How do I use "real()" on symbolic expressions to give me an acceptable result?

I am trying to get matlab to give me the real (or imaginary) part of a complicated rational expression with several complex variables. However, it won't return an answer when I use the "real()" command. How can I get the actual expression? I tried making sure the variables are assumed to be real and positive, but it doesn't seem to help. I have pasted a sample of my code below.

syms Rp Lp Cp Rm Rox Cox x Z0 real;
syms Rp Lp Cp Rm Rox Cox x Z0 positive;
Z_A = 1i*x*Lp + Rp;
Z_B = 1/(1i*x*Cp);
Z_C = Rm + Rox/(1i*x*Cox*Rox + 1);

Z11 = Z_A + Z_B;
Z21 = Z_B;
Z12 = Z_B;
Z22 = Z_B + Z_C;

S11_denominator = (Z11+Z0)*(Z22+Z0)-Z12*Z21;
S11_numerator = ((Z11-Z0)*(Z22+Z0) - Z12*Z21);
S11 = S11_numerator/S11_denominator;

And the output is:

ans =

real(((Rp - Z0 + Lp*x*i - i/(Cp*x))*(Rm + Z0 - i/(Cp*x) + Rox/(Cox*Rox*x*i + 1)) + 1/(Cp^2*x^2))/((Rp + Z0 + Lp*x*i - i/(Cp*x))*(Rm + Z0 - i/(Cp*x) + Rox/(Cox*Rox*x*i + 1)) + 1/(Cp^2*x^2)))

Answer 1

[Star Strider]

Add after your S11 assignment:

S11 = simplify(collect(S11))
S11R = real(S11)
S11I = imag(S11)

It seems to work for me, and contains no functions of ‘i’ in either.