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Re: re: modeling qha



John,

    My experience would say your model is probably pretty good.  You just
about need a high caliber machine shop to produce/reproduce a good
Quatrifial Helical Antenna.  Some antennas do not lend themselves to easy
home construction the QHA seems to be one of them.  Difficulty is in the eye
of the builder.  I love the articles in QST that say something to the effect
of "It was easy I just machined it out of a block of solid aluminum."

    The saving grace is usually it is not that hard to produce an okay to
fair reproduction.  Look at how many S-Band patches and helix feeds yielded
many AO-40 contacts, and later turned out to be pretty poor antennas when
measured.

73,
Joe


----- Original Message ----- 
From: "John Mock" <kd6pag@qsl.net>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, April 19, 2004 2:44 PM
Subject: [amsat-bb] 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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