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function smith(r = [0, 0.2, 0.5, 1, 2, 5], x = [0.2, 0.5, 1, 2, 5])
## SMITH Draws a Smith chart and sets hold to on so that you can plot the
## reflection coefficient on top of it.
## This is a quick little hack that draws a simple Smith chart, a diagram
## that is very frequently used in high frequency engineering. It maps
## the normalized impedance z to the reflection coefficient gamma:
## gamma = (z-1)/(z+1) with z = r+i*x .
## You can customize the function by specifying the r and x parameters
## 3-13-95 Frank Wiedmann for the matlab verion wiedmann@com.enst.fr
## 8-31-99 Eric Bouchar for the update to euler Eric.Bouchare@wanadoo.fr
_setplot([-1,1,-1,1]);
hold on;
h = textheight();
w = textwidth();
tr = 2*pi*(0:.01:1);
plot([-1,1],[0,0]);
.. r = [0 .2 .5 1 2 5]; .. specify the r=const lines you want to draw
for i=1 to length(r);
rr = 1/(r{i}+1);
cr = 1-rr;
plot(cr+rr*cos(tr),rr*sin(tr));
p = toscreen([cr-rr,0]);
if r{i}==0;
rtext(printf("%g",r{i}),[p{1}-(w/2),p{2}-(h/3)]);
else
rtext(printf("%g",r{i}),[p{1}-(w/2),p{2}]);
endif;
end;
.. x = [.2 .5 1 2 5]; .. specify the x=const lines you want to draw
for i=1 to length(x);
rx = 1/x{i};
cx = rx;
tx = 2*atan(x{i})*(0:.01:1);
plot(1-rx*sin(tx),cx-rx*cos(tx));
u = 1-rx*sin(2*atan(x{i}));
v = cx-rx*cos(2*atan(x{i}));
p=toscreen([u,v]);
if u<0;
rtext(printf("%g",x{i}),[p{1}-(w/2),p{2}-(h/3+h*v)]);
elseif u>0
text(printf("%g",x{i}),[p{1}+(w/2),p{2}-(h/3+h*v)]);
else
ctext(printf("%g",x{i}),[p{1},p{2}-(h+h/3)]);
endif;
plot(1-rx*sin(tx),-cx+rx*cos(tx));
u = 1-rx*sin(2*atan(x{i}));
v = -cx+rx*cos(2*atan(x{i}));
p=toscreen([u,v]);
if u<0;
rtext(printf("%g",x{i}),[p{1}-(w/2),p{2}]);
elseif u>0
text(printf("%g",x{i}),[p{1}+(w/2),p{2}]);
else
ctext(printf("%g",x{i}),[p{1},p{2}+(h/3)]);
endif;
end;
hold off;
return 0;
endfunction
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