Get intersection value from two lines

Hi all,

The following code generates the graph below it:

clear all;

   prompt = {'Height Antennas:','Height Tags:','Step:','Distance:','Frequency (MHz):'};                                                                                    
   dlg_title = 'Input values'; 
   num_lines = 1; 
   def = {'0','1','0.1','2','867'};  
   answer = inputdlg(prompt,dlg_title,num_lines,def);
   h1=str2double(answer(1));
   h2=str2double(answer(2)); 
   st=str2double(answer(3));
   d=str2double(answer(4));
   f=str2double(answer(5));
  %n=h2/st;
  v=0.72/st;
  n= ceil(v);

y(1)= h2-0.36;

R1(1) = sqrt(d^2+(y(1)-h1)^2);

R2(1) = sqrt(d^2+(y(1)+h1)^2); 

x(1)=R2(1)-R1(1);

for k=2:n,

      y(k)=y(k-1)+st;

      R1(k) = sqrt(d^2+(y(k)-h1)^2); 

      R2(k) = sqrt(d^2+(y(k)+h1)^2); 

      x(k)=R2(k)-R1(k);

end

c=3e8;

f=f*1e6;

lambda=c/f;

plot(x,y,'-rd')

for m=1:5,

     lamb(m)=m*lambda; 

     str = sprintf ('l%d',m);

     str = line([lamb(m) lamb(m)], [0 y(k)]);

     lam = sprintf ('%d\\Lambda',m);

     text(lamb(m),0, lam );

end

%lambda/2

lamb2=lambda/2;

l12 = line([lamb2 lamb2], [0 y(k)]);

lam2 = sprintf ('\\Lambda/2');

text(lamb2,0, lam2 );

% 3lambda/2

lamb32=3*(lambda/2);

l32= line([lamb32 lamb32], [0 y(k)]);

lam32 = sprintf ('3\\Lambda/2');

text(lamb32,0, lam32 );

% 5lambda/2

lamb52=5*(lambda/2);

l52 = line([lamb52 lamb52], [0 y(k)]);

lam52 = sprintf ('5\\Lambda/2');

text(lamb52,0, lam52 );

% 7lambda/2

lamb72=7*(lambda/2);

l72 = line([lamb72 lamb72], [0 y(k)]);

lam72 = sprintf ('7\\Lambda/2');

text(lamb72,0, lam72 );

% 9lambda/2

lamb92=9*(lambda/2);

l92 = line([lamb92 lamb92], [0 y(k)]);
lam92 = sprintf ('9\\Lambda/2');

text(lamb92,0, lam92 );

% 3lambda/4

lamb34=3*(lambda/4);

l34 = line([lamb34 lamb34], [0 y(k)]);

lam34 = sprintf ('3\\Lambda/4');

text(lamb34,0, lam34 );

% 5lambda/4

lamb54=5*(lambda/4);

l54 = line([lamb54 lamb54], [0 y(k)]);

lam54 = sprintf ('5\\Lambda/4');

text(lamb54,0, lam54 );

% 7lambda/4

lamb74=7*(lambda/4);

l74 = line([lamb74 lamb74], [0 y(k)]);

lam74 = sprintf ('7\\Lambda/4');

text(lamb74,0, lam74 );

% 9lambda/4

lamb94=9*(lambda/4);

l94 = line([lamb94 lamb94], [0 y(k)]);

lam94 = sprintf ('9\\Lambda/4');

text(lamb94,0, lam94 );

% 11lambda/4

lamb114=11*(lambda/4);

l114 = line([lamb114 lamb114], [0 y(k)]);

lam114 = sprintf ('11\\Lambda/4');

text(lamb114,0, lam114 );

% 13lambda/4

lamb134=13*(lambda/4);

l134 = line([lamb134 lamb134], [0 y(k)]);

lam134 = sprintf ('13\\Lambda/4');

text(lamb134,0, lam134 );

ylabel('Height (m)', 'FontSize',10)

xlabel('Lambda (m)', 'FontSize',10)

grid on

IMAGE: https://www.dropbox.com/s/xshir43qz2aepvt/2%2C5_0.8_0%2C05.jpg

But I would like to get the value from the y axis when the blue vertical line is intersected with the read line.

The read line is created from two double arrays x and y. However, the blue lines are created using the command line.

Is it clear?

Thanks a lot.

Raúl.

Answer 1

[Walter Roberson]

check_at = [2:14, 16, 18, 20] / 4;
intersect_points = interp1( x, y, (check_at * lambda)), 'linear' );

You should get NaN at any point outside of the red line.

You can also use check_at values to construct the labels.

[N, D] = rat(check_at);
for K = 1 : length(N)
  text(check_at(K)*lambda, 0, sprintf('%dLambda/%d', N(K), D(K)) );
end

You could of course tighten this up for the case where D(K) is 1 (the integer multiples)