005, Gary Schafer wrote:


>Tom Rauch wrote:
> > All of the time a practice like this severely increases
> > fading where signal levels from the two antennas are close.
> > It may reduce the number of people operating on or near the
> > TX frequency, but everyone should consider it also increases
> > fading in a broad unpredictable area.
> >
> >
> > Doesn't work. What you get is a skewed pattern with wider
> > beamwidth and less defined polarization. In every direction
> > the resulting pattern is a single polarization that is the
> > vector sum of the two fields in that direction.
> >
>How do FM stations manage to transmit dual polarity?


They don't.  They transmit circular polarization (or more likely, 
elliptical polarization) which helps reduce the effect of multipath and ghosts.




> > The ionosphere will scramble the polarization, anyway...what
> >
> >>he's doing is simultaneously exciting high and low
> >
> > angles...(assuming
> >
> >>the vertical has a decent ground system.)  He's also
> >
> > destroying the dipole
> >
> >>pattern.
> >
> >
> > As the above statement points out, the concept of thinking
> > of this as two totally isolated and independent signals is
> > wrong.
> > The actual result is it simply tips or tilts the
> > polarization of the pattern in every area where each antenna
> > has significant radiation.
> >
>What do you think I was seeing when I phased vertical and horizontal
>antennas together on 10 meters? I found that with severe rapid fading on
>either vertical or horizontal alone, when I switched to both phased
>together that the fading smoothed out. No more deep fades into the noise.

This is an expected behavior.  There's been some interesting research in 
France on various schemes for combining co-located antennas with different 
polarization patterns, etc.

Yes, a straight passive combiner will produce a pattern with all sorts of 
wierd nulls and bumps and polarization effects.  So, what you're probably 
seeing is the ordinary and extraordinary rays (which fade differently and 
have different received polarizations) moving in and out of various 
nulls.  You essentially got lucky.




>I did find that by phasing the two as right hand or left hand circular
>that the signal was rapidly shifting between left hand and right hand as
>confirmed by swapping sense. But with the antennas in phase at those
>times it worked quite well.

Implying that you were actually receiving two signals (paths of the same 
signal) of different polarizations, likely with different incidence angles, 
with the path delay on each signal changing a bit, so it appears to be 
changing from RCP to LCP.





>But here you are talking about multipath propagation.

Which is exactly what you have in skywave propagation.  There's typically 
two dominant paths. The ionosphere is a anisotropic medium, much like those 
classic demonstrations with twinning calcite crystals (where you put it 
over a line on paper and get a double image).