Monday, November 19, 2012

SWR, VSWR, ISWR, PSWR


Let there be math. In the political brouhaha of the election season there was a lot of rhetoric and precious little math. The season is over but I have sought refuge in real, inarguable answers in the land of math. So let's talk about VSWR. In radio, the standing wave ratio (SWR) is the ratio between the amplitude of a standing wave at it's maximum, to the amplitude at the minimum. This is usually defined as a voltage ratio called the VSWR. Some radio geeks actually pronounce the acronym as "viswar."  This ratio is expressed like any other ratio, A VSWR of  2:1 describes a standing wave amplitude that is twice it's minimum standing wave value.  VSWR values are always a positive integers (or infinity), and the ratio can be no lower than 1:1, which actually is ideal. Effectively this describes the power reflected from the antenna. In that case  the voltage would have a constant magnitude.

We also can describe this ratio in terms of current which is an IWSR. At first that makes no sense because Current is spelled with a "C." But by convention, the symbol for current is "I" and you can blame the French for it. the conventional variable originates in the phrase "intensité de courant" which was abbreviated as "I" by  André-Marie Ampère as early as 1820. PWSR is more simply the Power Standing Wave Ratio, which is just the square of the VSWR.

But this all has a purpose. VSWR is used as an efficiency measure for everything conveying RF includes transmission lines, electrical cables and even the signal in the air. In application this figure describes how well the antenna is impedance matched to the radio or transmission line it is connected to. In a connected line an impedance mismatch can cause reflection, which is just what it sounds likea wave bouncing back and going the wrong direction. The result of the opposing waves is a standing wave. This diminishes the power the antenna receives and can use to broadcast. It can even burn out a transmitter. More here.

There are other important bits to be measured in antenna efficiency: the reflection coefficient, the mismatch loss, and the return loss to name a few. VSWR isn't the end-all be-all of antenna theory, but it's important. Thankfully they make meters for this stuff.