Research On RF Characteristics Of Ka-band Millimeter-Wave Traveling-Wave Tubes

In this dissertation, the RF characteristics of Ka-band millimeter-waveTraveling-Wave Tubes (TWT) are studied systematically on the topic of dispersionand impedance of Slow-Wave System (SWS), Backward Oscillation (BWO) andcomputer simulation investigation method. Also, the wide-band output coupler ofKa-band millimeter-wave TWT and novel contrawound helix SWS have beeninvestigated and designed in this dissertation. The research provides supports to theinvestigation and project implement of millimeter-wave TWT. The important andcreative results are listed as follows.1. Investigation and design of wide-band output coupler of Ka bandmillimeter-wave TWT. The difficult of the design of output coupler which causes bythe small dimension is analyzed, according to the need of project implement. Then,first design was finished in theoretical, presenting the design of coaxial cable todouble ridge waveguide to rectangle waveguide, and HFSS was used to optimize andinvestigate the dimension of the structure for minimizing VSWR. The results showthat VSWR＜1.75 in the band 26.5-40GHz, and the requirement on technologicprecision is brought down. The goal of wide-band transform is accomplished.2. Simulation investigation of RF characteristics of the SWS. The researchmethods on RF characteristics, including theory calculation, experiment and computersimulation, are summarized and compared. Then the resonance method, quasi-periodboundary method, perturbation method and direct field extraction method of computersimulation methods for RF characteristics are compared and summarized, pointing outthat direct field extraction method is convenient to get the whole RF field of SWS, which plays a key role in the investigation of BWO and space harmonics,3. Investigation and design of novel contrawound helix SWS. A novelcontrawound helix slow-wave system with different helix radius is presented toreduce the difficulty of manufacture. CST MWS is used to simulate and investigatethe influence of dimension on the dispersion and interaction impedance. The resultsshow that the radius of contrawound helix can be larger than that of helical withoutcausing a sharp decrease of interaction impedance and the decrease of tape width helpbring down the phase velocity and improve the interaction impedance. The resultsalso indicate that, in certain range, the reduction of pitch increase the phase velocity.Finally, a design of the SWS is done for 8 mm millimeter-wave TWT, the interactionimpedance is improved, and the risk of BWO is low. Also, the requirement on electronoptics system is brought down because of large electron path radius, and theintroduction of longitudinal vane loads is a great help for the band-wide of the TWT.4. Simulation investigation of backward wave oscillation of millimeter-waveTWT. HFSS was used to simulate helical, contrawound helix, novel contrawoundhelix and ferruleless coupled-cavity slow-wave system, which work in Ka-band, toinvestigate the backward oscillation of millimeter-wave traveling-wave tube. Theresults show that, in helical slow-wave system, space harmonics, which axial spaceharmonic number and azimuth space harmonic number are unequal, are evident, andthe SWS runs the risk of backward oscillation around theπmode. Relatively,contrawound helix and novel contrawound helix show excellent ability in restrainbackward oscillation by improving interaction impedance and the normalizedfrequency which can work without the risk of BWO. And ferruleless coupled-cavitycircuit will run the risk of BWO when phase shifts per cavity areπor 2π.5. The application of quasi-period boundary method on the study of RFcharacteristics of coupled-cavity structure. The dissertation points out that the fakesolutions appear when the quasi-period boundary method is applied on thecoupled-cavity structure. Phase method and energy method were presented to identifythe fake solutions. Then, the cause of fake solutions is made out. Theory analysis andphase method were presented to eliminate the fake solutions. Finally, the application of the method on Huges coupled-cavity structure was carried out as an example. Thestudy is a great help for the application of the method on RF characteristics ofcoupled-cavity structure.