The Researches On Microwave And Millimeter-wave Spatial Power-Combining Technology Based On Waveguide

With the rapid advancements of military and commercial communications systems in recent years, the demand for high-power solid-state power amplifiers with high efficiency and wide bandwidth has greatly increased. As the output power from an individual solid-state device is rather modest at microwave and millimeter-wave frequencies, methods for combining output powers from a number of solid-state devices have been extensively studied in order to obtain higher and higher output power. This has motivated considerable research activities to develop broadband and efficient power-dividing/combining circuits at these frequencies. A wide variety of power dividing and combining techniques have been demonstrated. Conventional hybrid-type power-combining circuits, such as the Wilkson power divider, Lange coupler, and branch-line coupler, suffer from low power-combining efficiency and narrow bandwidth at microwave and millimeter frequencies. To meet these requirements, various techniques, such as quasi-optical and waveguide-based spatial power-combining approaches, have been proposed at these frequencies. These techniques are preferred over traditional corporate combiners because of their low insertion loss, and their high power-combining efficiency. Based on conventional spatial power-combining technique, several novel waveguide-based spatial power-dividing/combining circuits have been presented in this dissertation, and detailed theoretical study, circuit design, electromagnetic simulation and experiment study have been made. The major achievements are listed as the followings:1. For the first time, a novel oversized coaxial waveguide power dividing/combining circuit using probe array that achieved low-loss probe-to-waveguide transitions is presented. For one hand, the coaxial combiner is N-way combiner that sums the power of the N devices directly in one step without having to proceed through several combining stages. So this power combiner has low insertion loss and high power-combining efficiency. For another hand, efficient heat sinking of the power amplifiers was achieved by mounting the devices out of coaxial waveguide, and the high output power can be obtained. The coaxial environment supports a TEM mode that has no lower frequency cutoff. This coaxial power divider/combiner has broad frequency characteristics, which can break through the limit of waveguide dimensions at millimeter-wave and sub-millimeter-wave frequencies, and will be used widely at high-power microwave and millimeter-wave system.2. Firstly, Analysis based on equivalent circuits gives the design formula for this novel coaxial power dividing/combining circuits successfully. According to the electromagnetic theory and operating mode of this coaxial structure, the simple electromagnetic modeling of this power dividing/combining structure has been developed. The transitions from standard coaxial waveguide to oversized coaxial waveguide have been studyed, and two transitions have been obtained, namely coaxial taper and stepped impedance transformer, which have been analyzed using small reflection theory and equivalent-circuit method respectively. The simulated and measured results of this power divider/combiner demonstrate the advantages of simple structure, broadband, low insertion loss, sufficient heat sinking for the active devices, uniform phase responses and power of output port.3. A multiple-port radial waveguide power divider/combiner has been given a further study, and a simple and effective method has been presented firstly. Due to the symmetric field distribution, the simple equivalent-circuit model for the radial waveguide power dividing/combining circuits has been developed using equivalent-circuit method. In this model, the entire cavity structure has been divided into two volumes: a central cylindrical volume comprising the central probe and a cavity region (comprising the peripheral probes) outside the central cylindrical volume. These two regions have been matched on a reference plane, and then the input/output can be well matched.4. Several novel multiple-port substrate integrated waveguide (SIW) power dividing/combining circuits have been proposed firstly. The difference between this SIW power divider/combiner and others is that the probe-to-waveguide transition is achieved with the help of a current probe (metallic via), which achieve power dividing/combining. It can be fabricated by standard printed circuit boards (PCBs) process, and takes the advantages of simple structure, small size, low weight and ease of integration. It is possible for the power-dividing circuit to divide an input power equally and to receive a uniform phase response at output port because of the symmetry of the electromagnetic field.5. This compact multiple-port SIW power dividing/combining circuits have been analyzed firstly by employing the equivalent-circuit method, and the simple model has also been presented. The design formulas have been developed for this SIW power dividing/combining circuits. Good agreement between simulated and measured results is found for the proposed passive power divider/combiner.