NEC-LIST: Resistance of non-circular conductors

The question of the resistance of non-circular conductors came up on
the rec.radio.amateur.antenna newsgroup not long ago. I questioned
whether the "effective diameter" used for antenna calculations (one
that leads to the same antenna impedance if a circular conductor is
substituted) is applicable also to conductor loss calculations. In
response, Kevin Schmidt did an excellent analysis. Among the
interesting conclusions is that the RF resistance of a thin strip is
equal to that of a round conductor with a diameter of about 1/4 of the
strip's width, while the well-known equivalent diameter for impedance
purposes is 1/2 the strip's width.

What this means is that wire conductivity has to be modified when
modeling conductors with other than a round cross-section,
particularly flat strips. The procedure would be to use the
conventional, or "self-impedance", equivalent radius for the wire
specification in order to get the correct impedance. But the conductor
DC conductivity has to be reduced by the square of the ratio of the
"self-impedance" equivalent radius to "resistance" equivalent radius
(in order to reduce the RF conductivity by the ratio) to correctly
model loss. In the case of a flat strip, the DC conductivity would
have to be reduced by a factor of about four.

Kevin's analysis can be found at http://fermi.la.asu.edu/w9cf/, under
"Approximate R.F. Resistance of Rectangular Cross Section Conductors"
and "Analytic expressions for the equivalent diameters of rectangular
cross section conductors".

Roy Lewallen
Received on Tue Jan 05 1999 - 10:05:11 EST