Study On Microstrip Slow Wave Structure

Millimeter wave power module is a new concept of millimeter wave amplifierdeveloped from microwave power module that has both the advantages of a miniaturemillimeter-wave traveling wave tube (TWT) and a solid state amplifier. Millimeterwave power module is designed for a wide variety of military and commercialapplications, due to its size, weight and efficiency advantages compared to traditionalsolid-state and traveling wave amplifiers.So far, the helix TW is a main choice for microwave power module because of itsinherently wide bandwidth, low operating voltage and relatively high RF efficiency.While, as the operating frequency increases to V-band or above, the dimensionalparameters of the helix traveling wave tube become rather small. So, the traditionalmachining precision is insufficient for helix. Micro-fabrication technique, as a way tosolve this problem, is more naturally applicable to two-dimensional structures.Therefore, as a three-dimensional structure, the fabrication process of the helix circuitwill be rather complicated and difficult. It is essential for us to quest a new type ofslow-wave structure which retains the advantages of helix TWT but overcomes thefabrication problems.A slow-wave transmission line, of the microstrip type, called microstrip meander-lineslow-wave structure, is now proposed for a low voltage, wide bandwidth and moderatepower millimeter-wave traveling-wave tube. Its fabrication is very convenient byutilizing the two-dimensional micro-fabrication technology, and this type of slow wavestructure can be well-matched with the solid-state circuits. Therefore, it can be used forthe vacuum power booster of MMPM. The development of MMPM will benefit fromthe study of microstrip meander-line slow-wave structure, when its operating frequencyincreases to short millimeter or above. In this thesis, some important and valuableresults which bring forth some new ideas are accomplished and listed as the followings:1. Based on the traditional structure, the U-shaped microstrip meander-line slowwave structure is proposed. The beam-wave interaction model, including the slow-wavestructure, attenuator and the energy coupled structures, has been designed for Ka-band U-shaped microstrip meander-line millimeter-wave TWT. The simulation results showthat the instantaneous3-dB bandwidth of this millimeter-wave power amplifier is about9GHz, ranging from29GHz to38GHz. Its peak output power is about200watts withthe corresponding gain of33dB and averaged electronic efficiency of14.4%at34GHz.2. In order to extend the operating bandwidth of microstrip meander-line slow-wavestructure, a novel V-shaped microstrip meander-line slow wave structure is proposed.The high frequency properties comparison between U-shaped structure and V-shapedstructure has been put forward with all the dimensional parameters set to be equal. Then,the beam-wave interaction of this V-shaped microstrip meander-line millimeter-waveTWT is investigated in detail. The simulation results show that the instantaneous3-dBbandwidth of the V-shaped microstrip meander-line TWT is about20GHz and the peakoutput power is about90watts at97GHz.3. In order to reduce the electron beam density, a symmetric double V-shapedmicrostrip meander-line SWS is proposed. With all the dimensional and electricalparameters ste to be equal, the performance comparison between the double V-shapedmicrostrip meander-line TWT and the single V-shaped microstrip meander-line TWThas been carried out. The simulation results show that the instantaneous3-dB bandwidthof double V-shaped TWT is about18GHz and the peak output power is about110watts.However, the instantaneous3-dB bandwidth of single V-shaped TWT is about15GHzand the peak output power is only25watts. Obviously, the performance of doubleV-shaped TWT is superior to that of single V-shaped TWT.4. The thermal analysis of the V-shaped microstrip meander-line slow wave structureis put forward at W-band. The thermal analysis is an effective way to improve thestability and reliability of the TWT. Moreover, it is essential for us to perfect the designof TWT, especially for the millimeter wave TWT with new type slow-wave structure.The temperature distribution and heat flux vector of the V-shaped microstripmeander-line slow wave structure are obtained by utilizing the ANSYS software.5. The transmission properties of the microstrip meander-line slow wave structuresare measured. In the thesis, the design and fabrication of the microstrip meander-lineslow wave structures including the energy coupled structures are described in detail.According to the experimental results, the transmission parameter S21of U-shapedmicrostrip meander-line is>-3dB, and its reflection parameter is <-10dB in the frequency range of32-40GHz. The transmission parameter S21of V-shaped microstripmeander-line is about-3dB, and its reflection parameter is <-14dB in the frequencyrange of32-40GHz.