Rat-Race Coupler Overview
A rat-race (ring hybrid or 180° hybrid) is a 4-port device consisting of a 3λ/2 ring with 4 ports spaced λ/4 apart. It provides two outputs with 0° and 180° phase relationship (unlike the 90° hybrid branch-line coupler).
Rat-Race Ideal S-Parameters
Port 1 = Sum input, Port 2 = In-phase output (0°) Port 3 = Isolated, Port 4 = Anti-phase output (180°) S21 = −3 dB, 0° phase S41 = −3 dB, −180° phase S11 = 0 (matched) S31 = −∞ (isolated from Port 1)
Analysis in RF View
- Load .s4p rat-race file
- Extract Port1→Port2, Port1→Port4, Port1→Port3 as S2P files
- Overlay S21 (0° output) and S41 (180° output) magnitudes — should both be −3 dB
- Phase view: Phase(S41) − Phase(S21) = −180° ± 5° (balance check)
- S31: verify <−20 dB isolation
- Bandwidth: 0° and 180° balance maintained over ~20% of f₀
RF View: Load rat-race .s4p, extract port pairs, overlay S21 and S41 magnitude and phase. Delta marker quantifies 180° phase balance error. Free on Android.