Abstract-This paper describes the investigation of broadband interaction and harmonic suppression. A special dispersion shape used in broadband traveling-wave tubes (TWT) is obtained. The theoretical and simulation studies of negative dispersion are presented. On the basis of these studies, a broadband TWT used in microwave power module (MPM) is designed. Compared with the old TWT with flat dispersion, the new one with negative dispersion decreases the second harmonic content about 10 dB and improves the fundamental efficiency with a maximum increase of about 5% at the lowest frequency 2 GHz. The new one operates with the beam voltage of 3600 V and current of 250 mA. The modified TWT is fabricated and the simulation results meet the measurements very well.