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{{refimprove|date=May 2011}}
[[Image:Para dynamics swrmeter dec07.jpg|thumb|right| An SWR meter to be used with CB radio equipment]]
The '''SWR meter''' or '''VSWR (voltage standing wave ratio) meter''' measures the [[standing wave ratio]] in a transmission line. The meter can be used to indicate the degree of mismatch between a [[transmission line]] and its [[Electrical load|load]] (usually a [[antenna (radio)|radio antenna]]), or evaluate the effectiveness of [[impedance matching]] efforts.

==Directional SWR Meter==
A directional SWR meter measures the magnitude of the forward & reflected waves by sensing each one individually, with [[directional coupler]]s. A calculation can then be performed to arrive at the SWR.

[[File:SWR Meter.svg|400px|centre|thumb|A simple directional SWR meter]]

Referring to the above diagram, the transmitter ('''TX''') and antenna ('''ANT''') terminals are connected via an internal transmission line. This main line is electromagnetically coupled to two smaller sense lines (directional couplers) which are terminated with resistors at one end, and diode rectifiers at the other. Sometimes a [[printed circuit board]] using three parallel traces is used to make the transmission line and the two sensing lines. The resistors are chosen to match the characteristic [[Electrical impedance|impedance]] of the sense lines. The diodes convert the magnitudes of the forward & reverse waves to '''FWD''' and '''REV''' DC voltages, respectively, which are then smoothed by the capacitors.<ref>ARRL Antenna Book, 2007, p. 27-21.</ref>

To calculate the VSWR, first calculate the reflection coefficient:
:<math>\Gamma =  \frac{V_{rev}}{V_{fwd}}</math>

Then calculate the VSWR:
:<math>VSWR  =  \frac{1 + \Gamma}{1 - \Gamma}</math>

In a passive meter, this is usually indicated on a non-linear scale.
[[File:SWR & power meter inner view.jpg|thumb|right|Interior view of an SWR meter. The three parallel coupled lines are visible. Diodes, capcitors and termination resistors are mounted at the ends of the sense lines.]]

==SWR Bridge==
SWR can also be measured using an impedance [[bridge circuit]]. 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. [[Radio frequency|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 the characteristic impedance of the line is known to be 50 ohms, we know the magnitude and phase of the forward wave; it is the same wave present on the other side of the detector. Subtracting this known wave from the wave present at the line input yields the reflected wave. Properly designed, a bridge circuit can be used not only to indicate a match, but the degree of mismatch - thus 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.<ref>ARRL Antenna Book, 2007, p. 27-3.</ref>

==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, an [[antenna analyzer]] or other similar RF measuring device is required. For accurate readings, the SWR meter must be matched to the line impedance, usually 50 or 75 ohms. To accommodate multiple impedances, some SWR meters have switches on the rear, to select the resistance appropriate for the sense lines.

An SWR meter should be connected to the line as close as possible to the antenna: All practical transmission lines have a certain amount of loss, which causes the reflected wave to be attenuated 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.<ref>ARRL Antenna Book, 2007, p. 28-7.</ref>

The internal diodes of an SWR meter may generate objectionable [[harmonic]]s when transmitting and produce [[intermodulation]] products during reception, if it is left in the system after measurement. Active SWR meters do not usually suffer from this effect, and may be left in without causing such problems.

==Notes==
{{Reflist}}

==References==
* {{Citation
  |url=http://www.arrl.org/catalog/?category=Antennas,+Transmission+Lines+%26+Propagation#6133
  |title= The ARRL Antenna Book
  |edition= 21st
  |year= 2007
  |publisher= The American Radio Relay League, Inc.
  |isbn= 0-87259-987-6
  |doi=}}
{{Commons category|SWR meters}}

[[Category:Measuring instruments]]
[[Category:Radio electronics]]