Hi Charlie,
You are modeling in an area of segment size where the segment center (node)
space can affect the results.
Reduce the segment size in the model to the region of six, five or four
inches, and apply that to the entire antenna. Start with segment counts for
wires which give the region of four inchs and only lengthen if the segment
count is too high for your program version. This should be done in YO as
well, though it uses a different mechanism for establishing what amounts to
segment size.
On 160m I use fine grain one foot segments until the design is set. For
passing along the design to others, I reduce the number of segments until
the results start to change, and then back off a bit to get the fewest
segments that **still get the same results as the 1 foot segments**. While
I am using EZNEC Pro which supports gobs of segments, even the early EZNEC
will support much finer segments than you are using.
On your second method, what was the rationale for dropping from inch
diameter tubing to number 12? That what actually happens?
There is a modeling gotcha advisory to not place a source in a segment
adjacent to a junction of more than two wires or adjacent to a wire with a
widely divergent diameter, or widely divergent segment length. While this
does not effect all models, it clearly poxes enough to stay away from it.
Remember that models are creatures of algorithms and math. Most of these
strange issues have to do with the need to do many things with the same
math, just to have a program which is affordable to code. There are just
places where the math can't deal with an issue, without the expense of
writing exception code to handle it.
73, Guy K2AV
On Wed, Jul 9, 2014 at 7:39 AM, Charlie Ocker via Antennaware <
antennaware@contesting.com> wrote:
> Hi gang,
>
> I'm trying to verify a YO generated 20m yagi in EZNEC (version 3.x).
>
> I am using tapered tubing.
>
> The first taper on the DE is 36" of 1.25" diameter tubing. The DE is
> split, and there is 1.0" spacing between the tubing halves. The DE is
> insulated from the boom.
>
> Here's the deal - when I model the DE as a single wire, say, from -37" to
> +37", and make it 3 segments, and excite the model at the center of this
> wire, the results agree very well with what YO predicts.
>
> But, when I model the DE as two separate wires (1.0" to 37.0"), with the
> actual spacing between, and insert a 2" (from -1.0" to 1.0") wire of #12
> with 1 segment, and connect this wire to the two DE wires, then excite the
> model at the center of this #12 diameter wire, the input impedance is way
> lower than I expect.
>
> What is the accepted practice when modelling split DE's?
>
> Many thanks in advance!
>
> 73,
> Charlie N9CO
>
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