Rob's web

Using a noise bridge to measure coaxial-cable impedance

In Transmission Line Transformers,(1) Jerry Sevick, W2FMI, describes a simple method of measuring the characteristic impedance of a transmission line. This method makes use of the fact that when a cable is terminated in a resistance equal to the cable's characteristic impedance, the cable looks like a pure resistance. But when the cable is terminated in a resistance of a different value, the cable appears reactive. The null of a resistance bridge is very sensitive to the reactive component. When looking at a pure resistance, the bridge gives a deep null; if reactance is present, the null is shallow. This method of measurement is accurate and is invaluable when making parallel or twisted-wire lines for balun transformers.

Sevick uses a simple resistance bridge with a signal source and detector. It's also possible to make the same measurements using a noise bridge and a receiver. I tried this and was impressed with the accuracy I obtained.

Fig 1

Fig 1 shows the test setup. Here's the procedure:

  1. Tune the receiver to the measurement frequency. (Sevick suggests working at frequencies between 10 and 20 MHz.)
  2. Connect a 50-0 load to the UNKNOWN terminal of the noise bridge. Carefully adjust the X (or reactance) knob setting for a deep null in the noise. Hereafter, don't change the X knob setting.
  3. Remove the resistor from the UNKNOWN terminal and connect the cable under test (CUT) to that terminal. (You can measure a whole roll of cable, or a sample piece just a foot or two long.)
  4. Set the noise bridge R (resistance) knob at the resistance corresponding to the characteristic impedance you expect to find.
  5. Adjust R1 for a null.
  6. Change the setting of the R knob.
  7. Repeat steps 5 and 6 until you find the deepest null. As you approach the cable impedance, the null gets deeper. If you go beyond it, the null again becomes shallow.
  8. After finding the deepest null, read the noise bridge R dial, or measure the resistance of R1. The result obtained is the cable's impedance.

K6NY, Jack Althouse.

Notes

  1. J. Sevick, Transmission line transformers, 2nd ed. ARRL 1990, pp 12-16 to 12-18.