You said "Z=73.13+j*42.55+j*We*k*delta", but, actually the real part of Z (=73.13) is also a function of delta, right?
Yes, it’s an approximation considering that the change of the real part is relatively small compared with that of the imaginary part.
O.K. We assume that. And under that assumption, can you say that we get the best (=minimum) VSWR value if the antenna is in resonance?
That is a good question, which I can answer using a Smith Chart, but before doing that, you must remember that VSWR=(1+rho)/(1-rho), where rho is the absolute value of the reflection coefficient, gamma. And that it is a monotonically increasing function of rho, which means VSWR becomes minimum when rho is minimum.
