Re: NEC-LIST: Re: resonance and Q of an antenna and scattering system

I had just suggested the CAD package. With FEM or FDTD, such a
computation should not be too bad. You do need to be careful in that the
stored energy is not simply the maximum E or maximum H energy. You need
to consider total energy as a time variation (makes FDTD look like a good
choice). If you limit the energy consideration to the near field of a
small antenna, then using the max of the E or H field energies is a good
approximation and is consistent with the fundamental limit approaches
typically taken. However, for larger systems, the stored energy is not
stored in just the E or the H at any one time and a more complete
development is needed.

However, from a practical viewpoint, I have to agree with Alan that using
the impedance bandwidth is a simpler approach to the problem. That is
easy to obtain with almost any CAD package. This is what I had suggested
in my original mail also. Computing the stored energy is for the most
part of more academic interest than it is practical in most cases. The
computation is just too much extra work for a general problem, though
with FEM or FDTD it should not be too much effort. I am not sure that
the CAD packages actually include these options and you might actually
have to use the data to compute it externally. Good luck if you decide
to try it.

Bill Davis

> If using a CAD package is the solution to computing stored energy, then it
> would be interesting to know what approach the people that design/build CAD
> packages use to compute the stored energy. Any ideas?
>
> Malcolm M Bibby
>
> ----- Original Message -----
> Subject: Re: NEC-LIST: Re: resonance and Q of an antenna and scattering
> system
>
>
> > There is not a simple way to compute the stored energy needed for this
> > computation. It has been the subject of work for several years and led
> > about a half-century ago to the "fundamental limits" of antennas. The
> > limits computed are based on lower bounds of the stored energy and thus
> > give a lower bound on the Q. To get a smaller Q, you have to add loss,
> > that of course cuts into the efficiency and results in a false measure of
> > performance.

-- 
Dr. William A. Davis, Director of VT Antenna Lab
The Bradley Dept of Elec & Comp Engr, Virginia Tech
Blacksburg, VA 24061-0111  [(540)231-6307, 231-3362FAX]
http://antenna.ece.vt.edu
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