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Re: [TowerTalk] interaction problem...

To: "David Gilbert" <xdavid@cis-broadband.com>
Subject: Re: [TowerTalk] interaction problem...
From: "StellarCAT" <RXDesign@ssvecnet.com>
Date: Sun, 26 Nov 2006 12:10:56 -0700
List-post: <mailto:towertalk@contesting.com>
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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