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

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 ($\epsilon_{r}$) of 2.2. The measured characteristics exhibit minimal phase and amplitude imbalances of $\pm 1.5^{0}$ and $0.2 d B$, respectively, over a bandwidth of $1.7-3.6 \text{GHz}$ corresponding to a 70 % fractional bandwidth. These characteristics represent a significant improvement over conventional high-frequency narrowband and broadband $\mathbf{1 8 0}$-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 $2-6 \text{GHz}$ band.