Wideband 180-Degree Coupler for S-Band Precision Cosmology Experiments

Abstract

In this work, we propose a compact planar 180-degree coupler based on a Magic-T configuration, implemented using microstrip-to-slotline transition. The coupler has been designed and fabricated on a low-loss Rogers Ultralam substrate with a dielectric constant (ϵr) of 2.2. The measured characteristics exhibit minimal phase and amplitude imbalances of ±1.50 and 0.2dB, respectively, over a bandwidth of 1.7−3.6GHz corresponding to a 70 % fractional bandwidth. These characteristics represent a significant improvement over conventional high-frequency narrowband and broadband 180-degree couplers, which typically show higher phase and amplitude imbalances. This coupler is designed for the APSERa (Array of Precision Spectrometers for Epoch of Recombination) cosmology experiment, a ground-based radiometer designed to detect hydrogen and helium recombination signals from the epoch of recombination in the early universe within the 2−6GHz band.