La Viruela

APPLICATION NOTE

Distance To Fault
Site Master™, Cell Master™, VNA Master™

Distance To Fault Measurements
for Cable and Antenna Installation
and Maintenance

Introduction

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) measurementtechnique. 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 preciselylocate 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 arequickly identified. Since
DTF automatically accounts for attenuation versus distance, the display accurately
indicates the return loss or VSWR of the antenna.

Cable Problems
Cable Discontinuities
Damaged/Dented Ground Shield
Moisture and Corrosion
Fasteners Pinching Cables

Connector Problems
Corroded Connectors
Low Quality Connectors
Poor Center Pin Contact

Typical CommunicationSystems Problems

2

Antenna Problems
Out of Specification
Storm/Shipping Damage
Damaged Lightning Arrestor

Reduce Maintenance
Time and Expense

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 atower 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 theonly 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 timesthey 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 thetransmission line.
In a Wireless Communication System, tower mounted transmission lines and
cables are replaced frequently, perhaps every five to ten years, in some cases.
Usually, all the site’s cables are replaced based upon the assumption that
maintenance calls are imminent on other feeds in addition to the problem
cable. This practice may be precipitated by the cable installer, who would...