I assume those modeling programs are the same as EZNEC since all three
use the same NEC-2 engine, and EZNEC does not allow stepped diameter
correction if you have loaded elements unless the load is at the
center. EZNEC doesn't give you any kind of error message ... it just
ignores it, although you can see how (or whether) it got implemented in
one of the drop down options in the Wires dialog box. I'm not sure if
this is discussed in the documentation for NEC-Win and MultiNEC, but
below is the extract from the EZNEC help file.
73,
Dave AB7E
<extract>
Stepped Diameter Correction Overview
The following information applies only to the NEC-2 calculating
engine(s), which are the only choices for all EZNEC program types except
EZNEC Pro/4. This problem doesn't occur when using NEC-4 with
EZNEC Pro/4 for calculations unless there are extreme differences in the
diameters of connected wires. In general, if you have EZNEC Pro/4, you
should use an NEC-4 calculating engine whenever wires of different
diameters are connected.
NEC-2 is known to be inaccurate in modeling connected wires having
different diameters. (Note: This is sometimes called being "tapered". It
shouldn't be confused with EZNEC's segment length tapering
<Segment_Length_Tapering.htm>.) The problem gets worse as the segments
near the junctions get shorter and as the diameter difference gets
larger. The error is small enough that it's not important for many
applications, such as evaluating a tower top-loaded with a beam.
However, if the stepped diameters are on a parasitic element or
exceptionally high-Q antenna, the inaccuracy can cause significantly bad
results. EZNEC incorporates a method developed by Dave Leeson and
described in his book Physical Design of Yagi Antennas (ARRL, 1992) for
calculating an equivalent wire of constant diameter to replace a group
of wires of different diameters. The method is valid only under a narrow
range of circumstances, however, and EZNEC will apply it only under
those circumstances. The requirements are that:
- There must be at least two wires in the group.
- At least two of the wires must have different diameters.
- All wires in the group must be collinear (in a straight line).
- All wires must be connected to each other.
- Both ends of the group must be open, or one end open and one connected
to ground.
- The group must be nearly resonant (within about 15% of half-wave
resonance if both ends are open, or within about 15% of quarter-wave
resonance if one end is grounded).
- Only one source is permitted in the group, and it must be at the
center if the ends are open, or at the bottom segment if the group is
grounded. If the ends are open and the center of the group is a wire or
segment junction, the source must be a split source. If the group is
grounded, the source must not be a split source.
- The rules for loads are the same as for sources, except that two
equal loads must be used wherever a split source would be used.
- A single transmission line can be connected to the group. If the ends
of the group are open, the center of the group must be a segment center
-- not segment or wire junction -- and the transmission line can be
connected only to this segment. If the group is grounded, the
transmission line can be connected only to the bottom segment.
These criteria apply to typical Yagi elements made from telescoping
tubing, one of the most demanding cases where the correction is needed.
The correction won't be applied to a gamma match, another sensitive
application, so attempting to model gamma matches isn't recommended if
very accurate results are required.
A bright notice will appear on the EZNEC screen during calculation when
the stepped-diameter correction is being used for one or more groups of
wires. You can see exactly what the substitutions are from the Wires
Window by selecting Other/Show Stepped Dia Correction. (See Using The
Stepped Diameter Correction Display
<Using_the_Stepped_Diameter_Correction_Display.htm>.)
The correction can be disabled from the Options Menu if desired, but
this isn't recommended.
In situations where wires of different diameters are connected but
EZNEC's stepped-diameter correction doesn't apply, a technique can be
used to minimize the error. This is to use the minimum number of
segments possible, and do not use segment length tapering. (Exception:
wires very near ground. See Elevated Radial Systems
<Elevated_Radial_Systems.htm>. The accuracy of NEC-2 is worst in the
presence of stepped diameters when the segment length/diameter ratio is
small. Especially when confronted with a large difference in diameters,
the best accuracy will be obtained if you use the automatic segmentation
feature in the Wires Window, and select (M)in. recommended. The accuracy
is still not likely to be good enough for accurate modeling of parasitic
elements if the built-in correction can't be applied.
The inaccuracy of NEC-2 in the presence of large steps in diameter
typically shows up as an incorrect reactance. If you design an antenna
with parasitic elements having connected wires with largely differing
diameters, EZNEC will give an accurate idea of the antenna performance.
However, it will show the performance occurring at not quite the correct
frequency. When you actually build the antenna, you can expect the
predicted performance, but may need to adjust parasitic element lengths
slightly to achieve that performance at the desired frequency. For
example, if EZNEC shows your "X-beam" to have a gain of 6 dBi,
front/back ratio of 20 dB, and feedpoint SWR of 1.5:1 at 14 MHz, you
might find when you build it that the front/back ratio is much worse
than predicted at 14 MHz but is very good at 14.5. In this case, you
would need to lengthen the parasitic element until you get the best
front/back ratio at 14 MHz. The beam will then have very nearly 6 dBi of
gain and 1.5:1 SWR at 14 MHz.
</extract>
Scott McClements wrote:
> Hi All,
>
> I was modeling a Cushcraft 402CD loaded 40m yagi. I use Nec-Win Plus
> and MultiNEC. I enabled stepped diameter correction in both programs
> and noticed that in both programs a taper correction didn't occur. The
> output from NEC-Win Plus during the taper correction seemed unhappy
> because the element resonated around 11Mhz, but the simulation
> frequency range was 7 Mhz.
>
> I am not sure where the taper corrections are derived from in these
> two programs, but is there a proper way to do this?
>
> -Scott, WU2X
> _______________________________________________
>
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