The measurement for transmitter inter-modulation (IM) is developed

taking a directional coupler and an adjustable attenuator as core equipment. The IM signal models at each port of the directional coupler are established; and the regularity of how the IM signal level at coupler ports change with the attenuation of the adjustable attenuator is analyzed. According to theoretical analysis

the paper investigates methods for elimination of several error factors

like the first-order reflected IM signal from the interference signal source

and the second-order reflected IM signal from the transmitter under test and coupling signal power; and then gives the detail measurement procedures relies on the attenuation adjustment of attenuator. A practical measurement of a broadcasting station indicates that using the proposed method

a very good agreement of measurement and theoretical analysis can be achieved

which validates the rationality and accuracy of proposed method.