The ultrawideband (UWB) photonic THz source presented employs an InP-based modified uni-traveling-carrier photodiode (MUTC-PD) operating from 100 to 600 GHz. The MUTC-PD is hybrid integrated with a UWB high-resistivity Si dielectric rod waveguide (DRW) with a Vivaldi interface to couple optically generated THz waves to a device under test. This demonstration waveguide-based scalar transmission measurement system avoids multiple frequency extenders required for conventional electronics-based VNA measurements over the UWB frequency range to enable simpler cost-effective THz device testing. This work represents a significant step toward the development of a future photonic VNA.