Sorry Jim - you've misunderstood where I am going .... I'm using SWR as a
'worst offender" indicator. I am not at all worried about SWR, or at least
not in it as a single indicator - but in all the modeling I've done the most
sensitive factor is FB followed by SWR and finally gain. Gain one almost has
to work to reduce - and SWR is often a good bit behind FB. For example when
modeling the effects of guy wires I can see changes in the FB of say 5 - 8
db before there's any real noticeable impact on SWR ...
So my point is: If the SWR is that low - I'm sure there will be interaction
with the FB and potentially with the gain. So I'm most definitely in
agreement how to continue that analysis and I've done just as you indicate.
But here we have the real world actual situation where someone measures a 40
meter antenna and finds it to have an SWR on 15 of 1.2:1 - THAT antenna WILL
interact I'm sure (although I have no proof of it either through modeling
that can replicate it or through real world as I'm not aware of a single
instance where someone has checked it using a thorough range test setup!)
Gary
-----Original Message-----
From: Jim Brown
Sent: Thursday, June 2, 2016 12:43 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Modeling question - for the experts!
On Thu,6/2/2016 5:38 AM, StellarCAT wrote:
So here are the questions:
1) WHY when I model a full size 2 element 40 does it not have a low SWR on
15 when I know in the real world people have said it should/does?
At least to some extent, you're asking the wrong question. Interaction
is FAR more than VSWR. Interaction is PATTERN DISTORTION and/or
performance degradation. Pattern distortion can be in the vertical
plane, the horizontal plane, or both. To see it, we must look at the far
field pattern with and without the interfering antenna, and carefully
compare the plots on the same drawing.
One method I've found to do this is to model the "primary" antenna, save
the model, and save plots for H and V field strength, then add the
interfering antenna, look at the H & V plots, and with each, add the
saved plots to the display. NOW, you can see very clearly the effect of
the interfering antenna, if there is any.
Another method that I've used is to build the model with both antennas
and compare H and V plots of the primary antenna as I rotate (or raise
and lower) the "wires" defining the interfering antenna. Again, to see
the interference, we must look at plots "with and without" the
interfering antenna on the same graph.
When I evaluate an antenna, I typically look at the vertical pattern on
axis, and the horizontal pattern at 5, 10, and 15 degrees elevation.
When a second antenna (or wires in the vicinity) are interacting, the
"pretty" pattern of the primary antenna will be less pretty. :)
73, Jim K9YC
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