Re: NEC-LIST: Efficiency of Short Dipoles

Hal Smith wrote:
>
> Peter Excell wrote:
> > >
> > > My information indicates that mu(r)= 1 at HF frequencies for most
> > > steels, irons and other non-laminated metals.(gyromagnetic limiting
> > > frequency approx 20 kHz).
> >
> > I looked into this some years ago and the magnetics textbooks (I recall
> > Bozorth in particular) seemed to me to be saying that ferromagnetic
> > behaviour endured up to a few hundred MHz at least.
>
> Peter
>
> Obviously there has to be ferromagnetic behaviour at VHF and above given
> the ferrite materials used in inductor cores, hybrids, etc. That may not
> be totally relevant since the thread is about solid stainless
> structures, etc. but at least in the case of a ceramic/fine magnetic
> powder construction there are ferromagnetic effects at high RF.
>
> Do any of the NECs model ferrites?
>
> Sincerely
> Hal Smith

Tnx. This is a useful observation that disproves the principle of what
was originally being suggested, which was caused by a mix-up between
ferromagnetic effects and eddy-current effects. The latter are
irrelevant (by being seamlessly included) when considering RF surface
impedance. NEC can only model lossy wires (and lossy ground): you have
to work out the effective resistance of the wires for the material and
frequency concerned and then put this value into the code input.

Peter Excell
Received on Tue Jan 06 1998 - 09:36:36 EST