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

Hi Doug:

Q is defined as 2 * omega * stored energy / radiated power.

Approximately determining Q from Q = 2/HPBW or Q = 1/HPBW is only a function
of how bandwidth is defined. Q should be the same in either case.

For Q = 2/HPBW: HPBW is defined as the 1/2 power matched VSWR bandwidth as
defined in my previous posting.

For Q = 1/HPBW: HPBW is the 1/2 power or conductance bandwidth. HPBW in this
case would be determined by making the antenna resonant with an L or C and
then determining the 1/2 power bandwidth with a constant voltage feeding the
antenna.

If you take a self resonant antenna as an easy example, and approximate Q
using either method, you end up with essentially the same result.

Steve Best

> Q=2/HPBW is unloaded Q. Loaded Q is 1/HPBW. HPBW=Half-power bandwidth.
> As can be seen, this is treating an antenna as a circuit element, and the Q
> is only with respect to its terminal impedance. The HPBW is also a circuit
> definition. Other kinds of BW are often defined, which are not directly
> related to Q, such as pattern bandwidth. Amateur transmitters often can't
> stand VSWR>2, so they use BW as the VSWR<2 bandwidth. This is probably
> related to the HPBW, but probably not simply. I have no idea if people in
> the scattering community use the Q concept.
>
> Doug Miron
> --
> The NEC-List mailing list <nec-list_at_gweep.ca>
> http://www.gweep.ca/mailman/listinfo.cgi/nec-list

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