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

The Q of an antenna (2 omega W/Prad) can be accurately approximated from the
1/2 power matched VSWR bandwidth.

To determine the 1/2 power matched VSWR bandwidth (BW), a reactive element (L
or C) must be first added in series to the antenna impedance to make it
resonant at the frequency for which Q is to be determined. The BW is then
determined with respect to this now resonant frequency. Note that the
characteristic impedance used to determine the VSWR about this resonant
frequency must be equal to the resonant resistance. Additionally, the L or C
of the reactive element must be included in the VSWR calculations at all
frequencies. A 1/2 power VSWR is equal to 5.83:1.

Once the BW is determined, the Q of the antenna can be approximated as

Q = 2/BW

Steve Best
> I always thought of antenna Q as the BW where SWR is 2:1 or better, but
> thats a ham radio definition. Perhaps the bandwidth where the return
> loss is equivalent to 3dB of loss from the feed cable to the antenna
> would be better. I think that would be SWR of 5.8:1 Note that if you
> include the feedline with an antenna SWR of 5.8:1, you will have much
> higher loss due to increased voltage and current in the cable. See the
> Amateur Radio Handbook for details. rob
>
> Li Long wrote:
> >
> > Hello All
> >
> > It is well known that the relationship of resonance with Q is very compact
> in the circuit theory. But in the antenna and scattering theory, what is the
> definition of the resonance of antennas or scatterings? The previous
> discussionson on Q of small antenna is very interesting. But what is the
> relationship of resonance with Q of antennas? Now I am doing some work on the
> resonance of open systems. Any discussion and suggestions are welcomed.
> >
> > Happy new year!
> >
> > LI Long
> > ----------------------------------------------------------------
> > LI Long
> > National Key Lab. of Antennas and Microwave Technology
> > Xidian University
> > P.O.Box 223,
> > Xi'an, Shaanxi,710071,
> > P.R.China
> > Tel: +86-29-8204068
> > FAX: +86-29-8201164
> > Email: LL_at_mwp.xidian.edu.cn
> > ----------------------------------------------------------------
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