SWR Meter: 9 Important Explanations

Cover Image BY: Theonlysilentbob at English Wikipedia, CC BY-SA 3.0, Link

Points of Discussion

Introduction to SWR Meter | What is an SWR meter?

To know about Standing Wave Ratio Meters, we should know what is SWR at first. SWR is an Acronym of “Standing Wave Ratio”, and it is defined as follow.

SWR or Standing Wave Ratio: Standing Wave ratio is defined as the ratio of the maximum RF voltage to the minimum RF voltage of a transmission line.

When the ratio is calculated with respect to the AC voltage, then the parameter will be called voltage SWR, and if the ratio is calculated with respect to current, then the SWR will be known as current SWR.

Standing waves are physically stationary waves but not like typical ones as the amplitude doesn’t change with respect to time. SWR is necessary for the measurement of impedance matching for loads of transmission lines in Microwave Engineering.

What is an SWR Meter?

SWR Meter: SWR Meter or Standing Wave Ratio meter or Wave Ratio Meter or VSWR Meter (Voltage Wave Standing Ratio Meter) measures the value of Standing Wave ratio of a transmission line.

A typical Standing Wave Ratio Meter, Image BY: Theonlysilentbob at English Wikipedia, CC BY-SA 3.0, Link

The Standing Wave Ratio Meter actually measures the amount of mismatch present between the load and the transmission line associated with it. It also determines the amount of RF energy reflected by the transmitter.

The most common type of standing wave ratio meter contains a dual directional coupler which other samples out some amount of power in a direction. After that, a diode does some rectifications and applies to the meter.

This method of operation finds out a comparison between the minimum and maximum level of voltages. The standing wave ratio meters is applicable and useful for signal ranging from very high frequencies and above. It cannot be used for low-frequency signals.

VSWR meters measures voltage Standing Wave Ratio and ISWR meters measure current standing wave ratio.

Methods of Measurement of SWR

There many different methods available for measurement of SWR. The simplest method is the method of using a slotted line. The slotted line is a part or component of transmission lines with an uncluttered slot through which a probe gets passed. The probe does the main thing by allowing to measure voltages at various points.

Real World Applications of SWR Meter

Standing Wave Ratio Meter is one of the most critical and crucial devices for Microwave Engineering. SWR meters are widely used for setting up Antennas and connecting the antennas with their transmission lines. SWR Meters are also used for medical applications which are based on microwave engineering.

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Working of an SWR Meter | How does an SWR work?

Let us discuss how an SWR Meter works or how a directional SWR works. A directional SWR meter is necessary to measure the amplitude of the transmitted wave as well as the amplitude of the reflected wave.

Directional SWR Meter
Directional SWR , Image Credit: InductiveloadSWR Meter, marked as public domain, more details on Wikimedia Commons

As the image shows, there is a transmitter (Tx) and an antenna (ANT) terminal connected with the help of a transmission line. Here, the significant line electromagnetically couples with the directional couplers. Resistors terminate the lines at one of the ends and diodes are connected at another end for rectification purposes.

The resistors help to match the characteristic impedance of the transmission lines, and the diodes allow the conversion of the amplitudes of waves to their equivalent DC voltage. At last, capacitors smoothen the final DC voltage. There are also connected amplifiers with the forward and reverse terminals. They function as the needed drain resistor and help to determine the Dwell Time.

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How to use an SWR meter?

VSWR meters or typical Standing Wave Ratio meters are easy to use and measure the standing wave ratio. The process of using an Standing Wave Ratio meter while doing an experiment or applying for other purposes are listed below. The steps will help to interpret the result from the meters.

An important point to be noted before using VSWR is that: VSWR should be used at low power and in the clear channel, primarily if the experiment aims to measure an antenna’s performance.

  • Step 1: Find a clear frequency channel – The frequency channel should be clear enough or noiseless enough so that the transmitted signal from both ends could be interpreted from both sides.
  • Step 2: Reduction of Power – The transmitted power should not exceed a specific power range so that the signal causes distortion at the output devices.
  • Step 3: Set up the Mode – The mode of operation should be set using the options available on the meter. Like – Amplitude Modulation, Frequency Modulation etc.
  • Step 4: Set up of Meter – Now we need to set the Standing Wave Ratio meter to the forward mode. To do so, check the front panel. Also, switch the adjustment knob downwards. It will help to restrict overloading.
  • Step 5: Adjustment of the Forward Reading – After the transmitter starts its transmitting job, keep adjusting the CAL knob to ensure a full-scale reading of the experiment.
  • Step 6: Set up of Meter – Now the Standing Wave Ratio Meter is set up again. This time the knob on the front panel is changed to ‘Reverse’ direction. This is done after the meter is set for forwarding power.
  • Step 7: Restrict the transmission – The transmission is stopped as soon as possible to restrict the VSWR meter’s overloading.
  • Step 8: Repeat the above steps for various frequencies – Took the readings for several other frequencies by following the same steps.

SWR Bridge

Impedance bridge is also capable of measuring Standing wave ratio. Impedance bridge is an LCR meter. When the given impedance gets matched with the reference impedance, the bridge gets balanced. If a transmission line gets mismatched, there is some deviation of input impedance, and that could lose the bridge’s balance. That is how a bridge can measure if there is some amount of SWR present in the connection.

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SWR Meter Reading Explanations

Let us discuss the reading collected by Standing Wave Ratio Meters. Different values of Standing Wave Ratio meters describe different conditions.

Range of valueExplanation
SWR 1 to 1.5It is considered as the ideal range of value. The reading can be decreased to 1 from 1.5 by doing some additional tuning.
SWR 1.5 to 1.9It is not the best range, but fair enough. Such readings come from vehicle variables or installations faults. It is an acceptable range.
SWR 2.0 to 2.4It is not a good range of value. There are scopes for improvement. This range of values come from poor antenna mounting location. Fixing that problem might improve your meter’s SWR value.
SWR 2.5 to 2.9This range of value affects performance and lead to wrong impressions. The transmitter may also get a damaged—poor quality of equipment and needy mounting causes this range of values.
SWR >  3Operation with this range of values is risky. The device will be damaged within a few moments. The transmission should be stopped at this range. The reason behind such worse values is major installation problems. Attach a proper ground with the device. This range can also indicate poor quality of antenna and faulty coaxial cables.

Point to be noted: Do not transmit a signal if the SWR range exceeds the range of 1.5 to 2. It will damage the transmitter. If you observe the reading is more than 2.5, shut down the transmitter as soon as possible.

Essential Formulas For calculation of SWR using SWR Meter

The formula for calculating VSWR is Vmax / Vmin.

Also, the expression for VSWR using the forward and reverse wave voltages can be written as: VSWR = (VFWD + VREV) / (VFWD – VREV)

There is another formula for calculation of SWR.

SWR = | 1 + Г| / | 1 – Г|

Digital SWR Meter

Nowadays, most of the analogue SWR meters are replaced by Digital SWR Meters. Digital SWR Meters are easier to use, take less time to get the result, smaller in size, and lower the maintenance cost than an analogue meter.

Limitations of Standing Wave Ratio (SWR) Meters

SWR Meters does not measure the physical impedance present for a load. Instead, it measures a ratio which gives us the idea of mismatch. The perfect impedance for the load can be measured using a separate device known as – “Antenna Analyzer”.  The measurement is only possible if the SWR meter is set perfectly with the transmission line itself. It is matched with the characteristic impedance of the transmission line (generally 50 to 70 ohms).

SWR meters must be set up as close as possible concerning the transmission line. Otherwise, SWR creates some false impression regarding the readings.

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