[Antennaware] K9AY Loop modeling in NEC-2

[Guy Olinger, K2AV]

Wow. This belongs in a book and on a web page somewhere. Great post.

----- Original Message ----- 
From: "K9AY" <k9ay at k9ay.com>
Sent: Thursday, November 13, 2003 7:51 PM
Subject: [Antennaware] K9AY Loop modeling in NEC-2


> Pete and all,
>
> Here is what I have developed for K9AY Loop modeling...  Remember,
although
> the antenna was "discovered" by modeling, all the hard data on
appropriate
> terminating resistance and pattern shape were obtained through
extensive
> listening tests on the first unit.  I have not found a model that
does not
> need to be "tweaked" to match the observations.
>
>
> Method #1
> ---------------
> I model the loop exactly as it is constructed, connected to "Real"
ground
> using the Sommerfield-Norton "High Accuracy" ground type. This is
NOT a
> recommended procedure for NEC, because the currents in/out of ground
can be
> unpredictable. However, using the segmentation on the list below and
the
> "Average" ground conductivity, I get modeled results that match the
observed
> performance.
>
> Because this model goes against NEC modeling guidlines, I rarely
publicize
> it.  It suits my purposes, within strict limits based on LOTS of
experience
> with this particular antenna type.  I believe it is an acceptable
model --
> just as with Beverages and EWEs when they are modeled with
connections to
> High Accuracy Ground.
>
> (Dimensions in feet)
> Wire 1   0,0,0   0,-15,4   #14   9 segments
> Wire 2   0,-15,4   0,0,25   #14   13 segments
> Wire 3   0,0,25   0,15,4   #14   13 segments
> Wire 4   0,15,4   0,0,0   #14   9 segments
> Ground -- "REAL" "HIGH ACCURACY" "AVERAGE" (.005, 13)
> Source:  Wire 1, 0%
> Load:  Wire 4, 100%, 400 ohms
> Freq: 1.83 MHz
>
>
> Method #2
> ---------------
> Instead of the two long wires used by W7EL, a model using "High
Accuracy"
> ground can be created using (8) radials 30 to 50 feet, less than one
foot
> above ground. Length doesn't matter too much, as long as the radials
enclose
> an area about twice the footprint of the antenna.  Be sure the
radial field
> include two that are exactly in line with the loop.
>
> This model requires a terminating resistance much higher than what
was
> empirically determined, but it "follows the rules" and the pattern
shape is
> in the right ballpark.  I never use this model -- it was developed
as a
> "first cut" at making a model that met NEC usage guidelines.  It is
possible
> that this is an accurate model for a loop installed over radials,
but that's
> not how I make them.
>
>
> Method #3
> ---------------
> I have not yet thoroughly tested this modeling method, but it gives
> consistent results and appears to match real-world behavior.
>
> 1. Use the REAL/MININEC ground
> 2. Raise loop 1 foot above ground
> 3. Add a wire from ground to the junction of Wires 1 and 4 (which
were
> connected to Ground in the above wire list)
> 4. Feed/terminate the loop at the same points as above -- inner ends
(first
> pulse) of the semi-horizontal bottom wires.
> 5. Add a 150 ohm resistive load in the new wire between ground and
the loop
> feed/term junction.
>
> MININEC ground models antenna impedance as if connected to perfect
ground. I
> believe the resistor in the vertical wire represents the reduced
currents
> in/out of lossy ground. 150 ohms is a "trial-and-error" value, and
the right
> value probably varies with local ground conditions. In any case, the
pattern
> shape is right, dBi gain and feedpoint impedance are pretty close to
the
> High Accuracy Ground model I've been using.
>
> 73, Gary
> K9AY
>
>
>
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