Bill
The ATU-10 doesn't have a bypass mode in the traditional sense. After a reset, it will wait unti it sees TX power that causes the directional coupler to report a VSWR greater than the programmed value - I believe the default is 1.3? Because of this, you won't be able to create a useful VSWR curve across the band.
You're better off obtaining a bridge or a NanoVNA.
73, Al, K2ZN
Al Scanandoah