"1 Introduction
Millimeter wave rectangular waveguide bridge plays an important role in the power divider of millimeter wave radar and communication system. In millimeter wave band, rectangular waveguide coupler can make up for the defects of too large loss and too small isolation of microstrip power divider such as Wilkinson. It is an indispensable and important component in millimeter wave power divider. The power distribution / synthesis network based on rectangular waveguide has become the focus of researchers because of its low loss and high application frequency. In the millimeter wave band, for the binary power distribution waveguide unit structure, the 3-dB waveguide E-plane branch structure is mostly used. The waveguide branch structure has the characteristics of low loss, high isolation and compact structure. However, due to the influence of machining, the number of waveguide branches is limited, which will strictly limit the bandwidth of the waveguide branch structure in the millimeter wave band.
In the power synthesis technology, the power synthesis network with broadband characteristics can greatly improve the output power of the system. Louis w. Hendrick and other scholars proposed a broadband bridge with narrow arms and short slots. This broadband coupling is due to the gradual disappearance of te30 mode in a common area of the coupling, which will lead to the change of the electrical length of odd and even mode circuits, so as to realize the broadband characteristics. Using this structure, compact structure and broadband characteristics are realized.
Based on the theory of the scholar, this paper proposes a Ka band broadband power synthesis network based on the waveguide H-plane. The power division network can achieve a bandwidth of nearly 7.5ghz. In the range of 29.5-37ghz, the amplitude difference between the two ports is less than 0.4db and the return loss is less than - 19.5db.
2 theoretical analysis of Ka band broadband power synthesis network based on waveguide H-plane
The structure proposed in this paper is analyzed by H-plane step discontinuity multimode equivalent circuit model. The example shown in Fig. 1 is a directional coupler with two steps on the H plane. Since the circuit structure changes only on the H plane, the direction of the magnetic field is parallel to the H plane due to the excitation of TE10 mode. In addition, the circuit has two foldable symmetry planes: AA and BB, so the four ports can be analyzed as four same ports. The division of the port is based on the division of the electric field and magnetic field boundary along the two symmetry planes according to the odd and even mode excitation of the plane (as shown in Figure 2). In this case, the scattering matrix element can be derived from the reflection coefficient of four ports:
Subscripts E and O represent odd and even modules of the symmetry plane respectively, and the first subscript corresponding to this corresponds to AA plane and BB plane.
Fig. 1 two step directional coupler
Next, consider the circuit of one quarter of one port as shown in Fig. 2. This structure is composed of a two-stage ladder structure. This ladder junction is composed of three waveguide narrow arms of electric field or magnetic field. Three multi-mode guidance cascades will produce step discontinuities. Therefore, the multimode equivalent circuit model as shown in Fig. 3 can be derived. The equivalent mode voltage and current guided in the step discontinuity are coupled with each other as follows:
Figure 2 quarter port section
Figure 3 multimode equivalent circuit model
The subscript represents the modulus of each waveguide H-plane and the model coupling coefficient of the connected waveguide, which is determined by the interface mode matching. If it is assumed that the high-order modes disappearing at the input waveguide end are terminated with their characteristic impedance, and the propagation and non propagation modes at the output waveguide end are terminated with the output impedance seen from the short-circuit or open circuit terminal, the reflection coefficient under each excitation can be derived, so as to obtain the scattering matrix of the whole circuit.
3 design and Simulation of Ka band broadband power synthesis network based on waveguide H-plane
Through the theoretical analysis of stepped directional coupling based on waveguide H-plane, a one-step directional coupler based on waveguide H-plane is designed according to its theory. In this paper, the method of designing the rectangular waveguide bridge is mainly designed by HFSS three-dimensional electromagnetic field simulation software. The final structure size is obtained by establishing the model, setting the boundary, scanning and optimizing the parameters, data processing and drawing the simulation graphics. The core of this method is parameter scanning and optimization, which needs a lot of time and energy. However, due to the simplicity of the method, a large number of calculations are completed by computer software, especially for the design of millimeter wave waveguide devices. The simulation structure using HFSS is shown in Figure 4 as follows:
Fig. 4 first step directional coupler based on waveguide H plane
Fig. 5 power division amplitude of synthetic network
The simulation results are shown in figures 5 and 6. In the 29ghz-37ghz broadband range, the four channel amplitude imbalance of the power division network is less than 0.5dB, the phase is also well consistent, and the return loss at the input is less than - 17dB. The simulation results show that although it meets the requirements of the required broadband characteristics, the effect of amplitude imbalance and return loss is not very good in the whole frequency band.
Figure 6 return loss and isolation of synthetic network
Based on the method and theory of one-stage step design, this paper proposes a two-stage step directional coupler. The difference from the one-stage step coupler is that in the area of the common coupler, the one-stage step becomes a two-stage step. Through this change, the impedance in the coupling area changes gradually, so as to achieve low return loss and improve the amplitude imbalance. HFSS simulation is shown in Figure 7:
Fig. 7 two step directional coupler based on waveguide H plane
Fig. 8 power division amplitude of synthetic network
The simulation results are shown in Figures 8 and 9. In the range of 29.5ghz-37ghz, the four channel amplitude imbalance of the power division network is less than 0.4db, the phase is also well consistent, and the return loss at the input is less than - 19.5db.
Figure 9 return loss and isolation of synthetic network
Through the comparison of the simulation structures of the above two structures, the two-stage ladder coupling has significantly improved the performance of power division amplitude imbalance, input return loss and isolation compared with the one-stage ladder structure.
4 Conclusion
Based on Louis w. Hendrick and other scholars, a Ka band broadband power synthesis network based on waveguide H-plane is proposed in this paper. The synthesis network is based on the H-plane step discontinuity multimode equivalent circuit model. Through the design and analysis of the step directional coupler, the step directional coupler based on the waveguide H-plane has wide-band characteristics, and its performance is significantly improved with the increase of the number of stages. From the simulation results, the structure has the characteristics of wide frequency band, low return loss and high isolation, which are one of the necessary conditions for efficient synthesis., Read the full text, technical section
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