Standing Wave Ratio

 

Standing wave ratio, earlier known as voltage standing wave ratio is the ratio of the difference between the maximum and minimum of a voltage when a transmission happens in a channel. The feedline has a characteristic impedance that depends on the physical construction of transmission media. When the antenna and the characteristic impedance of both match, the energy is transmitted properly, and if there is a mismatch, then some of the signals travel back toward the source. This backward flow of energy constructively and destructively interferes with the forward signals and thus forms standing waves. As mentioned earlier, the standing wave ratio is the difference between the maximum and minimum voltages. If the maximum voltage is 9 volts and the minimum voltage is 4.5 volts, the SWR turns out to be 2. Here we need to understand if the reflected energy is large, resulting in large peaks, and troughs would have a large difference. This results in a large difference between maximum and minimum points and consequently translate into a high SWR, potentially causing damage to the source transmitter. For this reason, protection and monitoring circuits are now installed to keep track of SWR.

To prevent SWR from reflection energy is to use a device called an antenna tuner, the tuners adjust the signal energy to match the forward energy to lower the SWR, usually a pair of these devices one close to the transmitter source and the other one close to the transmission feed line to get the best performance. The ideal ratio for SWR is considered to be 1:1, neglecting the line losses and frequency distortion. This optimum condition can only be achieved when the characteristic impedance of the transmission source and the line feeds match.

 

Expression To Calculate SWR:

The standing wave ratio is given by the ratio of maximum voltage to the minimum voltage as below:

However, it can also be calculated using the reflection coefficient as the SWR is deeply related to this factor. The following equation explains the relationship between SWR and reflection coefficient:

SWR And System Efficiency:

For the ideal efficiency, the impedance between the source and the transmission feed needs to be in sync to perform optimally; however, there is always a mismatch due to the reflected power towards the source in real systems. These mustaches are mainly due to destructive interference and occur due to the distance between the feeding lines. SWR can be reduced by using the matching characteristic impedance. The equipment used also needs maintenance as old equipment can degrade the performance resulting in an impedance mismatch.