The ultra-wideband spectroradiometer instrument aims at measuring the brightness temperature gradient in the upper lunar regolith using a wideband passive microwave spectrometer covering a continuous band from 300 MHz to 6 GHz. As a part of the Development and Advancement of Lunar Instrumentation (DALI) program, the designed ultra-wideband spectrometer is expected to provide lunar heat flux measurements. Difficulty in RF matching across the ultra-wideband and lack of isolators covering the large bandwidth make it challenging to design and calibrate the instrument. The heat-flow spectroradiometer instruments employ internal calibration sources for tracking and detecting mismatch changes in addition to gain variations measurements for stable and reliable radiometric operation.