"Vector must be strictly increasing or strictly decreasing with no repeated values" error

Hello,

I'm trying to use contour to represent some data, but I get this error.

Thank you in advance.

R_a=3;
N=100;


phi=linspace(0,2*pi,N);
rho= linspace(0,R_a,N);

r = randi([50 70],1,length(rho));
A=ones(length(rho)).*r;

[x_c,y_c] = pol2cart(phi,rho);
[Ro_tag, Phi_tag]=meshgrid(rho,phi);

contour(x_c,y_c,A);

[Adam Danz]

Polar axes do not support contour plots.

Here are some alternatives:

https://www.mathworks.com/matlabcentral/answers/442432-how-to-plot-the-contour-polar-map-in-matlab#answer_358902

[Michael Elman]

In my original problem i need to define a circular grid (aperture of the antenna) and then to calculate a radiation pattern, for this purpose I must have radius and angle vectors to be defined from 0 to 3 and from 0 to 2*pi respectively. Matrix A is calculated from cartesian matrices (cartesian coordiantes of the apperture) is there another way to define these parameters? Thank you.

[Adam Danz]

Shot in the dark,

R_a=3;
N=100;

phi=linspace(0,2*pi,N);
rho= linspace(0,R_a,N);

r = randi([50 70],1,length(rho));
A=ones(length(rho)).*r;

[Ro_tag, Phi_tag]=meshgrid(rho,phi);
[x_c,y_c] = pol2cart(Ro_tag,Phi_tag);

contour(x_c,y_c,A);
axis equal

[Michael Elman]

Looks a little bit weird to me, I will try the link you posted before.

Answer 1

[Matt J]

Your meshgrid must be Cartesian:

R_a=3;
N=100;

[X,Y]=meshgrid( linspace(-R_a,+R_a, N) );

[Phi,Rho]=cart2pol(X,Y);             
             
A= ... computaton based on Phi and Rho ...

contour(X,Y,A);

[Michael Elman]

My problem is a little bit more complicated than it looks. I need to compute a far field pattern of the circular antenna, that's why I defined Ro_tag and Phi_tag in this way (values of the radius vary from 0 to 3 and angles vary from 0 to 2*pi). In addition the matrix A is computed from other two matrices X_c and Y_c, where,

[X_c Y_c]=meshgrid(x_c,y_c)

[Matt J]

Very well, then I modify my answer to,

R_a=3;
N=100;

[X,Y]=meshgrid( linspace(-R_a,+R_a, N) );

A= ... computaton based on X and Y ...

contour(X,Y,A);

The point is, A needs to be sampled on a normal Cartesian meshgrid for the purposes of doing the contour plot. You cannot, as a starting point, set up your sample locations in polar coordinates because they will not remain monotonic when transformed by pol2cart().

[Michael Elman]

contour(X,Y,A);

I think, this will do the trick, thank you.