A Ka-band broadband power synthesis network based on waveguide H-plane

　　1 Introduction

　　Millimeter-wave rectangular waveguide couplers play a very important role in the power dividers of millimeter-wave radar and communication systems. In the millimeter-wave band, rectangular waveguide couplers can make up for the defects of excessive loss and low isolation of microstrip power dividers such as Wilkinson, and are an indispensable and important component in millimeter-wave power dividers. The power distribution/synthesis network based on rectangular waveguides has become the focus of researchers due to its low loss and high application frequency. In the millimeter-wave band, the 3-dB waveguide E-plane branch structure is mostly used for the binary power distribution waveguide unit structure. This waveguide branch structure has the characteristics of low loss, high isolation and compact structure. However, due to the influence of mechanical processing, the number of waveguide branches is limited to a certain extent, which will lead to strict restrictions on the bandwidth of the waveguide branch structure in the millimeter-wave band.

　　In power synthesis technology, the use of a power synthesis network with wide-band characteristics can greatly increase the output power of the system. Scholars such as Louis W. Hendrick proposed a broadband bridge with narrow arms and short slots. This broadband coupling is due to the gradual disappearance of the TE30 mode in a common area of ​​coupling, which will cause the change of the electrical length of the odd and even mode circuits, thereby achieving broadband characteristics. Using this structure, a compact structure and broadband characteristics are achieved.

　　Based on the scholar's theory, this paper proposes a Ka-band wideband power synthesis network based on waveguide H-plane. This power division network can achieve a bandwidth of nearly 7.5 GHz. In the range of 29.5-37 GHz, the amplitude difference between the two ports is less than 0.4 dB, and the return loss is less than -19.5 dB.

　　2 Theoretical analysis of Ka-band broadband power synthesis network based on waveguide H-surface

　　The structure proposed in this paper is obtained by analyzing the multi-mode equivalent circuit model of H-plane step discontinuity. The example shown in Figure 1 is a directional coupler with two steps of 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 the TE10 mode. In addition, the circuit has two foldable symmetry planes: AA and BB, so the four ports can be analyzed as four identical ports. The division of the ports is based on the division of the electric field and magnetic field boundaries 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 unit can be obtained by the reflection coefficient of the four ports:

It is deduced that:

　　The subscripts e and o represent the odd and even modes of the symmetry planes, respectively, and the first subscript corresponds to the AA and BB planes.

　　Figure 1 Two-stage ladder directional coupler

　　Next, we consider the circuit of one-quarter port as shown in Figure 2. This structure is composed of a two-stage ladder structure. This ladder junction is composed of three narrow electric arms or magnetic arms of waveguides. The cascade of three multi-mode guides will produce ladder discontinuities. Therefore, the multi-mode equivalent circuit model shown in Figure 3 can be derived. The equivalent mode voltage and current of each guide at the ladder discontinuity are coupled to each other as follows:

　　Figure 2 Quarter-port cross-section

　　Figure 3 Multimode equivalent circuit model

　　Subscript

Represent the mode number of each waveguide H surface,

represents the model coupling coefficient of the connected waveguides, which is determined by the interface mode matching. If it is assumed that the evanescent higher-order modes at the input waveguide end are terminated with their characteristic impedance, and the propagating and non-propagating modes at the output waveguide end are terminated with the output impedance seen from the short-circuited or open circuit terminal, then the reflection coefficient for each excitation can be derived, and thus the scattering matrix of the entire circuit.

　　3 Design and simulation of Ka-band broadband power synthesis network based on waveguide H-plane

　　Through the theoretical analysis of the step-type directional coupler based on the waveguide H-surface, a first-stage step directional coupler based on the waveguide H-surface is designed according to the theory. The method of designing the rectangular waveguide bridge in this paper is mainly to use HFSS three-dimensional electromagnetic field simulation software for design. By establishing a model, setting boundaries, performing parameter scanning and optimization, data processing, and drawing simulation graphics, the final structure size is obtained. The core of this method is mainly parameter scanning and optimization, which takes a lot of time and effort. However, because the method is relatively simple, a large number of calculations are completed using computer software, which is particularly useful for the design of millimeter-wave waveguide structure devices. The structure simulated using HFSS is shown in Figure 4 as follows:

　　Figure 4: One-step directional coupler based on waveguide H-plane

　　Figure 5 Power division amplitude of the synthesis network 　　The simulation results are shown in Figures 5 and 6. In the wide frequency band of 29GHz-37GHz, the four-way amplitude imbalance of the power division network is less than 0.5dB, the phase also achieves good consistency, and the return loss at the input end is less than -17dB. It can be seen from the simulation results that although it has achieved the required wide-band characteristics, the amplitude imbalance and return loss are not very good in the entire frequency band.

　　Figure 6 Return loss and isolation of the synthetic network

　　Based on the method and theory of the first-step design, this paper proposes a second-step directional coupler. The difference between the first-step coupler and the first-step coupler is that the first-step becomes a second-step in the common coupler area. Through this change, the impedance gradient in the coupling area can be achieved, so that lower return loss can be achieved and the amplitude imbalance is also improved. The HFSS simulation is shown in Figure 7:

　　Figure 7 Two-stage step directional coupler based on waveguide H-plane

　　Figure 8 Power division amplitude of the synthesis network

　　The simulation results are shown in Figures 8 and 9. In the range of 29.5GHz-37GHz, the four-way amplitude imbalance of the power division network is less than 0.4dB, the phase also achieves good consistency, and the return loss at the input end is less than -19.5dB.

　　Figure 9 Synthesized network return loss and isolation

　　Through the simulation structure comparison of the above two structures, the two-stage ladder coupling has obvious improvements in power division amplitude imbalance, input end return loss and isolation performance compared with the one-stage ladder structure.

　　4 Conclusion

　　Based on the work of Louis W. Hendrick and other scholars, this paper proposes a Ka-band broadband power synthesis network based on waveguide H-plane. This synthesis network is proposed based on the multi-mode equivalent circuit model of H-plane step discontinuity. Through the design and analysis of the step directional coupler, the step directional coupler based on waveguide H-plane has broadband 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 broadband, low return loss, and high isolation, which are one of the necessary conditions for obtaining high-efficiency synthesis.