I'm inclined to think that multipath was at the
root of the variations...in the sense that a varying
series of angular arrivals swept across the response
of the antenna. This is pretty normal.
(I've seen some very extreme multi-path prop's on 40m,
using a vertical, from east coast US to JA, in the spring,
where both our dawn short path, and the skew path to the southwest
produced delay differences long enough to make cw
above 15wpm impossible to copy. But I'm guessing
that's NOT what was going on in this case.)
I read it as vertical arrival angle differences...
not polarization. Somebody convince me I'm wrong.
n2ea
jimjarvis@ieee.org
-----Original Message-----
From: Jim Lux [mailto:jimlux@earthlink.net]
Sent: Monday, April 04, 2005 14:44
To: garyschafer@comcast.net; jimjarvis@ieee.org
Cc: towertalk@contesting.com; Tom Rauch
Subject: Re: [TowerTalk] simultaneous Horizontal and Vertical antennas
At 11:24 AM 4/4/2005, Gary Schafer wrote:
>Hi Jim,
>
>The antenna I used was a ta33 and I mounted a vertical dipole out on the
>end of the boom near the director. I realize it was kind of a crude way
>of doing it as the beam had more gain than the vertical dipole but the
>basic properties of vertical and horizontal were there.
>
>I ran equal length cables and made an additional length of cable the
>proper electrical length to compensate for the physical spacing
>difference between the vertical element and the driven element of the
>beam. That put them both electrically at the same point in time.
>
>I used an additional 90 degree length of line to switch into either feed
>line to get either right hand or left hand circular polarization.
Since the antennas aren't equal gain, you had more of an elliptical
polarization.
The TA33 probably has fairly sharp nulls and is fairly directive. The
dipole does not, but is lower gain, so what it will do is effectively "fill
in" the nulls.
>I found that when signals were fading rapidly, 5 to 10 seconds apart,
Implying strong multipath (the attenuation (absorptiona and reflectivity)
of the ionosphere doesn't change all that fast).. however, the polarization
properties DO change on that sort of time scale. It's extremely noticeable
on AM shortwave, where the two sidebands fade differently.
>It was not multi path. The two antennas were separated by 6 or 8 feet
>but in line with the other station and the delay line put the two at the
>same point in time electrically.
Not multipath in the "ground reflections" sense (like you would with a VHF
link).. the multipath is at a grander scale and more diffuse.
>The path was about 2000 miles to a friend. We met on several days and
>always found the same results.
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