The default phasor angle symbol in LaTeX is obtained by

\angle F(z)


which produces

However, to get the bottom leg of the angle symbol to underline the entire quantity you want to take the angle of, you must enable the steinmetz package in the preamble:

 \usepackage{steinmetz}

You can then use the  \phase command to get correct usage of the phasor angle symbol:


\text{Phasor angle } \phase{F(z)} \\