Technical Insights > RF + Microwave

How to Find the Fault in Your Transmission Line

2020-05-29  |  6 min read 

Transmission lines, like coaxial cables or twisted pairs, play an integral role in electrical engineering, carrying information and power for many applications. Transmission lines are distributed elements whose electrical characteristics depend on physical geometry. As traveling electromagnetic waves interact with geometrical disruptions along a line, they produce reflections that give you information on circuit behavior.

Let’s say you have a one-mile-long  transmission line buried in the ground and you suspect you have a disruption or fault causing performance issues. How do you know where to dig to repair or replace your line?

In this blog, I discuss two important cable and antenna tester measurements that help you: 

  • Figure out  the length of your cable without physically measuring the cable
  • Verify cable performance at the touch of a button
  • Determine if failures exist in your transmission line and the nature of those failures without visually inspecting the cable
  1. Distance-to-Fault (DTF) measurement

In simple terms, a DTF measurement involves sending an RF signal through a cable then measuring how much power reflects back. The display shows the reflected signals along the cable as spikes in power and the measurement appears as return loss in dB versus distance. For an accurate measurement, you need to input the velocity factor of your cable which is either a standard, known value for common cable types, or provided on the spec sheet.

Ideally, the largest peak in the center of the display represents the end of the cable, as shown in Figure 1. Placing a marker there gives you the cable length. However, the largest peak might not occur in the center if there is a significant fault in the cable before the end of the transmission line. If a suspicious peak appears before the end of your transmission line, you can place a marker on that peak to find out the approximate location of the fault.

Figure 1. A DTF measurement made using a FieldFox handheld analyzer. The yellow trace shows a cable with an open end, and the blue trace, stored to memory, shows the same cable terminated in a 50 Ω load.
Figure 1. A DTF measurement made using a FieldFox handheld analyzer. The yellow trace shows a cable with an open end, and the blue trace, stored to memory, shows the same cable terminated in a 50 Ω load.

     2. Time-Domain Reflectometry (TDR) measurement

TDR measurements are similar to  DTF measurements in that they help you determine if there is a fault in your cable. A TDR measurement also sends an RF pulse through the cable, but instead of reading the amount of power reflected back, it measures a mismatch of impedance, shown in Figure 2. The measurement shows reflections along the transmission line and appears as impedance (ohms) versus time.

Placing a marker on the reflection peaks shown in a TDR measurement will give you the approximate location of a discontinuity. In addition to location, the TDR measurement also gives you valuable information about the kind of discontinuity you are dealing with. For instance, a reflection from an open or a cut in the cable appears inductive because open faults have a higher resistance than the cable impedance. Open faults will appear as upward, positive reflections in a TDR measurement. On the other hand, a reflection from a short (or water in the cable ) appears capacitive. Because a short has less resistance than the cable impedance, its reflection appears downward, or negative. Knowing this kind of information helps you better understand the issue with your cable and how to approach the problem from a repair or replacement standpoint.

Figure 2 . A TDR measurement showing no reflections from mismatched impedance. To watch a demo of DTF and TDR measurements, check out this video.
Figure 2. A TDR measurement showing no reflections from mismatched impedance. To watch a demo of DTF and TDR measurements, check out this video.

DTF and TDR measurements are invaluable tools to have in your field kit. They save you time and effort by giving you information like cable length, fault location, and fault type without having to visually inspect the entire cable. This is especially helpful when a cable is too long, buried underground, or elevated in a cell tower.

To find out more about DTF, TDR, and other important transmission line measurements, read the app note “Transmission Lines and Reflected Signals.