Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - 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 shows the test setup. Here's the procedure:
K6NY, Jack Althouse.