How to Locate RF Cable Faults
Posted in How To's/Tips
- Can you hear me now? Cellular base station towers facilitate communication between user equipment and a wireless network. Towers are typically several hundred feet high. The antenna at the top of the tower is connected to the transceiver equipment at the bottom via a series of long cables. And approximately half of base station problems are due to faulty cables, connectors or antenna.
Cable faults can happen both during site installation and over time. Despite the best effort to protect these cables, they are still susceptible to the various environmental stresses and sometimes, extreme weather conditions. Eventually, degradation of cable performances or even physical damages occur. Since the combined length of the cables between the antenna and transceiver equipment could reach hundreds of feet, the sooner you know where faults occur along the line, the sooner you can resolve the problem, and the better your customer satisfaction ratings.
A faulty cable is one where the insertion loss is too high, or return loss and VSWR is out of specifications.
Telecommunications test equipment 101
You can locate the physical position of a cable fault by using equipment typically called the Cable and Antenna Tester (CAT) with Distance-to-Fault (DTF) measurement capability. Unlike traditional Time-Domain Reflectometry (TDR) which uses fast rising pulses, DTF measurement usually employs Frequency-Domain Reflectometry (FDR) which utilizes swept frequency response measurements. The swept frequency response of the FDR is then inversely Fourier transformed into time-domain information. Due to its narrowband operation, FDR has the advantage of having better dynamic range compared to TDR.
Basic procedures to make DTF measurements vary, depending on the test equipment model(s) used, but generally follow these steps:
- Equipment and Information needed:
- Suitable adapter to mate the cable to the CAT port (if the connector of the cable does not mate directly with the CAT).
- A load termination to terminate the other end of the cable (if it is not terminated).
- Knowledge of cable length, cable type, cable loss and velocity factor. Velocity factor can be obtained from the datasheet or can be calculated as 1/√εr, where εr is the relative permittivity of the dielectric in the cable.
- Connect the cable to the CAT, using any adapter if necessary.
- If you are using combination analyzers and CAT ensure that you are using CAT mode before you begin.
- Activate the DTF measurement feature.
- Enter the maximum/stop distance parameter as longer than the known cable length.
- Load the cable specifications profile, if available. Otherwise manually enter the cable loss and velocity factor. The velocity factor information is crucial in correctly determining the distance of the fault because it deals with the propagation speed of the signal.
- Optimize the frequency range as per the manufacturer’s recommendation. Wider frequency span will give you better resolution but shorter maximum distance.
- Start the measurement and view the screen for the measurement results.
Essentially, what appears on the screen will be the magnitude of the transmitted signal being reflected back to the CAT along the distance of the cable. Unusually high peaks generally suggest a problem such as a loose or broken connection, which causes more energy to be reflected back, while lesser magnitude may be due to bending or faulty joints. Once you have identified the abnormal peaks, place markers there to get a reading of the distance to fault.
Need help determining the best CAT equipment to locate cable faults? Are you waiting too long for calibrations and routine service on your test equipment? Contact MATsolutions to discuss your needs.