Title: Broadband Planar 180-degree Hybrid Coupler for S-Band Precision Cosmology Experiments

In RF systems, 1800 hybrid couplers are essential for creating equally power-divided signals with an 180 0 phase difference. These couplers play a pivotal role in bandpass calibration and additive noise cancellation in broadband S-band radiometers designed for the APSERa Cosmology experiment. However, maintaining a constant phase difference and power division across a wide frequency band poses a formidable challenge. To address this issue, we propose a compact planar 180 0 coupler designed based on a Magic-T configuration, implemented using microstrip to slotline transitions. The coupler exhibits minimal phase and amplitude imbalances of ±1° and 0.03dB, respectively over a band of 1.7 – 3.6 GHz, achieving a 70% fractional bandwidth. This is a significant improvement compared to conventional 180 0 couplers, which typically exhibit less than 10% fractional bandwidth.