Voltage Standing Wave Ratio (VSWR)


...As in all phases of electricity, to produce maximum transference of energy from one circuit to another the impedance of the two circuits must match. When the transmission line does not match the load impedance, not all of the energy fed down the line flows into the load. Some is reflected back, forming standing waves on the line. At every half wave point along the line, high-E and low-I points appear. Halfway between these points will be low-E and high-I points. The ratio of voltage across the line at the high-E points to that of the low-E points is known as the "Voltage Standing Wave Ratio", or VSWR. The SWR is also the ratio of the current values at the high and low points on the line.

...When the load impedance matches the line impedance, there will be no standing waves. The current at all points along the line is the same. The SWR is 1:1 and the line is said to be flat. "Changing the length of the line under these conditions will have no effect on its SWR".

...If standing waves appear on an antenna transmission line that should be flat, it is necessary either to change the transmission line impedance until it matches the antenna or to change the antenna impedance until it matches the line. This is important if optimum transmission or reception is desired from an antenna.

...73,
...NAO911/Jack


to continue this tutorial visit:: Transmission Line Dynamics and SWR


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