Sinusoidal plot at constant value

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Hi all,

I am trying to plot a sinusoidal line across all points where the function "phi" equals 5450, but I am unsure how to go about this. My first thought was to calculate phi for all values of x and y using a for loop, and then use the find function to determine the index of those points where phi=5450. Is there a way to make matlab plot the points at which the function is equal to a set value?

phi0=5.5*10^4;
U=20; %m/s
y=(-1000*10^3):1000:(1000*10^3); 
x=(-2000*10^3):1000:(2000*10^3); 
Lx=4*10^6; 
Ly=4*10^6; 
k=(2*pi)./Lx; 
l=(2*pi)./Ly; 
A=8*10^6; 
f=10^-4; 
for i = 1:length(y)
    for j = 1:length(x)
        phi(i,j) = phi0 - (f*U*y(i)) + (f*A*sin(k*x(j))*cos(l*y(i)));
    end
end
id5450 = find(phi==5450);