What Is Port Isolation?
Port isolation is S21 measured between two ports that should be isolated — the TX and RX ports of a duplexer, the ON and OFF ports of a switch, or the input and isolated port of a coupler. The more negative the S21 (in dB), the better the isolation.
VNA Dynamic Range for Isolation Measurement
Dynamic range = maximum S-parameter magnitude measurable: VNA dynamic range = source power − noise floor Typical: −10 dBm source − (−110 dBm floor) = 100 dB dynamic range Maximum measurable isolation: ~90 dB (accounting for crosstalk) For measuring 50 dB TX→RX duplexer isolation: easily within range For measuring 80 dB switch isolation: need averaging + IF BW narrowing
Isolation Measurement Setup
- VNA SOLT calibration at cable ends (removes cable insertion loss from measurement)
- For duplexer isolation: connect Port 1 cable to TX input, Port 2 to RX output
- Terminate ANT port in 50Ω load (critical — must match real operating condition)
- Measure S21 from TX to RX — this IS the TX-to-RX isolation path
- Sweep across RX band → find minimum isolation frequency (worst case)
Common Isolation Measurement Errors
| Error | Symptom | Fix |
|---|---|---|
| ANT port not terminated | S21 level changes when touching ANT port | Use precision 50Ω load on ANT |
| VNA noise floor exceeded | S21 shows noisy floor, not real device | Narrow IF BW, add averaging |
| Cable coupling | S21 doesn't improve past certain level | Separate Port 1 and Port 2 cables physically |
RF View Isolation Analysis: Load duplexer isolation path .s2p (extracted from .s3p), view S21 across RX band, use single marker to find worst-case isolation frequency. Free on Android.