To estimate the seriousness of spurious te0n mode generation in a long‐distance millimeter waveguide communication system requires a measurement of small echoes in the impulse response with delays up to about 500 ns. We report the results of such a measurement made as part of a recent 14‐km field test of the wt4 communication system with a novel test set operating near 41 GHz. The results show that the wt4 system can perform satisfactorily without any filters of spurious te0n modes.