vector fields in Matlab

I am trying to create a vector field of a diferential equation system, I know what is the real vector field, but with my code I get another. I paste my code, the system, my vector field, which is wrong, and the good vector field, someone knows what part of my code is wrong?

my code:

x1=0;
x2=2;
y1=0;
y2=2;
N = 20;
x = linspace(x1,x2,N);
y = linspace(y1,y2,N);
[X,Y]= meshgrid(x,y);
m1 =   a.*x.*y/b.*x.*y;
S1 = atan (m1);
U = - cos (S1);
V = - sin (S1);
[U,V]=meshgrid(U,V);
quiver (app.Axes2, X,Y,U,V);

the system

my vector field, that is wrong

the good one

Answer 1

[darova]

Here is the mistake

[Alvaro Mª Zumalacarregui Delgado]

thanks because after correct that the field looks better, but if you look at the bottom of 'the good one' vectors are point and I have vectors like the top of 'the good one', so maybe the value of m1 is wrong, you know how to correct it?