function s = exwire(a,b,d,m,c)
% s = exwire(a,b,d,m,c)
% Ecited Wire 
% a, b :  Vectors [ax ay az] and [bx by bz] of wire end coordinates
% d    :  wire diameter in m  (default = 0.002 m)
% m    :  number of segments
% c    :  the wire will be excited at its midpoint with this complex 
%         amplitude c
% s    :  data structure of antenna model 
