function an = hb9cv(f,h,l1,l2)
% HB9CV antenna for frequency f [MHz] at height h [m].
% l1 and l2 are the half lengths of the elements introduced 
% here for optimization. Normally the function is called by 
% hb9cv(f) or hb9cv(f,h) using the defaults.

% Defaults
% ========
if nargin<1   f = 14.1;   end
if nargin<2   h = 12.0;   end
if f<50
   if nargin<3  l1 = 71.15/f; end
   if nargin<4  l2 = 77.52/f; end
else
   if nargin<3  l1 = 71.07/f; end
   if nargin<4  l2 = 77.33/f; end
end

% Antenna model
% =============
dx = 37.5/f;
d  = 0.58/f;
d1 = exwire([0 -l1 h],[0 +l1 h],d,15);
d2 = wire([-dx -l2 h],[-dx +l2 h],d,15);
tr = transmission([0 0 h],[-dx 0 h],dx,-72.5);
an = comment(construct(d1,d2,tr),{'HB9CV';[num2str(f) ' MHz']});
