| Directional coupler is one of the most significant passive components in microwave and millimeter wave system,which influences the performance of the whole system.Especially 0d B directional coupler,is widely used in measurement and signal source shielding and isolation system.The traditional directional coupler has many problems,such as hign loss,narrow bandwidth,large size and difficult packaging,which can’t meet the development needs of modern communication system miniaturization and integration.In this paper,firstly,the development status of directional coupler and periodic structure is briefly introduced,then the characteristics of periodic structure regulating the electromagnetic characteristics of circuit are theoretically analyzed,and then the theory and implementation method of forward-wave directional coupler(FWDC)are systematically analyzed by odd/even mode analysis.A stripline FWDC is realized by using periodic structure loading.In order to adapt to the development trend of millimeter-wave directional coupler,a millimeter-wave broadband directional coupler with low loss is designed by using the new transmission line technology of gap waveguide.The main research works of this paper can be listed as follows:1.Research on slow wave characteristics and electromagnetic band gap characteristics of periodic structure loading: The electromagnetic characteristics of transmission lines loaded with periodic structures are studied theoretically: the periodic boundary conditions and the Floquet mode field loaded with periodic structures are analyzed with Floquet—Bloch theory,and the lumped parameter equivalent circuit model loaded with periodic structures is given.Using ABCD matrix and S-parameter matrix,the calculation formula of Bloch impedance and phase of transmission line loaded with periodic structure is derived,which provides theoretical research basis for the following design.2.Analysis and design of stripline FWDC based on periodic mushroom structure and short-circuit branch line loading: Four coupling units are designed.By using the extraction formula of Bloch impedance and phase of odd-even mode derived in the previous chapter,a unit that can realize larger phase difference of odd-even mode is selected.The model is analyzed by lumped parameter equivalent circuit,and the coupling unit with larger phase difference of odd-even mode is designed,and the transition structure from stripline to coaxial line is realized.Finally,six coupling units are cascaded to realize a stripline FWDC.The proposed FWDC achieves a coupling degree of 2.5 d B in the frequency band of 6.1-6.7GHz.The in-band return loss and isolation level are better than 20 d B,and the size is only1.28 λg.It solves the difficult problems of narrow bandwidth and strong coupling of traditional FWDC,and has the characteristics of self-shielding and self-packaging,which can be applied to the integrated application of Apollo beam scanning antenna and RF front end.The actual test results are basically the same with the simulation results,which validates the rationality and correctness of the design.3.Analysis and design of millimeter-wave broadband low-loss directional coupler based on groove waveguide: The proposed coupler is realized by opening five coupling slots with the same size and symmetrical distribution in the common wall of two slot-gap waveguides.For the convenience of testing,the transition structure from broadband slot gap waveguide to standard WR-28 waveguide is designed.The proposed directional coupler realizes flat 0 d B coupling in the frequency band of 27.1-31.5 GHz,the maximum coupling level in the band is about 0.5 d B,the return loss and isolation are less than-10 d B,and the relative bandwidth of the coupler is 14.7%(the center frequency is 30 GHz).The actual test results are in great match to the simulation results,and the correctness of the design is validated by experiments. |
PDF Full Text Request