SWR Meters

A SWR meter, or standing wave ratio meter, is a device used to measure the amount of reflected power on a transmission line. It is typically used in radio frequency (RF) systems to ensure that the antenna is properly matched to the transmitter.

SWR meters work by using a directional coupler to sample the forward and reflected power on the transmission line. The directional coupler is a device that allows a small portion of the RF signal to be sampled without disrupting the main signal. The sampled signal is then detected and rectified to produce a DC voltage. The DC voltage is proportional to the power level of the RF signal.

The resulting Voltage SWR (VSWR) meter, the most common type, calculates the SWR by dividing the forward voltage by the reflected voltage giving the reflection constant:

Then

A SWR of 1:1 indicates that there is no reflected power, and the antenna is perfectly matched to the transmitter. A higher SWR indicates that there is more reflected power, and the antenna is not as well matched to the transmitter.

SWR meters are typically used as follows:

If the SWR is too high, it may be necessary to adjust the antenna or use an impedance matching device to improve the match.

SWR meters are important tools for ensuring optimal performance of RF systems. By measuring the SWR, operators can identify and correct impedance mismatches that can reduce transmit power and damage equipment.

Here are some of the applications of SWR meters:

Figure 1. Two wires can be inserted in a piece of foam insulated coaxial cable to make the directional coupler. 

Figure 2. A SWR Meter circuit using toroidal cores suitable for lower frequencies (100 KHz to 5 MHz). Forward and Reverse are connected to an AC voltmeter.

Figure 3. An example RF Volt Meter you could use to create a more sensitive SWR meter.