Portion of the CH₃OD microwave spectrum dominated by the m = 0 K = 4± ← 3∓ Q-branch transitions in the 548.0–549.8 GHz range. The observed spectrum is shown in the lower panel, and the calculated one is in the upper panel. The spectrum was recorded in the second derivative detection mode of the Köln spectrometer. The J quantum numbers of the Q-branch transitions given at the top decrease with frequency for the K = 4+ ← 3− transitions and increase with frequency for the K = 4− ← 3+ transitions. The arrows in progressions of J quantum numbers shown in the top panel indicate the successive J values (from four to seven for K = 4+ ← 3− and from four to 11 for K = 4− ← 3+) that we were not able to show separately within the selected scale of the figure. Also, we have marked the m = 1, 12_1,11 ← 11_1,10 R-type transition, which has a comparable intensity with dominating Q-branch transitions in this range. We observed that the experimental frequencies and the intensity pattern are rather well reproduced by our model for the spectral features dominating this frequency range.