Distance To Fault Measurements For Cable And Antenna Installation And Maintenance

Distance To Fault (DTF) is a performance verification and failure analysis tool used for antenna and transmission line service and maintenance. It uses the Frequency Domain Reflectometry (FDR) measurement technique. FDR is a transmission line fault isolation method which precisely identifies signal path degradation for coax and waveguide transmission lines. Although the acronyms are similar, FDR technology is different from traditional Time Domain Reflectometry (TDR) techniques. The FDR technique uses a swept RF signal instead of TDR DC pulses. FDR is far more sensitive than TDR and can precisely locate faults and degradation in system performance, not just DC open or short circuit conditions. This dual role of predicting future failure conditions and isolating existing problems makes DTF an important part of service and maintenance on transmission lines.

DTF displays RF return loss or VSWR data versus distance. The effects of poor connections, damaged cables, or faulty antennas are quickly identified. Since DTF automatically accounts for attenuation versus distance, the display accurately indicates the return loss or VSWR of the antenna.

For the majority of transmission lines and antennas, the absence of DTF capability severely impacts the time to repair transmission lines and renders preventative maintenance procedures impractical. RF failure conditions at the top of a tower or through a bulkhead frequently are not measurable with traditional tools such as TDR and spectrum analyzers with tracking generators. A TDR cannot detect small performance changes at RF frequencies, so it is not possible to monitor performance degradation between maintenance intervals with these traditional methods. Without FDR techniques, the "Fix after Failure" philosophy becomes the only alternative.

Many components can cause problems in a communication system. Transmission lines are typically the most common failure point. Tower mounted transmission lines are exposed to weather, and will degrade over time. Lightning can sever a portion of the antenna or damage the in-line lightning arrestor. Sunlight exposure can change the dielectric properties of the antenna housing, causing the antenna bandwidth to drift. Antennas and transmission lines used on-board ships and aircraft may be degraded due to salt water corrosion. These common problems can cause unwanted signal reflections. Poorly tightened connectors and poor environmental seals are exacerbated by acid rain corrosion. Eventually these problems cause intermittent outage and failures at exactly the times they are least welcome, such as during storms or during extreme periods of cold. With DTF available, the root causes of RF problems can be identified. For example, connector corrosion can be detected early and weather seals replaced before moisture destroys expensive cables. DTF finds these problems because the FDR technique can accurately detect very small performance changes within the transmission line.