I work for the Navy, doing testing and repair of Electronic Warfare (RADAR detection and jamming) systems. A fundamental understanding of Radio Frequency (RF) Wave Propagation is required to effectively troubleshoot system operations. Part of this process is conduct Voltage Standing Wave Ratio (VSWR) testing of RF pathways, such as millimeter Waveguides and Heliax.
VSWR is the measure of how efficiently RF power is transmitted in to a load. For example, if a power amplifier is connected to an antenna through a transmission line, such as a Waveguide, ideally there will be no reflections and all the signal from the power amplifier will be transmitted to the antenna. However, in the real world, there will be some mismatches that will cause some of the signal to be reflected back in to the transmission line. VSWR is the measure of how much signal is reflected back in to the system. It is the ratio between transmitted and reflected waves. A high VSWR indicates poor transmission-line efficiency and performance, in most cases requiring component or pathway replacement.
To calculate VSWR, a specialized Signal generator and Spectrum Analyzer are used, as well as an assortment of loads and couplers. Some of the basic formulas for VSWR include:
Where (r) is the Reflection Coefficient, (RL) is the Return Loss, and (ML) is the Mismatch Loss (Microwave101, 2022).
To be honest though, I’ve really never looked at nor understood the formulas required for the calculations. The test equipment I use is designed to generate the needed signals, measure the reflected energy for a given frequency range, and then provide a graph with the resulting calculations. From there, I compare the loss values based on the specifications for the pathway or antenna under test.
Microwave101. (2022, November 01). Voltage standing wave ratio (VSWR). Retrieved from microwave101.com: https://www.microwaves101.com/encyclopedias/voltage-standing-wave-ratio-vswr