Standing Wave Ratio explained

Imagine water flowing through a flexible pipe. Assume that the water is being pumped by a motor and the other end of the pipe is with us. Now the amount of water coming out of the other end can at best be equal to the amount of water sucked by the pump. Now consider there is a mild twist in the pipe. What does this mean to flowing water? Water sees a portion on pipe that is resisting flow and eventually sending back a portion of water coming towards the twist/bend. This is electrical terms can be told "Water faces a high resistance." In other words, until the bend the water was facing very low resistance and all of a sudden, at the bend it begins to face a much higher value of resistance. This mismatch in resistance is causing a portion of water coming in to flow back eventually causing what is known as "reflections" of water. In a nutshell, resistance mismatch on the line( caused by the bend) resulted in reflections and in the process it is worth to note that not all water sucked by the pump is reaching the other end.

Let us now migrate to transmission line parlance and connect the above scenario with that world. Energy sent down the transmission line is guaranteed to reach the other end "maximum" only when there is no change in the impedance as seen through out the line. When there is a impedance change on the line, there is said to be an impedance mismatch. And when there is an impedance mismatch, RF energy is partially or completely reflected back. A way to quantify these reflections is Standing wave ratio. Reflected waves interact with incident-forward travelling waves to form what is known as standing wave in which the positions of maximum and minimum are constant. For a symmetrical wave sent down, the ratio of maximum and minimum should be the same in magnitude. This ratio of maximum to minimum is called as standing wave ratio. Lower the standing wave ratio, better is the matching and higher is the power delivered. Higher the standing wave ratio, worse is the matching, higher are the reflections and lesser is the power delivered.

Waveguides

In this write up, I attempt to demystify propagation of energy in waveguides without touching upon complex mathematics which anyway is found in literature. Traditionally, at lower frequencies upto VHF and UHF, coaxial cable has been obvious choice for transmission of energy. However, skin effect severely degrades the performance of such lines at high frequencies and this presses us to look at an alternative that could let us transmit energy not in the form of currents but in the form of fields. Simply using an antenna to radiate fields in to free space is not viable due to the inherent path losses involved. We look for a setup that could 'guide' energy in the form of fields from one point to another.

It is well known that the reflection coefficient of a surface made out of pure conductor is -1. Which means that an EM wave incident on a metallic surface is turned back completely with its phase inverted by 180 degrees. If we could find an arrangement such that the EM waves are completely reflected back into a region from 'all' sides and the arrangement is such that the waves are also carried forward, what we arranged is a waveguide.

This is precisely what a waveguide does. It is made out of a nearly perfect conductor to make the reflection coefficient as close to -1 as possible. This would mean little penetration of fields into the thickness of the conductor and hence little losses in the walls.

Having established that wave propagation is indeed possible by multiple reflections from all sides, Iam rather tempted to assert at this point that, not all frequencies are willing to go down the waveguide of given dimensions.

In my coming posts, I will attempt to explain further as what kind of arrangement accommodates a given frequency. I hope you enjoyed reading this. A rather lucid explanation is found in the book "Electronic communication systems" by John.F.Kennedy.