Techniques for Precise Cable and Antenna Measurements in the Field
Application Notes
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
What Is Insertion Loss?
Insertion loss is defined as the amount of signal power lost when a signal passes through a transmission line or component, such as a coaxial cable. It is typically expressed in decibels (dB) and represents the ratio of the input power to the output power. In practical terms, insertion loss quantifies how much signal is attenuated due to resistive, dielectric, and mismatch losses within the cable or device under test (DUT).
In the context of RF and microwave systems, insertion loss is a critical parameter for evaluating the performance of cables and connectors. It includes not only the inherent losses of the cable material but also losses due to reflections caused by impedance mismatches between the source, cable, and load. For example, a coaxial cable with an air dielectric typically has lower insertion loss compared to one with a solid dielectric.
Why Is Cable and Antenna Insertion Loss Testing Important in RF Systems?
Insertion loss testing is essential for maintaining the performance and reliability of RF and microwave systems. Over time, cables and antennas can degrade due to environmental exposure (e.g., rain, snow, lightning) or physical damage (e.g., bending, crushing, or poor routing). These issues often lead to signal degradation, poor coverage, and increased handovers in cellular systems. Insertion loss tests help identify such faults early, ensuring system integrity. Tools like the Keysight FieldFox analyzer enable technicians to perform quick and accurate insertion loss tests and return loss measurements, even in harsh field conditions.
How Is Cable Insertion Loss Test Performed Using One-Port and Two-Port Methods?
Cable insertion loss tests can be conducted using either a two-port or one-port method. The two-port method connects both ends of the cable to the analyzer, measuring the signal loss directly from input to output. This method is highly accurate and ideal when both ends are accessible. The one-port method, useful for long or inaccessible cables, involves sending a signal into one end and measuring the reflected signal from an open or shorted termination. FieldFox uses its internal reflectometer and CalReady calibration to calculate insertion loss from the round-trip signal. This flexibility makes insertion loss testing feasible in a wide range of field scenarios.
What Role Does Insertion Loss Test Play in Antenna Return Loss and VSWR Measurements?
Insertion loss testing complements return loss and VSWR measurements in evaluating antenna performance. Return loss indicates how much signal is reflected back from the antenna, while VSWR expresses this reflection as a ratio. A high return loss (e.g., 15 dB or more) and a low VSWR (e.g., 1.5:1 or better) signify good impedance matching and minimal signal loss. However, if the cable between the analyzer and antenna has high insertion loss, it can mask the true return loss of the antenna. Therefore, accurate insertion loss tests are crucial to isolate antenna performance from cable effects.
How Is Insertion Loss Test Used to Measure Antenna-to-Antenna Isolation?
In systems with multiple antennas, insertion loss testing is used to measure antenna-to-antenna isolation. Poor isolation can lead to intermodulation distortion and receiver desensitization. FieldFox performs this test using a two-port insertion loss configuration, sweeping across frequency bands to determine how much signal from one antenna couples into another. Proper calibration and spacing are essential to ensure accurate insertion loss test results and maintain isolation levels, which are often required to be 60 dB or higher in shared infrastructure environments.
How Can Insertion Loss Test Help Locate Faults Along Transmission Lines?
FieldFox’s Distance-to-Fault (DTF) feature enhances insertion loss testing by identifying the physical location of faults along a transmission line. DTF sends a signal down the cable and analyzes reflections caused by impedance mismatches. These reflections are converted into distance measurements, helping technicians pinpoint issues like damaged connectors, splices, or crushed cables. Accurate DTF results depend on entering the correct velocity factor for the cable type, which is critical for precise insertion loss test diagnostics.
The next few sections of this application note include examples showing techniques for testing 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.