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