VSWR / Return Loss

In any wave system, whether it be physical (percussion/sound, water) waves, or electromagnetic, the interaction between any wave and a boundary will result in a reflection. To illustrate this concept, the simplest example might be a sound wave hitting a thin wall, whereby some of the wave's energy is reflected off the wall back towards the source, with the remainder passing through the wall to be heard in the next room. The amount of energy being reflected vs amount transmitted is measured with VSWR (when expressed as a ratio), or Return Loss (when measured in decibels).

The simplest way to understand the parameter is considering these parameters as describing how well the transmission is 'absorbed' by the component. Return Loss, sometimes referred to as S11, is a little more intuitive, which expresses the absorption (or transfer) as a number of decibels - to provide a simple example, an antenna with a Return Loss of 10 dB can be thought of saying that the reflected energy is 10 dB lower than the input. Putting some numbers behind this example, a 20 W input would have a 2 W reflection and as such 18 W is absorbed by the antenna.

VSWR is slightly less intuitive but refers to the same measurement. In our example above, the power (dB) of the reflected sound versus the power (dB) of the sound present in the other room can be expressed as a ratio. Should almost all of the sound's energy pass through the wall we would have a ratio approaching 1:1, conversely if almost all sound was reflected back as an echo we would have an extremely high ratio, perhaps 10:1. These ratios can be directly converted into Return Loss, for example a perfectly matched system with a VSWR of 1.1:1 would have a Return Loss of 26 dB. Conversely a poorly matched system with a VSWR of 10:1 would have an RL of only 1.7 dB.

Understanding these concepts in our more familiar world of RF is not a particularly big mental leap. In the case of antennas; our transmission is an electromagnetic wave rather than a percussion wave, our medium now copper wire, and our interface boundary is the metal-on-metal contact between connection points whether that be between two coaxial connectors or a solder contact on a PCB. The physics however remains the same.

Intuitively metal contacts are substantially more efficient at transferring energy, but less intuitive is the susceptibility of the much higher frequencies we are now operating at. To stress how important VSWR and RL are to your network, consider a reduction in performance from VSWR of 1.3:1 to 1.5:1 - this is a change in Return Loss of 16 dB to 13 dB. This 3 dB reduction in radiating/receiving performance means that to compensate your radio must now work harder, and consequently in the case of IoT systems reducing battery life (by having to broadcast louder) and reducing network capacity and performance (due to the lower modulation and coding scheme).

Accordingly matching of the antenna to its transmission line is absolutely critical to the performance of not just the individual device, but the entire network.

Understanding VSWR and Return Loss is essential for anyone involved in the design, installation, or maintenance of radio frequency systems. These parameters serve as critical indicators of how well an antenna is matched with its transmission line and other components. Poor matching can lead to significant performance degradation, resulting in decreased network capacity, reduced battery life in IoT devices, and even potential damage to components due to increased heat from reflected energy.

By giving attention to these parameters, you can optimise the efficiency and reliability of your RF system. Whether you're working on a large-scale telecommunications network or a smaller IoT setup, achieving low VSWR and high Return Loss values is key to ensuring robust and reliable wireless communication.

The below table can be used to convert between VSWR and Return Loss

It's important to understand the practical consequences of VSWR and Return Loss on system performance, in which the effect is realised through Insertion Loss. For a given power input PI, VSWR will cause a reflection, and accordingly a loss of power PR, resulting in the effective transfer of power PO.

For poorly tuned antennas PR represents a significant loss of system efficiency, resulting in reduced communications range.