Section of 10 MHz of the FTMW spectrum of HDCCN showing 36 hyperfine components of the N_Ka,Kc= 1_0, 1–0_0, 0 pure rotational transition. The spectrum was achieved by 100 shots of accumulation and a step scan of 0.5 MHz with a repetition rate of 10 Hz. The coaxial arrangement of the adiabatic expansion and resonator axis produces an instrumental Doppler doubling. The resonances frequencies are calculated as the average of the two Doppler components.