RE: NEC-LIST: Re: resonance and Q of an antenna and scattering s ystem

The Q of an antenna is usually of most interest for electrically small
antennas, where the bandwidth is an important practical issue. Small
antennas tend to have a reactive input impedance, and when the reactance is
(say) 5 times larger than the resistance, the Q is accurately estimated as
the reactance divided by the resistance. This is consistent with the basic
definition of Q as the peak stored energy divided by the energy dissipated
per radian. With a small antenna the peak stored energy can also be
computed by integrating the energy in the fields outside the antenna, and if
that is done (not so easy) the answer is the same.

If a CAD package is being used, it is easier to avoid computing the stored
energy and to compute the input impedance of the antenna over a set of
frequencies which include the band required, then to calculate the vswr at
the input port, over the frequency range, after tuning and matching
components have been included.

Alan

-----Original Message-----
From: Bibby [mailto:bibby_at_attbi.com]
Sent: 02 January 2003 12:34
To: wadavis_at_vt.edu; nec-list_at_gweep.ca
Subject: Re: NEC-LIST: Re: resonance and Q of an antenna and scattering
system

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 -----
From: <wadavis_at_vt.edu>
To: <nec-list_at_gweep.ca>
Sent: Thursday, January 02, 2003 12:04 AM
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.
>
> At resonance, the stored energy acts much like a tuned circuit where the
> energy simply varies between the field components in time and maintains
> the stored energy as constant. About the only way to get at the Q for a
> specific situation is to compute the energy in a CAD package or to
> estimate it by a measure of the impedance bandwidth.
>
> Bill Davis
>
> > Happy new year, everyone.
> >
> > A dicussion of the definition of Q for an antenna is helpful to us.
The
> > conpect of the ratio of energy stored to the energy dissipated per cycle
is
> > according to the definition of the circuit theory. John B. Wood pointed
out
> > that in the case of an antenna, the energy "dissipated" consists of
that
> > dissipated as heat in the antenna structure and radiated into the
medium, and
> > the stored energy is that contained in the reactive near field of the
antenna.
> > The other important problems are what is the definition of resonance of
an
> > antenna or an antenna array, and how to determine the stored energy
contained
> > in the reactive near field of the antennas. Thank you for you kind
attention.
> >
> > Sincerely
> >
> > Long Li
> --
> The NEC-List mailing list <nec-list_at_gweep.ca>
> http://www.gweep.ca/mailman/listinfo.cgi/nec-list

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