How to get complex reflection coefficients?
Terminate with an inductor or capacitor. 

$$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$

\$Z_0\$ is char. impedance i.e. 50 Ohm. \$Z_L\$ is complex if you use inductor or capacitor; thus \$\Gamma\$ is complex.