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re: modeling qha



I tried for a long time to model a Quatrifial Helical Antenna using NEC2
without much success.  Sure, i got one which had a pattern and impedance 
that looked somewhat plausible at the design frequency:

    http://www.qsl.net/kd6pag/nec/apt1g.html

But it did not appear to be very numerically stable to me (i.e. small
changes in parameters yielded large changes in the model; see also the
variation of impedence vs. frequency in the above example).  Hence, i 
never trusted the results and pursued other antenna.  Perhaps someone
else has come up with a better NEC2 model for this antenna (which i
would be interested in seeing).

I suspect the problem with simulating these with NEC2 is that this antenna
may involve cancelling fields and that's probably what impairs the numerical
stability of the model i tried to use.  The difficulty most likely involves
the subtracting two floating point numbers which are nearly equal, results
of which are rather approximate at best...  Perhaps someone familiar with
computational analysis and numerical modeling can explain this better than
i can.
			         -- KD6PAG


Note: If you receive something purportedly from <kd6pag@qsl.net> that isn't
signed with my callsign, then it probably isn't from me.
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