The rotational K-structure (K = 0, ... 6) of the a-type transition (J = 65 - 64) of the symmetric top molecule methylcyanide, CH3CN, has been recorded at 1.2 THz employing a newly designed quasi-optical frequency multiplier. A planar Schottky diode is used as non-linear element for producing the harmonic power and is mounted in the focus of an off-axis antenna across a short single ridged waveguide. The 2nd harmonic of the fundamental frequency is filtered with adichroic plate and is focused onto the InSb bolometer after passing an absorption cell. The multiplier is driven by a high power backward wave oscillator with a fundamental frequency near 600 GHz.