SWR meters are used to measure the standing wave ratio (SWR) in radio equipment. They are sometimes called VSWR meters, a reference to voltage standing wave ratio (VSWR). SWR is the ratio of the amplitude of a partial standing-wave at maximum amplitude to the amplitude at an adjacent node. In telecommunications applications, standing wave ratio is a measure of the quality of the match between the antenna and the receiving system. VSWR is the ratio of the maximum voltage to the minimum voltage on an electrical transmission line. In other words, it is a measure of the impedance mismatch between a transmission line and load.
Selecting SWR Meters
Selecting SWR meters and VSWR meters requires an analysis of form factors. Benchtop instruments are designed to sit atop a bench or table, typically in a laboratory setting. Free-standing devices have a full case or cabinet and integral interface. SWR meters with a clamp meter form-factor measure current through wires that are still connected to a live circuit. Rack-mounted SWR meters come with hardware such as rail guides, flanges and tabs. They are designed to be mounted in a telecommunications rack. Handheld standing wave ratio meters are designed for use while held in the hand. SWR meters with a computer-board form factor are printed circuit boards (PCBs) that plug into computer motherboards or backplanes. Electronic test equipment with other form factors is also available.
Output Interface and Available Features
SWR meters and VSWR meters differ in terms of output interface and available features. Choices for output interface include universal serial bus (USB), general-purpose interface bus (GPIB), RS-232, binary coded decimal (BCD), and digital-to-analog (D/A). Some SWR meters have an adjustable sampling rate, alarm lights, auto-ranging features, or application software. Others provide data acquisition, data storage or logging, decibel reading, or external triggering capabilities. External shunts can be used to extend the current input range. Integrating functions allow the active power and current to be integrated. Mirrored scales facilitate readings to a given accuracy and help operators avoid parallax errors. Range switches can be used to select the range of units to measure. SWR meters with overload protection, filters, scaling functions, and temperature compensation are also available.
SWR meters and VSWR meters may bear quality marks and comply with various international standards. The CE Mark indicates that compliance with the requirements of European Union (EU) directives that uphold standards for health, safety, and environmental protection. These standards include requirements from the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronics Equipment (WEEE) directives. SWR meters that bear a CSA Mark have been tested by the Canadian Standards Association (CSA) and meet applicable standards from organizations such as Underwriters Laboratories (UL). Recognized standards for safety and performance also include IEC 61010 from the International Electrotechnical Commission (IEC).