- SWR Meter Definition: An SWR meter, also known as a standing wave ratio meter, measures the mismatch between a transmission line and its load, typically an antenna, aiding technicians in impedance matching.
- Standing Wave Ratio (SWR): SWR measures the ratio of maximum to minimum voltage or current along a transmission line, reflecting the efficiency of impedance matching and indicating the presence of standing waves.
- Methods to Measure SWR: SWR can be measured using techniques such as slotted lines or directional SWR meters, which assess transmitted and reflected wave amplitudes to determine SWR values.
- How a Directional SWR Works: Directional SWR meters utilize directional couplers and diode bridges to measure the magnitudes of forward and reverse waves, assisting in determining impedance mismatches.
- How to Use an SWR Meter: To utilize an SWR meter effectively, users must find a clear frequency, reduce transmitter power, set the meter, adjust readings, switch between forward and reverse, and monitor interference and VSWR readings.
What is an SWR Meter?
A standing wave ratio meter, also called an SWR meter, ISWR meter (current I SWR) or VSWR meter (voltage SWR), measures the standing wave ratio on a transmission line. The reading shows how well a transmission line matches its load, usually an antenna. Technicians use that number to judge the impedance matching.
The meter compares radio-frequency energy sent toward the load with energy reflected back to the transmitter. A 1:1 ratio means a perfect match: no reflected wave on a lossless line.
The usual amateur-radio SWR meter uses a dual directional coupler. Each coupler samples a small amount of power travelling in one direction. A diode then rectifies that sample so the meter can display it.
A single coupler can be turned 180 degrees to sample either direction. Forward and reflected samples set the SWR. Two couplers, one for each direction, avoid that rotation.
A slotted line measures maximum and minimum voltage directly along the line. That method is used at VHF and above. At HF the slotted section would be too long for a bench instrument.
From HF through microwave, a directional coupler is the compact method. It does not need a long slotted section.
An ISWR meter reads current standing-wave ratio. A VSWR meter reads voltage standing-wave ratio. In radio work the two names are often used for the same instrument.
What is Standing Wave Ratio?
Standing wave ratio is the ratio of the highest radio-frequency voltage on the line to the lowest radio-frequency voltage on the same line. That voltage form of the ratio is called VSWR.
Current SWR is the ratio of the highest RF current on the line to the lowest RF current on the same line.
In physics these are stationary waves. They oscillate in time, but the peaks and nulls stay fixed along the line.
In radio and microwave work, SWR is the usual measure of impedance matching between a load and a transmission line. A mismatch creates a standing wave and raises line loss.
Operators also use SWR as a quick check on RF feeders, including some television and other coaxial runs. It reports match, not total system efficiency.
Methods to measure SWR
The most direct method uses a slotted line: a transmission-line section with a slot that lets a probe travel along it. The probe reads the voltage at each point on the line.
How does a Directional SWR work?
A directional SWR meter uses directional couplers to sample the amplitude of the forward and reflected waves.
In the diagram the transmitter and antenna share an internal line. Two directional couplers sit on that line. Each coupler feeds a resistors termination at one end and a diode bridge rectifiers detector at the other.
The resistors are chosen to match the coupler line impedance. The diodes convert the forward and reverse samples to DC voltages. capacitors then smooth those DC voltages.
How to use SWR meter?
These meters read standing-wave ratio on a transmitter feeder. Taking a reading is simple. Reading the number needs care, because feeder loss can hide a bad antenna match. In this use VSWR and SWR mean the same quantity.
When checking a new antenna, use low power and a clear channel. A typical meter procedure is:
- Finding a clear channel or frequency: Pick a frequency where you would not interrupt a two-way contact.
- Reduce power: Lower the transmitter output so a mismatch is less likely to stress the output stage.
- Set mode switch: Set CW, AM or FM as required by the radio and the test.
- Set VSWR meter: Set the meter switch to forward. Turn the CAL or sensitivity knob down so the needle is not driven off scale.
- Adjust forward reading: Transmit and turn CAL until the meter reads full scale.
- Switch Meter to Reverse: After that forward calibration, switch to reverse and read VSWR.
- Stop Transmitting: Unkey as soon as the reading is taken, to limit interference and transmitter stress.
- Check on other frequencies: If the antenna will be used across a band, repeat the check on other channels. VSWR usually changes with frequency.
How to setup and where to measure SWR?
Connect the meter in the feeder from the transmitter to the antenna. Place it at the transmitter end so the operator can watch the reading. That reading is the SWR at the meter, not automatically the SWR at the antenna.
Where the meter sits matters. Feeder loss changes the VSWR that the meter sees.
Feeder loss absorbs power in both directions. A lossy feeder reduces the wave on the way to the antenna and again on the way back. The reflected sample at the transmitter is then smaller than the reflection at the antenna.
A poorly matched antenna can therefore look acceptable at the transmitter end of a lossy feeder. The meter can read a low VSWR while the antenna still has a high VSWR.
SWR Formulas and Calculations
VSWR can be written from several measured quantities. By definition it is the ratio of maximum line voltage to minimum line voltage.
The same ratio can be written from the forward and reflected wave voltages.
Applications of SWR
The usual job for an SWR meter is installing and tuning a transmitting antenna.
SWR is also watched in microwave medical work such as ablation or electrosurgery. An antenna placed in tissue may not match the feedline, so a standing wave appears and reflected-power readings become less reliable.
