SWR meter

Last updated July 20, 2021

The standing wave ratio meter, SWR meter, ISWR meter (current " $I$ " SWR), or VSWR meter (voltage SWR) measures the standing wave ratio (SWR) in a transmission line.^{[lower-alpha 1]} The meter indirectly measures the degree of mismatch between a transmission line and its load (usually an antenna). Electronics technicians use it to adjust radio transmitters and their antennas and feedlines to be impedance matched so they work together properly, and evaluate the effectiveness of other impedance matching efforts.

Directional SWR meter

A directional SWR meter measures the magnitude of the forward and reflected waves by sensing each one individually, with directional couplers. A calculation then produces the SWR.

Referring to the above diagram, the transmitter (TX) and antenna (ANT) terminals connect via an internal transmission line. This main line is electromagnetically coupled to two smaller sense lines (directional couplers). These are terminated with resistors at one end and diode rectifiers at the other. Some meters use a printed circuit board with three parallel traces to make the transmission line and two sensing lines. The resistors match the characteristic impedance of the sense lines. The diodes convert the magnitudes of the forward and reverse waves to the terminals FWD and REV, respectively, as DC voltages, which are smoothed by the capacitors.^[1]^(p27‑21) The meter or amplifier (not shown) connected to the FWD and REV terminals acts as the required drain resistor, and determines the dwell-time of the meter reading.

To calculate the SWR, first calculate the reflection coefficient:

\Gamma ={\frac {V_{rev}}{\;V_{fwd}\;}}

(the voltages should include a relative phase factor).

Then calculate the SWR:

{\mathsf {SWR}}={\Biggl \|}\,{\frac {1+\Gamma }{\;1-\Gamma \;}}\,{\Biggr \|}~.

In a passive meter, this is usually indicated on a non-linear scale.

Radio operators' SWR meters

For decades ^[2]^[3] radio operators have built and used SWR meters as a simple tuning and diagnostic tool. With shielding compromised, a pair of coax or twinline transmission lines, placed close enough, suffer crosstalk. A wave moving in the driven line induces waves in the measurement line. Placed in parallel (straight or loosely coiled) a driven wave reinforces or cancels an induced wave in the same or opposite direction. If the cable pair exceeds half wavelength, cancellation is complete, and power dissipated in matched termination is approximately proportional to the forward and reflected power.

The approximation improves as crosstalk weakens and harmonic number increases. Over time, nonlinear high gain amplifiers have replaced nonlinear electro-mechanical movements – which replaced incandescent bulbs – to require less cross-talk and improve linear frequency range.

Because all frequencies above minimum contribute, the measured ratio is a single frequency quality measure, that increases with unintended harmonics and spurious emissions, as well as actual SWR. By analogy, the measurement cable is a crystal radio (non-discriminating receiver) representing all the radio receivers that might suffer interference from dirty emissions. Though called an SWR Meter, a low measured ratio indicates not only good match, but also clean A3, F3, or G3 emission without excessive harmonics nor spurious (out-of-channel) power.

SWR bridge

SWR can also be measured using an impedance bridge. The bridge is balanced (0 Volts across the detector) only when the test impedance exactly matches the reference impedance. When a transmission line is mismatched (SWR > 1:1), its input impedance deviates from its characteristic impedance; thus, a bridge can be used to determine the presence or absence of a low SWR.

To test for a match, the reference impedance of the bridge is set to the expected load impedance (for example, 50 Ohms), and the transmission line connected as the unknown impedance. RF power is applied to the circuit. The voltage at the line input represents the vector sum of the forward wave, and the wave reflected from the load. If we know the characteristic impedance of the line is 50 Ohms, we know the magnitude and phase of the forward wave. It's the same wave present on the other side of the detector. Subtracting this known wave from the wave at the line input yields the reflected wave. Properly designed, a bridge circuit can not only indicate a match, but the degree of mismatch – making it possible to calculate the SWR. This usually involves alternately connecting the reference wave and the reflected wave to a power meter, and comparing the magnitudes of the resulting deflections.^[1]^(p27‑03)

Limitations

An SWR meter does not measure the actual impedance of a load (the resistance and reactance), but only the mismatch ratio. To measure the actual impedance requires an antenna analyzer or other similar RF measuring device. For accurate readings, the SWR meter itself must also match the line's impedance (typically 50 or 75 Ohms). To accommodate multiple impedances, some SWR meters have switches that select the impedance appropriate for the sense lines.

An SWR meter should connect to the line as close as possible to the antenna: All practical transmission lines have a certain amount of loss, which attenuates the reflected wave as it travels back along the line. Thus, the SWR is highest closest to the load, and only improves as the distance from the load increases, creating the false impression of a matched system.^[1]^(p28‑07)

Footnotes

↑ The terms ISWR (current standing wave ratio) and VSWR (voltage standing wave ratio) are sometimes used to emphasized the method by which the measurement is made, however, in the absence of measurement errors, the two numbers are identical. The circumspect term SWR is preferred to avoid false precision.^[1]

Related Research Articles

In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected wave to the incident wave, with each expressed as phasors. For example, it is used in optics to calculate the amount of light that is reflected from a surface with a different index of refraction, such as a glass surface, or in an electrical transmission line to calculate how much of the electromagnetic wave is reflected by an impedance. The reflection coefficient is closely related to the transmission coefficient. The reflectance of a system is also sometimes called a "reflection coefficient".

In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be caused by a mismatch between the termination or load connected to the line and the characteristic impedance of the line. It is usually expressed as a ratio in decibels (dB);

In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line.

Waveguide Structure that guides waves, with minimal loss of energy by restricting the transmission of energy to one direction

A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities decrease according to the inverse square law as they expand into three dimensional space.

In electrical engineering, electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.

Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable cause impedance mismatches and non-linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected. In radio frequency (RF) practice this is often measured in a dimensionless ratio known as voltage standing wave ratio (VSWR) with a VSWR bridge. The ratio of energy bounced back depends on the impedance mismatch. Mathematically, it is defined using the reflection coefficient.

In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load. A source of electric power such as a generator, amplifier or radio transmitter has a source impedance equivalent to an electrical resistance in series with a frequency-dependent reactance. Likewise, an electrical load such as a light bulb, transmission line or antenna has an impedance equivalent to a resistance in series with a reactance.

Twin-lead Two-conductor flat cable used to carry radio frequency signals

Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic ribbon. The uniform spacing of the wires is the key to the cable's function as a transmission line; any abrupt changes in spacing would reflect some of the signal back toward the source. The plastic also covers and insulates the wires. It is available with several different values of characteristic impedance, the most common type is 300 ohm.

In electronics, electrical termination is the practice of ending a transmission line with a device that matches the characteristic impedance of the line. Termination prevents signals from reflecting off the end of the transmission line. Reflections at the ends of unterminated transmission lines cause distortion which can produce ambiguous digital signal levels and mis-operation of digital systems. Reflections in analog signal systems cause such effects as video ghosting, or power loss in radio transmitter transmission lines.

An antenna tuner is a device that is inserted between a radio transmitter and its antenna; when properly adjusted (tuned) it improves power transfer by matching the impedance of the radio to the impedance of the antenna, as it appears end of the feedline connected to the antenna tuner, with the other end connecting to the antenna.

The input impedance of an electrical network is the measure of the opposition to current (impedance), both static (resistance) and dynamic (reactance), into the load network that is external to the electrical source. The input admittance (1/impedance) is a measure of the load's propensity to draw current. The source network is the portion of the network that transmits power, and the load network is the portion of the network that consumes power.

Dummy load Device used to simulate an electrical load

A dummy load is a device used to simulate an electrical load, usually for testing purposes. In radio a dummy antenna is connected to the output of a radio transmitter and electrically simulates an antenna, to allow the transmitter to be adjusted and tested without radiating radio waves. In audio systems, a dummy load is connected to the output of an amplifier to electrically simulate a loudspeaker, allowing the amplifier to be tested without producing sound. Load banks are connected to electrical power supplies to simulate the supply's intended electrical load for testing purposes.

An antenna analyzer or in British aerial analyser is a device used for measuring the input impedance of antenna systems in radio electronics applications.

In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from the transmitter to the antenna, where it is radiated as radio waves. In a receiving antenna it transfers the tiny RF voltage induced in the antenna by the radio wave to the receiver. In order to carry RF current efficiently, feed lines are made of specialized types of cable called transmission line. The most widely used types of feed line are coaxial cable, twin-lead, ladder line, and at microwave frequencies, waveguide.

A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver with the antenna and makes the two devices compatible. In a radio transmitter, the transmitter generates an alternating current of radio frequency, and the feed system feeds the current to the antenna, which converts the power in the current to radio waves. In a radio receiver, the incoming radio waves excite tiny alternating currents in the antenna, and the feed system delivers this current to the receiver, which processes the signal.

A test probe is a physical device used to connect electronic test equipment to a device under test (DUT). Test probes range from very simple, robust devices to complex probes that are sophisticated, expensive, and fragile. Specific types include test prods, oscilloscope probes and current probes. A test probe is often supplied as a test lead, which includes the probe, cable and terminating connector.

In physics a null is a point in a field where the field quantity is zero as the result of two or more opposing quantities completely cancelling each other. The field may be scalar, vector or tensor in nature. Common situations where nulls arise are in the polar patterns of microphones and antennae, and nulls caused by reflections of waves.

Mismatch loss in transmission line theory is the amount of power expressed in decibels that will not be available on the output due to impedance mismatches and signal reflections. A transmission line that is properly terminated, that is, terminated with the same impedance as that of the characteristic impedance of the transmission line, will have no reflections and therefore no mismatch loss. Mismatch loss represents the amount of power wasted in the system. It can also be thought of as the amount of power gained if the system was perfectly matched. Impedance matching is an important part of RF system design; however, in practice there will likely be some degree of mismatch loss. In real systems, relatively little loss is due to mismatch loss and is often on the order of 1dB.

Slotted lines are used for microwave measurements and consist of a movable probe inserted into a slot in a transmission line. They are used in conjunction with a microwave power source and usually, in keeping with their low-cost application, a low cost Schottky diode detector and VSWR meter rather than an expensive microwave power meter.

References

1 2 3 The ARRL Antenna Book (21st ed.). The American Radio Relay League, Inc. 2007. ISBN 0-87259-987-6.
↑ Grebenkemper, John, KI6WX (1997). "The Tandem match – An accurate directional wattmeter". Handbook for Amateur Radio (PDF). The American Radio Relay League, Inc. Chapter 22: Station setup and accessory projects, pages 22.36–22.42.^{[ full citation needed ]}
↑ Kaune, Bill, W7IEQ (2012). "A modern directional power/SWR meter". Handbook for Amateur Radio (PDF). The American Radio Relay League, Inc.

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[2] The terms ISWR (current standing wave ratio) and VSWR (voltage standing wave ratio) are sometimes used to emphasized the method by which the measurement is made, however, in the absence of measurement errors, the two numbers are identical. The circumspect term SWR is preferred to avoid false precision.^[1]

[ARRL_Ant2007-1] 1 2 3 The ARRL Antenna Book (21st ed.). The American Radio Relay League, Inc. 2007. ISBN 0-87259-987-6.

[KI6WX_1997-3] Grebenkemper, John, KI6WX (1997). "The Tandem match – An accurate directional wattmeter". Handbook for Amateur Radio (PDF). The American Radio Relay League, Inc. Chapter 22: Station setup and accessory projects, pages 22.36–22.42.^{[ full citation needed ]}

[Kaune2012-4] Kaune, Bill, W7IEQ (2012). "A modern directional power/SWR meter". Handbook for Amateur Radio (PDF). The American Radio Relay League, Inc.

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