Design and development of high bandwidth coaxial RF combiner

Abstract The Low Energy High Intensity Proton Accelerator (LEHIPA) has been commissioned to 20 MeV at BARC, India. A non-interceptive, high bandwidth wall current monitor (WCM) with 8 feedthroughs has been designed and fabricated to measure beam bunch structure at 352 MHz in LEHIPA facility. For combining the wall current signals of the WCM, a 50 Ω, 2:1 coaxial RF combiner has been designed. This is an economically viable alternative to the commercial wideband RF combiners. The design of the coaxial combiner is carried out using CST- Microwave Studio software and geometry is optimized based on S-parameter studies, to obtain a high bandwidth of ∼ 10 GHz. The design incorporates open spaces in the middle and bottom sections for ease in the mechanical assembly of the combiner. The S-parameters for the optimized combiner design are: S 11 = -9.5 dB, S 21 = -3.5 dB and S 31 = -3.5 dB, over a wide bandwidth of 10 GHz. The combiner has been fabricated and characterised using Vector Network Analyser (VNA) and Time Domain Reflectometry (TDR) measurements. The TDR measurements showed a bandwidth of ∼ 9 GHz for the combiner and the measured insertion loss ( S 21 ) using VNA is around -3.5 dB. The measured values are in good agreement with the simulation results.