[BC] Skirt feeds.....

[Phil Alexander]

See comments interspersed.

>From: Poetter Bruehl <pbruehl at gmail.com>
>
>The problem with NEC analysis is that there are a number 
>of programs available that are based on NEC but do not 
>necessarily perform all of the analysis possible with fully
>functional code. 

Agreed. It was not until NEC-4 that most of the problems
were resolved. It is also important to understand that in
some respects NEC-4 has capabilities which should not be
used for broadcast work because the FCC assumptions embedded
in the rules will not always agree with the model unless the
FCC propagation assumptions are understood. For example, the
ground functions of NEC-4 are not useful because the FCC
methods assume a perfect earth with perfect conduction in
the near field.

<snip>

>The opposite 
>is the case for simple series fed tower where it can be
>approximated as a cylinder or very thick wire and still 
>produce reasonable results.

Yes, it is generally advisable to model the lattice except
in the case of a uniform cross-sectional guyed structure.
However, I have achieved very satisfactory results using
a cylinder substitute (for a uniform tower) in the case of
skirt/shunt fed unipoles. This does present a difficulty in 
the spider feed but I have found a workable solution that 
reduces the error to insignificance. 

>I suspect the author has never used a fully functional NEC 
>program that has the capability to analyze many of his 
>examples and hence his insistence that it is not possible.

While that may be true, the full version of NEC-2 is capable
of nearly everything found in NEC-4 with the exception of
real grounds, and this program has been available either
freely or inexpensively for some years. I cannot agree, under 
these circumstances, that ignorance is a satisfactory excuse
for knowledge of the state of the art, especially when one
chooses to condemn it.

>I agree that it is indeed not possible when using EZNEC but 
>easily accomplished with NEC4 so long as there is a proper 
>model constructed.

EZNEC Pro/4 v.5.0, which contains the LLNL core engine and
requires a NEC-4 license from the LLNL contractor, is a very
competent program and is far faster for model construction
than native NEC-4.

>NEC analysis as is also the case for many other things in 
>life is dangerous when used by someone who does not 
>understand the underlying theory and does not know 
>approximately what the result should be.  It is really 
>necessary to start out with a basic model that will produce
>a result that is close to a value already known to the 
>designer and then make it progressively more complex.  This
>reduces the possibility of there being a fundamental error 
>in the assumptions made when constructing the model.

Yes, I agree completely with the idea that a result that
does not agree with your experience is probably an incorrect
result. It is the experience and understanding that is key.
This is especially true in the case of modeling directional
antenna arrays and transmission systems. Having a good 
modeling program without a good understanding of the 
underlying principles of antennas, transmission and 
propagation can be a prescription for confusion and disaster.

>A good NEC program that can analyze complex antenna systems 
>is an investment that a casual user may be unwilling to make.

Indeed, especially if the LLNL NEC-4 license is part of the
purchase. As I recall, when I bought it several years ago,
the license was $1200 and may have increased since then. The
same is true of professional grade modeling programs which
are generally in the $700~$1000 range. Good tools are seldom
cheap.

Phil Alexander, CSRE, AMD