David,
Thanks for running this.... however what I am seeing is, much
more often than not, the dipole is only about 8 db below the
array - and at times almost equal ... this is not quite within
your run - I would assume the array would be on average ~12 db
greater and at times even more so! I have only seen it more than
this once. In addition the FB is about 8 db and the antenna
shows almost no variation as I rotate it around. There is no
forward peak to speak of (I actually saw it peak just off of
forward when looking at one station that was ~20 degrees angled
to a guy - it peaked with the antenna looking OVER the guy!).
Its this latter fact that bothers me (as well) - no real
directivity to speak of.
And the SWR varies from 1.3:1 to about 2:1 depending on where
the antenna is relative to the guys.
g.
----- Original Message -----
From: "David Gilbert" <xdavid@cis-broadband.com>
To: "StellarCAT" <RXDesign@ssvecnet.com>
Cc: "tower" <towertalk@contesting.com>
Sent: Sunday, November 26, 2006 11:35 AM
Subject: Re: [TowerTalk] interaction problem...
:
: Maybe I'm missing something, but I don't see anything all that
strange
: with your results. The WARC dipole, for receiving purposes,
might as
: well be a normal 10m dipole. Your stack of C31's at 121, 81,
and 41
: feet should work great on 20m, but they are are pretty high
and spaced
: pretty wide for 10m. I realize HFTA (the terrain modeling
program from
: N6BV that comes with the ARRL Antenna Book) may not be
perfectly
: accurate, but I loaded your situation into it and got some
interesting
: results. I don't think this reflector allows jpeg enclosures
so I'll
: send them to you separately (and I'll send them to anyone else
who asks).
:
: I ran the model for your stack at 121, 81, and 41 feet with
the
: comparison dipole at 91 feet. At certain elevation angles the
stack has
: a deep notch in the pattern and the dipole will even beat it.
:
: I also ran a plot for a stack at 105, 90, and 75 feet to show
the
: comparison for the stack with closer spacing at the same
height as the
: dipole. The peaks in the pattern match those of the dipole
but there is
: still a pretty deep notch just above 10 degrees takeoff. That
might be
: an issue at some times for some paths.
:
: Finally, I ran a plot for a stack at 70, 55, and 40 feet
(arbitrarily
: chosen) to show how a lower height for 10m actually improves
the pattern
: ... the lowest notch goes away.
:
: As an aside, the C31 specs show a gain of roughly 7.5 db over
a dipole.
: The maximum gain you can expect from stacking two identical
antennas
: (even with optimum spacing) is about 3 db over that of a
single
: antenna. Adding a third antenna to the stack will give you
maybe 1.5 db
: additional gain. When I add those numbers (7.5 + 3.0 + 1.5) I
get 12 db
: gain for your stack over the dipole if the spacings were
optimized, and
: in your case they are not. That doesn't sound too much
different than
: some of your observations. The primary benefit of an
optimized stack is
: more it's ability to remove notches from the takeoff pattern
than it is
: simply to add some gain. Removing a notch, as you can see
from the HFTA
: plots, might result in several S-units benefit for certain
conditions.
:
: Hope any of this helps. 73,
: Dave AB7E
:
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