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 (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\epsilon_{r}$</tex>) of 2.2. The measured characteristics exhibit minimal phase and amplitude imbalances of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 1.5^{0}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.2 d B$</tex>, respectively, over a bandwidth of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.7-3.6 \text{GHz}$</tex> corresponding to a 70 % fractional bandwidth. These characteristics represent a significant improvement over conventional high-frequency narrowband and broadband <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 8 0}$</tex>-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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2-6 \text{GHz}$</tex> band.