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Eight-way power divider/combiner circuits are vital to the operation of microwave signal-distribution and power-amplification systems, but constructing such circuits for wide-bandwidth use can be challenging. Fortunately, an eight-way power divider/combiner was developed using substrate-integrated-waveguide (SIW) and half-mode SIW techniques. By using a stepped coaxial line transformer with the SIW circuits, it was possible to achieve impedance matching over a broad bandwidth.

To benefit planar circuit integration, the combiner/divider was constructed by connecting two power dividers back-to-back with vertical microstrip transmission lines. This technique delivered good input return loss with low insertion loss over a wide bandwidth. To demonstrate the effectiveness of the design approach, the power divider/combiner was fabricated and compared with results from models within a commercial computer-aided-engineering (CAE) simulation software program, with very good agreement in the two sets of data across a wide frequency bandwidth.

Power dividers and combiners are particularly useful as combiners when higher power levels, such as for transmission, are needed and a single amplifier can not provide adequate output levels. Air-filled metal waveguide power dividers/combiners have traditionally provided low insertion loss with good high-power-handling capabilities, but such components tend to be relatively expensive to manufacture and are limited in application due to the transitions required for use with planar RF/microwave circuits.1 On the other hand, power dividers based on planar transmission-line technologies, such as microstrip, can suffer high insertion loss while being limited in power-handling capabilities.2,3

As alternatives, SIW and half-mode SIW (HMSIW) technologies have been proposed as practical alternative approaches for power divider/combiner circuits for their inherently low loss, low cost, compactness, and easy integration with planar components. Some SIW and HMSIW power dividers with very good performance have previously been detailed in the literature.4-6 For example, SIW-based power dividers using a resonant structure were described in ref. 7, but these components suffer from limited bandwidth. A broadband traveling-wave four-way power divider was described in ref. 8, but the input impedance matching for this component was relatively complicated to manufacture and its coaxial ports are difficult for microstrip integration with planar active components for compact power-combining applications.

As a possible solution, a broadband HMSIW eight-way power divider/combiner has been developed. Its topology is similar to that of the traveling-wave power divider described in ref. 8, but its input matching structure has been modified to simplify the manufacturing process while providing broadband impedance matching. Four broadband HMSIW Wilkinson power dividers were used to construct an eight-way power divider. Furthermore, by connecting two power dividers back-to-back by means of vertical microstrip transmission lines, this circuit can be directly integrated with active components, eliminating the loss associated with microstrip-to-coaxial transitions. For both modeled and fabricated versions of the proposed power divider/combiner approach, results reveal low insertion loss, wide bandwidths, and ease of manufacturing.

Figure 1 shows a top view of an eight-way power divider with an axially symmetric structure. It is centrally fed by a current probe through a stepped coaxial line. Four SIWs are used as arms for signal distribution. In each SIW, side walls are realized by arrays of metallic viaholes formed in relatively thin dielectric substrates. Viahole spacing of three times the viahole radius was chosen to minimize leakage losses while preventing an overload of the substrate.

HMSIW Methods Make Broadband Dividers, Fig. 1

To achieve high-performance signal division, HMSIW Wilkinson power dividers were integrated with SIW. The broadband monomode characteristics of the HMSIW transmission lines and direct connection to SIW ensure wideband performance. To minimize cross coupling between HMSIW circuits while also achieving good return loss, the gap lengths and widths between them should be optimized. Each HMSIW is matched to a microstrip port by means of a tapered transition. The angle θ between two HMSIW branches is 30 deg. If θ is too small, there will be some unpredicted coupling between adjacent output ports. On the other hand, when θ is large, return loss will be degraded.

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