How To Check or Verify Your Cable’s Velocity of Propagation Using the Bird Vector Network Analyzer

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For whatever reason you need to know the velocity of propagation (Vp), any RF test cable will have the cable type printed on the jacket of the cable length which you can then use in a web search to get this Vp detail. However, there could be an instance where the print is difficult to read or not printed. In these situations, you can use your Bird BN100+ or BN900 vector network analyzer to help you determine the mystery information.

  1. First, you will need to perform a basic 1-Port calibration. Because Vp tends to be specified at 1 GHz, ensure your start and stop points are set near this center point, perhaps a start stimulus of 990 MHz and a stop stimulus of 1.01 GHz. If you want additional samples for this span, you can increase the number of sweep points from the default of 201 to 1001.
  2. Once the calibration is complete, connect your cable to the calibrated port leaving its opposing end open.
  3. At this point, use the port extensions feature to capture the signal travel time to the end of the cable length.
    • Using the on-screen function buttons, navigate to Main Menu->Calibration->Port Extensions->Auto Port Extension.
          • 2024-02-15_AutoPort.png
    • Click on the Select Port option and select only the port where the measurement will be taken
          • 2024-02-15_SelectPort.png
    • Click on Measure Open to acquire a measurement.
    • Navigating up one level, you can see the measurement displayed in the function button associated with your port measurement selection.
  4. Next, use this value to calculate the velocity of propagation. Let's assume your cable has a 6-foot (1.8288 m) length:
          • 2024-02-15_Calc_01.png
  5. This will not look like the Vp given in the specs from the cable manufacturer.
          • 2024-02-15_MFG01.png
  6. Because the manufacturer specifies Vp as the percentage of the speed of light, you need to perform one more division as follows to achieve the equivalent, where c is the speed of light at 300e6 m/s:
          • 2024-02-15_Calc_02.png

 

If you want some added confidence in this procedure, you might choose to perform it again, perhaps using a 20’ piece of RGS-393 cable which the manufacturer states as having velocity of propagation of 70%:

                  2024-02-15_MFG02.png

 

In this case, the port extension measurement yields 28.785ns, so:

                  2024-02-15_Calc_03.png

 

 

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