Techniques for Precise Cable and Antenna Measurements in the Field
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Introduction
This application note introduces the practical aspects of cable and antenna testing, interpreting measurement results, and instrument operation. It looks at calibration options such as CalReady and QuickCal using a Keysight FieldFox handheld analyzer configured as a cable and antenna analyzer. Measurement examples show techniques for measuring insertion loss, measuring return loss, and locating faults in a transmission system.
Cable and antenna measurements are often necessary to verify and troubleshoot the electrical performance of radio-frequency (RF) and microwave transmission systems and antennas. In RF and microwave communications industries, measurements are often made along the coaxial cable connecting a transmitter to its antenna or between an antenna and its receiver. This process, called line sweeping, results in the measurement of the signal attenuation and return loss as a function of frequency. Line sweeping may also help estimate the physical location of a fault or damage along the transmission line using the distance-to-fault (DTF) measurement available on many RF and microwave analyzers, including FieldFox.
Testing the performance of transmission lines extends beyond coaxial cable types. You may characterize systems using waveguide and twisted-pair cables once you have installed the appropriate adapter between the transmission line and the coaxial interface on the analyzer. In addition, antenna measurements in the form of signal reflection or return loss and voltage standing wave ratio (VSWR) may help verify the performance of an antenna at the installation site. When the installation site requires multiple antennas, whether the antennas are associated with the same system or different systems, FieldFox may also help verify the antenna-to antenna isolation.
Figure 1 shows a typical return loss measurement (upper trace) and DTF measurement (lower trace) for an X-band antenna connected to a 3-meter length of coaxial cable. The antenna was mounted on a vertical mast at a rooftop installation and measured using FieldFox configured with a preset calibration called CalReady. The return loss measurement includes two frequency markers placed across the specified operating band at 8 GHz and 12.4 GHz. The DTF measurement includes a distance marker placed at the location of the antenna at 3.23 meters.
Table of Contents:
Why Is Cable and Antenna Testing Important?
Cable Insertion Loss Measurements
Antenna Return Loss and VSWR Measurements
Antenna-to-Antenna Isolation
Locating Faults Along Transmission Lines
Calibrating FieldFox’s Test Ports
Why Is Cable and Antenna Testing Important?
During the installation, operation, and maintenance of a telecommunications system, the connecting cables, adapters, and antennas may become damaged or show reduced performance over time. Faulty cables, connectors, and antennas cause the majority of cellular base station problems. Failure in these components causes poor coverage and unnecessary handovers in the cellular system. In any communication system, it is important to have a process and the appropriate instrumentation to verify the performance of the cables and antennas when issues arise at the system level.
Cabling and antennas may be installed in a variety of environments, including outdoors and indoors. Outdoor installations may include rooftop, tower-mounted, and underground cabling. Indoor installations may include equipment shelters, office buildings, shipboard, aircraft, and certain automotive applications. An outdoor installation of a wireless cellular system may expose the antenna and portions of the coaxial cabling to extreme weather conditions, including rain, snow, ice, wind, and lightning.
Severe environmental conditions may create physical damage to the system components. Damage may include failed waterproof sealing at interfaces between cables and connectors, failed sealing at cable splices, and weather-induced cracks in the insulation, allowing moisture into the cables and interfaces.
Sheltered installations can expose components to mishandling, stress, heat, vibration, oils, and other contaminants. Cable faults can form at interfaces between the cables and connectors where solder joints and cable crimps become fatigued and break. It is also possible that cables can be dented, crushed, or improperly routed during or after installation.
Under some conditions, the minimum bend radius may exceed the value specified by the cable manufacturer. For example, a low-loss coaxial cable may have a specified minimum bend radius of 1 inch to more than 10 inches. Bending the cable below the specification will cause damage to the cable and substantially degrade its electrical performance.
Verifying and maintaining the operation of the RF and microwave transmission system, including coaxial cables, waveguides, twisted-pair lines, and antennas, requires fast and accurate measurements of insertion loss and return loss as a function of frequency. Also important to field operations is the capability to quickly troubleshoot and locate faults and damage in cables and connectors. A modern handheld analyzer, such as FieldFox, configured for cable and antenna testing (CAT), is a useful tool for characterizing the entire transmission system as well as the performance of individual components in the system.
The next few sections of this application note include examples showing techniques for measuring insertion loss, measuring return loss, and locating faults in a transmission system. A FieldFox N991XA analyzer with a frequency range of up to 26.5 GHz made the measurements.
