Modeling and Design of High-Power Non-Isolating RF Power Combiners based on Transmission Lines

Many industrial radio-frequency (rf) power applications are characterized by high power levels, narrow frequency ranges, and the need to combine rf power from multiple sources. In the HF frequency range (3–30 MHz) and above, power combiners based on coupled transmission lines are attractive due to their small form factor and high efficiency. However, most available literature on rf power combining with transmission-line structures focuses on frequency response, with little consideration regarding losses or efficiency optimization. This paper explores the design and optimization of lossless narrow-band rf combiners using transmission-line conductors. We introduce a lumped-element circuit model suited towards this application space, and further propose a tuning technique that, by adding two capacitors, minimizes impedance distortion while preserving high efficiency within a specified operating frequency range. A 1 kW combiner designed to operate around 13.56 MHz is prototyped, and a custom testbench is developed to measure its loss. The model and tuning technique are validated with both small-signal measurements and high-power tests. Improvements in impedance distortion from 1.19:1 to 1.03:1 VSWR is achieved (with
$50\ \Omega$
load) through the proposed methods, and high combiner efficiency of >99.3% (<0.031 dB insertion loss) is maintained up to the full rated power.