Isolation (or isolation loss) is a critical parameter for coaxial switches, measuring signal leakage between ports when the switch is in the "off" state. Here’s a step-by-step guide to testing it accurately：

1. Test Equipment Required

• Vector Network Analyzer (VNA): Measures S-parameters (e.g., S12/S21 for isolation).

• Calibration Kit (SOLT): Ensures accurate VNA calibration up to the switch’s frequency range.

• RF Cables & Adapters: High-quality cables matching the switch’s impedance (typically 50Ω).

2. Test Setup

1. Calibrate the VNA using Short-Open-Load-Thru (SOLT) at the switch’s operating frequency (e.g., DC–18 GHz).

2. Connect the switch between VNA ports:

   • Port 1 → Switch Input (Common Port).

   • Port 2 → Switch Output (Isolated Port when "off").

3. Power Settings: Set VNA to low power (e.g., –10 dBm) to avoid damaging the switch.

3. Measurement Steps

• Step 1: Activate the switch to isolate the target port (e.g., disconnect Port 2).

• Step 2: Measure S21 (Forward Transmission)—this directly indicates isolation (in dB).

• Step 3: Repeat across the frequency range (e.g., 1 GHz steps) to capture worst-case values.

4. Key Considerations

• Baseline Noise: Ensure VNA noise floor is at least 10 dB below the expected isolation.

• Connector Torque: Use a torque wrench (e.g., 8 in-lb for SMA) to avoid signal leaks.

• Temperature Stability: Test at 25°C ±5°C (isolation may degrade at extremes).

5. Interpretation

• Typical Values:

  • Mechanical Switches: 60–90 dB (higher at lower frequencies).

  • Solid-State Switches: 40–70 dB (varies with frequency).

• Fail Criteria: If isolation drops >10 dB below spec, check for damaged contacts or contamination.

6. Advanced Tips

• Time-Domain Reflectometry (TDR): Locate impedance mismatches causing leakage.

• Repeatability Test: Cycle the switch 100+ times to detect wear-related degradation.