NEC modeling

1. Wire - defined by two of its end points and radius(P1,P2,r)
Wire modeling
1.1 Divide wire into segments using piece-wise linear fit on curves.
*Typically the segment length should be less than 0.1*lamda of the desired frequency.
For critical section, 0.05*lamda or less may be needed. However, extremely short
segment(<10^-3 lamda) should be avoided as it may lead to numerical inaccuracy.
1.2 wire radius-- only currents in the axial direction of the segment are considered.
two appx in NEC are available:
* thin-wire kernel--- The current on the surface of the wire is
reduced to a filament of current on the segment axis.
* extended thin-wire kernel ---assume the current uniformly distributed around
the segment.
* Delta/a>8 for thin-wire kernel, delta/a>2 for extended thin-wire kernel.