Feedline and SWR: What Every Ham Should Know

Ask a group of operators what SWR means and you will get a confident answer that is subtly wrong. The common belief is that a high SWR wastes power, and that a low SWR means a good antenna. The first is partly true and the second is not true at all.

What SWR actually measures

Standing wave ratio describes how well the impedance of the load matches the impedance of the feedline. A perfect match gives 1:1 and no reflected wave. A mismatch reflects part of the forward wave back toward the transmitter, and the two travelling waves interfere to produce a standing pattern on the line.

What SWR does not tell you is whether any of that power was radiated. A dummy load has a perfect 1:1 SWR and radiates nothing. A resonant antenna with a 1:1 match radiates everything it receives. The meter cannot tell them apart.

Why feedline loss matters more than SWR

The reason a mismatch is worth correcting is that feedline loss is not simply applied once. With a standing wave present, the loss on the line is multiplied by the additional loss caused by the mismatch, because the reflected wave makes a second pass through the same cable.

This is why a badly matched antenna on a long run of thin coax is genuinely worse than the same antenna on a short run of thick cable. The mismatch and the cable loss compound each other rather than adding.

Where to measure

An SWR meter measures at its own position in the line. A meter at the transmitter, with a tuner in circuit, is measuring the match the transmitter sees. The match on the feedline and at the antenna may be entirely different.

To understand the antenna itself, measure at the antenna end with an analyser that generates its own signal. This is the only measurement that tells you what the antenna is doing rather than what the tuner has hidden.