You picked up an RF directional coupler in the lab, but the ports are not labeled. What are the typical 4 ports of an RF directional coupler?

Zeng, Lingzhen, Lu, Wei-Chun, Grimes, Paul K., Chen, Tse-Jun, Chang, Yen-Pin, Tong, Cheuk-yu, and Wang, Ming-Jye. 2020. "A Silicon Chip-Based Waveguide Directional ...