> How do FM stations manage to transmit dual polarity?
With time delay caused either by a difference in distance,
antenna type (a small loop and a small dipole), or a
difference in excitation phase of 90 degrees for two
antennas with the same radiation centers but a 90 degree
tilt in polarization, you can generate a rotating EM field.
FM stations got away with circular polarization because the
target was mostly by direct wave and had a single plane
antenna. TV antennas were horizontal and car antennas
vertical. It does increase fading under cionditions of
multipath, but the tradeoff was more FS in close open areas
into vertical car antennas.
> 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.
I wouldn't have any idea what was going without knowing more
about how the system was configured. The original post was
about using random phase on two widely separated antennas,
one not even directional. Not a good idea.
> 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.
I expect it would, at times. "At times" being the key. You
are manually doing the voting. Maybe ten meters has slower
fading and less multipath than 160, I don't that much about
propagation on ten. I just know on bands below 40 meters it
doesn't work no matter how much controls are ridden.
My point was if someone just randomly mixes two antennas
they do not have diversity, and statistically a broader
pattern (especially one that supports two modes) has more
fading.
> > I bet that also. The reason broadcast stations quickly
> > abandoned 5/8th wave verticals is the combination of
high
> > and low angles caused by the high angle lobe that
appears
> > with the 5/8th led rise to severe fading outside of the
> > strong groundwave area of the antenna. This effect was
so
> > severe they actually called shorter verticals
"non-fading
> > verticals".
>
>
> But here you are talking about multipath propagation.
It's normal to have signals arriving from multiple angles
and with varying phase on HF skywave, just as it is in the
fringe areas of a BC station.
If I bend a dipole in a V or cross two dipoles with a common
center axis all I get is a plane wave that is tilted and not
the polarization of either wire. Some people claim that is
"diversity", like the Carolina Windom. It isn't. It is a
skewed polarization. 90 degrees from the alignment peak I
have a null. If I delay between the two right angle
radiators I can have a rotating polarization in some
directions. Unfortunately I don't know which rotation is
best.
You could at least pick and you basically found out what I
say is accurate...the rotation varies. You voted. How does
someone with a receiver on another antenna who is not even
listening on the TX antenna and who is working random
stations in random directions know his two amplifiers are in
proper phase? He doesn't!
At best it is like a coin toss, but more typically it would
increase fading. You got to pick, and found the rotation
was random.
73 Tom
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