Substrate-Integrated-Waveguide E-Plane 3-dB Power-Divider/Combiner Based on Resistive Layers

Several different microwave circuits, including beamforming networks and balanced amplifiers, make use of 3-dB power dividers and combiners. A well-known architecture able to work over large bandwidths, a critical request for many applications, is based on E-plane three-port waveguide structures, where a lossy element is added to overcome the inherently poor isolation given by lossless three-port junctions. While a standard implementation based on normal rectangular waveguides often results in large and heavy structures, an implementation based on substrate-integrated-waveguide (SIW) technology offers advantages in terms of compactness, weight reduction, cost minimization, and integration possibilities with active stages. This paper presents the design, fabrication, and characterization of two SIW E-plane 3-dB power divider/combiners where the lossy element is realized using a resistive layer. The prototypes cover the entire X-band from 8 to 12 GHz, and the resistive layers are realized according to two different manufacturing techniques to investigate the potentials of both approaches. The optimization of the resistive layer geometry and resistivity, a critical aspect for low-profile SIW circuits, is discussed in detail. In particular, an analytical formula is derived, which allows to determine the optimum values for the resistive layers' principal parameters, namely the length and resistivity, without the use of full-wave numerical solvers.