The instrument most commonly used to measure VSWR is a sloted length of waveguide section in which the electric field can be sampled by a movable probe. Fig 2.2 (a) shows the construction.
A small probe, connected to a diode detector, extends through the slot in the waveguide wall to sample the voltage on the line.
The probe and detector are mounted on a carriage which can be moved along to sample the voltage at different points along the line.