[BC] Skirt feeds.....

[Poetter Bruehl]

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. Some programs have 
difficulty with loops, multiple wire connections and buried grounds. The 
other problem is the accuracy of the model. Often shortcuts are taken 
which could be fine for a simplistic problem but fail when a more 
complex analysis is attempted. One example would be the inverse Unipole 
presented in the book being discussed. A perfectly accurate model of the 
tower is necessary to perform such an analysis and would require a large 
number of possible segments available. 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.

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. 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.

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.

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

Phil Alexander wrote:
> Having read the book, the problem I have with it is 
> his instance that these antennas require a special 
> math and insistence that they are not subject to 
> accurate analysis using the NEC. 
>
> It appears to me that the reason for this insistence is 
> the NEC results do not coincide with his pre-conceptions.
>
> Before the development of the NEC unique purposed math
> theories were the best we could do. Since its development
> the NEC has been proven in some of the most complex radiation applications, especially military designs such as stealth
> aircraft and ships to space vehicles. The NEC has been
> improved through four major major revisions over 40 years 
> since its inception. 
>
> IMHO saying there is something unique in the operation of 
> skirted unipoles that prevents their correct analysis with
> the NEC and requires using a unique math theory is laughable. 
> In my own experience modeling skirt/shunt excited unipoles
> works well provided the rules I have outlined earlier in this
> thread are observed. However, the NEC model does not seem to
> support some of the alternative configuration and design ideas 
> that have been described as successful in some cases although
> empirical proof of claims of success is generally lacking.
>