oops...I have changed the wording in a couple of places, and restated
problem below:

Let's say that I have a set of S parameter data in polar form. The
parameters are S11(ant) for an antenna, S11(line) and S21(line) are for
the transmission line. S12(line) and S21(line) are assumed equal.

S11(system) is for the antenna and cable connected together. How would the
S11(system) be calculated?

As a first stab at it, I would calculate:
S11(system)=S11(ant) * S21(line) * S12(line)

Is that valid, and/or does S11(line) need to be included?

Thanks