RE: NEC-LIST:Equivalent diameter - short tapered element

Jacob,
Searching around for differences in accuracy between NEC4 and NEC2 I came
across the 'steps diameters' for representing tapers - as your trying to
model. Conclusions from this analysis are as follows:

1. NEC-4 has limits in dealing with stepped-diameter elements, especially
under the following circumstances:
  a. Where the step in diameters between adjacent elements is large, and
  b. Where the large step occurs in the region of maximum element current.

Achieving convergence under these circumstances may require quite large
models relative to the number of antenna elements involved. These models
grow larger for every diameter step involved in the structure of the
element.

2. Inadequate segmentation in stepped-diameter elements in NEC-4 may result
in unrealistically high values for forward gain and low values for feedpoint
impedance. Adequacy of segmentation includes the following:
  a. Number of segments,
  b. Equalization of segment lengths within the element structural limits,
and
  c. Alignment of segments among the elements.

3. NEC-2 with stepped-diameter corrections can yield reasonable figures on
antenna performance. However, the best segmentation to achieve those results
is not self-identifying within NEC-2 due to the absence of a convergence
trend. In general, the best modeling region for achieving reasonable results
from NEC-2 with stepped-diameter corrections involves the following
constraints:
  a. Use the fewest segments possible within the limits of conservative NEC
guidelines; and
  b. Adjust segment lengths and alignment for equalization for the major
element section lengths, but not for very short lengths. The result will
be a model somewhat larger in segment numbers than the conservative minimum,
but smaller than a converged NEC-4 model.

4. NEC-2 without a stepped-diameter correction is highly unreliable. The
larger the diameter jump between adjacent section of the element, the more
unreliable the output figures. With small increments of diameter change and
long constant-diameter sections, the results may be tentatively usable if
the total number of segments is small. However, precision antenna analysis
is not possible.

Given the trends in the values of forward gain and front-to-back ratio (or
front-to-rear calculations) for the test Yagis with stepped-diameter
elements, it is entirely possible to generate misleading models in either
NEC-2 or NEC-4. Good practice calls for extreme care in developing models
and in reporting results. Not only should one present modeling results, but
as well one should reveal all the relevant details of the model used to
generate them. Likewise, those with modeling programs should check reported
figures for themselves.

Indeed, modeling is not in itself the solution to resolving issues of
reported antenna performance figures. Without adequate descriptions of the
models used (and the good engineering reasons for using those models), the
output of NEC programs can be as misleading as any other antenna performance
report. Properly used within their limitations and fairly reported, NEC
models can be a source of important and useful information not otherwise
easily obtained.

v/r
Ken Carrigan

-----Original Message-----
From: Jacob Schanker [mailto:schanker_at_frontiernet.net]
Sent: Thursday, July 11, 2002 2:48 PM
To: NEC-LIST
Subject: NEC-LIST:Equivalent diameter - short tapered element

Group:

I'm hoping someone will have a suggestion that will save me from going
through 15 years of ACES publications.

I am modeling a vertical monopole which is very short (about 18 meters)
compared to the wavelength of operation (LF). The monopole is linearly
tapered (not stepped) from about 0.3 m radius at the base to 0.05 m radius
at the top. In the past, I have used the average radius as the radius of an
equivalent, uniform radius element, with reasonable results, as compared to
actual measurements.

Now, I would like to get into the equivalence in a bit more depth. I don't
believe Leeson's Yagi methods apply here. Any insights or suggestions would
be welcomed.

Regards,

Jack

Jacob Z. Schanker, P.E.
65 Crandon Way
Rochester, NY 14618
Phone: 585 442 3909
Fax: 585 442 2182
j.schanker_at_ieee.org

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