We report for the first time on RF SiC BJTs fabricated on semi-insulating (SI) substrates with L-band performance. Small-periphery (4x150μm) devices were tested using on-wafer load-pull measurements up to 1.5GHz. Under pulsed conditions, the devices exhibited 10dB of power gain at 1GHz and a peak power density of 2.3W/mm (1.4W) with a 100μs pulse width and a 1% duty cycle. The power gain decreased to 8dB at 1GHz under CW conditions at a power density of 1.6W/mm (1W). The load-pull measurements were performed up to 125oC, which resulted in a 1 dB reduction of power gain compared to the room temperature performance. Results at 0.5 and 1.5 GHz are presented as well.