In MR coils or MR table there are multiple cables. Their presence is not desirable because they distort the B1 field; therefore multiple RF traps are distributed along the cable length to minimize induced cable currents. Tuning and performance of the MR cable traps’, also called cable baluns, on the cable are always difficult to predict, because the cable shape and position may vary. A coaxial half-wave cavity resonator is shown to be a precise tool to test cable balun assembly. It facilitates precise measurement of a balun’s coupling to the cable, its loss, and coupling to neighboring baluns.