Re: NEC-LIST: Modeling an EMP using NEC

On 2008 May 25 Justin L. McKinnerney wrote:

> I'm trying to model an EMP using NEC in order to calculate the
> coupling to an antenna.

Some useful information can be obtained using NEC to model EMP and other
complex Rf waveforms such as chaotic RF signals. The major limitation
is the lack of the time dimension. All signals are assumed to be
continuous, single frequency sine waves.

Any complex signal can be broken down into its component harmonics and
NEC will readily calculate each frequency/amplitude component
separately. For a continuous complex wave one can perform post-
processing to obtain the desired result, this can amount to summation of
the instantaneous amplitudes of all frequencies with the proper phase
and sign relationships observed.

For impulse waves the problem is more complex, since the frequency
content of the signal changes over time - high frequencies at the rise,
lower toward the fall. As a result an antenna's overall coupling and
pattern will effectively be determined by the frequency content at each
incremental time within the pulse.

In all - NEC for this purpose is better than nothing, but more suitable
methods do exist. Finite difference and similar methods can be used,
but these require substantial mathematical effort and skill.

Hope this helps - I wish you luck.

Darryl Holder

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