Gary (TT):
FM Stations transmit circular polarization,
which results in signals being 3dB down in
any single plane, but less susceptible to
selective fading/interference problems,
particularly with subcarrier content.
Not to forget, they're only concerned with line
of sight, which eliminates the problem of
ionospheric scrambling of the polarization.
As to your 10m experience, you didn't indicate
what the spatial relationship of the two antennas
might be. I assume they were the CB type, on a common
boom, if you actually fed them circularly.
My belief is that you experienced the diversity effect,
where the sum is down 3dB from peaks on either antenna,
but the signal dips are filled in. It's more likely
due to differences in arrival angles, rather than
polarization, by the way. Most stuff is coming in
horizontally polarized, once it goes through the ionosphere,
or so I've read.
N2EA
-----Original Message-----
From: Gary Schafer [mailto:garyschafer@comcast.net]
Sent: Monday, April 04, 2005 12:38
To: Tom Rauch
Cc: jimjarvis@ieee.org; towertalk@contesting.com
Subject: Re: [TowerTalk] simultaneous Horizontal and Vertical antennas
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.
>
> > Secondly....with respect to the original concept of
> running parallel
>
>>vertical and horizontal antennas to get multiple
>
> polarizations...
>
>>twaddle!
>
>
> 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?
> 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.
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.
> Sometimes we want to transmit a broad beamwidth signal, but
> the best solution is to just design an antenna that does
> that and understand doing so severely increases fading in
> the wide area (angle and direction) where both antennas have
> significant FS.
>
>
>>I'd bet he'd get better results by switching between them
>
> and picking
>
>>the better antenna at any given moment.
>
>
> 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.
>
> How you tune and load each amplifier plus other less
> esoteric things like feedline length control where the new
> nulls are as well as where the new main lobes are formed.
>
> 73 Tom
>
73
Gary K4FMX
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