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

On Thursday, January 02, 2003 6:43 AM, Alan Boswell wrote:

>
> 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.
>

    It is not at all unusual to achieve tuning Q's approaching 100 for loop
aerials having a circumference of an eighth of a wavelength, even more for
smaller diameters. And when coupled with a regenerative preamplifier
(receiving aerials), the Q's can go significantly higher. It's very
dependent upon the number of turns and the wire diameter.

Chris

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