What are the s-parameters of a rg58 coaxial cable?
The insertion loss and phase delay (\$S_{21}\$ and \$S_{12}\$) can be roughly calculated from the specifications of the cable. 

You can use 

$$S_{21} = S_{12} \approx Ae^{-j\theta}$$

where \$A\$ is calculated from the insertion loss (i.e. 10 dB insertion loss would give \$A = \sqrt{0.1}\$), and \$\theta\$ is the electrical length of the cable.

The return loss and phase of the reflection (\$S_{11}\$ and \$S_{22}\$) will depend more on the quality of the connectors you use than on the cable itself.