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PORT-Z-50-75-Omh.md

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PORT-Z-50-75-Omh.md

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50 Ohm and 75 Ohm Support (PORT-Z)

NanoVNA-X supports measurements in systems with different characteristic impedances (e.g., 75 Ohm for TV and video equipment) while maintaining the standard 50 Ohm calibration.

How it works

The hardware is designed for 50 Ohm impedance. All physical measurements are performed in a 50 Ohm environment. To work with a 75 Ohm load, a mathematical impedance renormalization function is used.

  1. Perform a standard SOLT calibration using a 50 Ohm calibration kit (Open, Short, Load 50 Ohm).

    • Important: Always perform calibration with a 50 Ohm calibration kit.
    • Do not use a 75 Ohm load for calibration. The firmware assumes a 50 Ohm standard for the calibration process.
    • The renormalization feature (PORT-Z) mathematically converts the 50 Ohm calibrated data to 75 Ohm.

  2. Change the port impedance (PORT-Z) to 75 Ohm in the device menu.

    • The device recalculates the measured S-parameters (Gamma) as if the measurement was taken with 75 Ohm ports.
    • The center of the Smith Chart shifts to the 75 Ohm point.
    • VSWR is recalculated relative to 75 Ohm.

Instruction: Setting up 75 Ohm

  1. Perform a standard calibration with 50 Ohm loads.
  2. Navigate to the menu: MEASURE.
  3. Locate the PORT-Z item. It defaults to 50 Ohm.
  4. Tap PORT-Z, enter 75, and press x1.
  5. All plots (Smith, LogMag, SWR) will now display values renormalized for a 75 Ohm system.

Physical Limitations

  • Using 50 Ohm connectors and cables introduces a small mismatch error. The renormalization mathematics attempts to compensate for the impedance change, but physical reflections at 50/75 Ohm interfaces remain.
  • For precise laboratory measurements, it is recommended to use 50-75 Ohm Matching Pads (Minimum Loss Pads). However, these introduce attenuation (approximately 5.7 dB), which must be compensated for in the calibration or by using S21 offset.
  • The software PORT-Z method is ideal for quick evaluation of 75 Ohm antennas and cables without additional hardware.