TABLE OF CONTENTS Doping out transmission line difficulties can be a tedious and difficult chore especially when the load end is not easily accessible. Although there are a number of different methods available, I want to discuss time-domain reflectometry. Although the official, bottled-in-bond, 100-proof time-domain reflectometer (TDR) is a terribly expensive piece of equipment, some TDR methods can be used by any amateur who has access to an oscilloscope. Will the results be as good as professional TDR equipment? In a word: no. Will the results be useful in troubleshooting a transmission line? In a single word: yes.
Time-domain reflectometry works on the principle that waves on a nonmatched transmission line reflect. The waveform seen at any given point along the line is the algebraic sum of the forward and reflected waveforms. In TDR measurements, we look at the waveform at the input end of the transmission line system.
Figure 24-1 shows the basic set-up for our impromptu TDR. A pulse generator, or other source of 1-MHz square waves, is applied simultaneously to the vertical input of an oscilloscope and the input end of the transmission line. This neat little trick is accomplished with an ordinary coaxial tee connector, either BNC or UHF, depending on your own situation. In my case, the pulse source and oscilloscope use BNC connectors, so I used a BNC tee connector. The PL-259 UHF connector on the end of the 200-foot length of RG-58 transmission line was converted using a SO-239 to BNC male adapter.
