FTMW spectra of HC₃S⁺ showing the J = 5 − 4 rotational transition at 27.3 GHz. The spectrum was achieved by 10 000 shots of accumulation at a repetition rate of 10 Hz. The coaxial arrangement of the adiabatic expansion and the resonator axis produces an instrumental Doppler doubling. The resonance frequency is calculated as the average of the two Doppler components. The feature marked with an asterisk is an artifact.