SWR Meter: What Is It & How To Use It

What is an SWR Meter?

A standing wave ratio meter – also known as an SWR meter, ISWR meter (current “I” SWR), or VSWR meter (voltage SWR) – is defined as a device that measures the standing wave ratio (SWR) in a transmission line. The SWR meter indirectly measures the degree of mismatch between a transmission line and its load (usually an antenna). This is helpful to know the effectiveness of the impedance matching performed by technicians.

SWR meter helps to determine how much of radiofrequency energy is reflected back to the transmitter compared to the amount of RF energy that is been sent out during operation. This ratio should not be high and the ideal rating is 1:1 such that the power reached the destination and reflects no power.

The common type of SWR meter used in amateur radio markets contains a dual directional coupler. Directional coupler samples a small amount of power in one direction. Later, a diode is used to rectify it before applying to the meter.

When using a single coupler, it can be rotated 180 degrees, to sample the power coming out from either direction. The forward and reflected power measured by the coupler is used to measure SWR. Else two couplers can be used one for each direction.

This method helps to measure the comparison between maximum and minimum voltage level values directly. It is used at VHF and higher frequencies. Cannot be used with low frequencies because the lines will be impractically long.

For HF through microwave frequencies, directional couplers can be used. They are more long and hence can be used at high frequencies.

The meter that is used to measure SWR is known as SWR meter. ISWR meter can measure current SWR and VSWR can measure voltage SWR.

What is Standing Wave Ratio?

The ratio of the maximum radio-frequency voltage to the minimum radio frequency voltage along the transmission line is known as Standing Wave Ratio (SWR). When SWR is recognised in terms of maximum and minimum AC voltage along the transmission lines, it is known as voltage SWR.

The ratio of the maximum RF current to the minimum RF current on the transmission line is known as current SWR.

Standing waves are known as stationary waves in physics. Such waves oscillate in time, but the amplitude does not move. Amplitude remains constant with times.

In microwave engineering and telecommunications, the measure of impedance matching of loads to the impedance of a transmission line is known as SWR. When there is a mismatch in the impedance, it results in standing waves along the transmission line, which increases the transmission line losses.

SWR is generally used to measure the efficiency of a communication line. This line can include other cables that allow radio frequency signals and TV cable signals.

Methods to measure SWR

Though there are various methods to measure SWR, the most intuitive method uses a slotted line, which is a section of transmission line. It contains an open slot, that allows to pass a probe to pass through. This probe helps to detect the actual voltage at various points all along the line.

How does a Directional SWR work?

Using directional couplers, a directional SWR meter is used to determine the amplitude of the transmitted and reflected waves.

From the above-given diagram, it is seen that the transmitter and antenna are connected through an internal transmission line. The line is electromagnetically coupled with two directional couplers. Then from there, it is connected to resistors at one end and diode bridge rectifiers at other ends.

The characteristic impedance of the lines can be matched with the help of resistors. Diodes are used to convert the magnitude of forwarding and reverse waves to their corresponding DC voltages. In the end, capacitors are used to smoothen the obtained DC voltages.

How to use SWR meter?

All forms of SWR meters measure SWR, standing wave ratio on a transmitter feeder. Using SWR meter is relatively easy, but one should be able to interpret the results when using it. In general, VSWR and SWR are the same.

When using VSWR meters to measure the the performance of a new antenna, always one should prefer to use low power and a clear channel. The following procedure can be used to use the meter.

Finding a clear channel or frequency: Should be able to hear one station in a two-way contract
Reduce power: The output power from the transmitter has to be reduced. This helps to reduce the damage to the output device of the transmitter.
Set mode switch: From the mode options like CW, AM or FM, the mode switch has to be set according to the application need.
Set VSWR meter: On the front panel, set VSWR meter switch to forward. Turn the CAL or adjustment knob down, to avoid overloading of the meter.
Adjust forward reading: When the transmitter transmits, adjust the knob on the CAL to obtain a full-scale reading
Switch Meter to Reverse: When the meter is calibrated for forward power, switch the meter to reverse position and read VSWR.
Stop Transmitting: To reduce the interference to other stations, and to reduce the overload on transmitter output, it is good to stop transmitting as soon as possible.
Check on other frequencies: When using a wide band of frequencies or channels, check the VSWR readings for other channels to be used, since VSWR tends to change over a range of frequencies.

How to setup and where to measure SWR?

The VSWR meter is connected in the feeder from the transmitter to antenna. For the feeder convenience, SWR meter is located at the transmitter end of the feeder. This helps to monitor the actual VSWR values.

One should know, where exactly SWR can be measured. There is always some degree of feeder loss, which has a major effect on the VSWR readings.

Feeder loss absorbs power in both directions. When the reflected signal is reduced, there is a high level of feeder loss. The reflected signal gets reduced as a forward signal to the antenna, later it’s reflected back to the transmitter.

Since the signal is reduced on its way to the transmitter, an antenna with poor match and high VSWR will be fine at the transmitter. Hence, when a feeder that induces a high loss is used, VSWR is good at the transmitter, but at the antenna, it may be poor.

SWR Formulas and Calculations

VSWR can be calculated from various parameters. By definition, VSWR is given as ratio of maximum voltage on the line to the minimum voltage.

$\begin{align*}VSWR=\frac{V_{max}}{V_{min}}\end{align*}$

The same can be expressed in terms of forward and reflected wave voltages.

$\begin{align*}VSWR=\frac{V_{fwd}+V_{ref}}{V_{fwd}-V_{ref}}\end{align*}$

Applications of SWR

The most common use of SWR Meters is during the installation and tuning of transmitting antennas.

SWR is being used in microwave-based medical applications and it has got a great impact on it. During microwave electrosurgery when an antenna is placed directly into the tissue, it may not always have an optimal match with the feedline that produces SWR.

Source: Electrical4u

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