SWR: what it actually means

SWR — standing wave ratio — is the number every new operator watches obsessively and very few can explain. Here is the version that actually helps.

Where standing waves come from

Your transmitter expects to see a 50 ohm load. Your feedline is 50 ohm coax. If the antenna at the far end also looks like 50 ohms, all the power travelling down the line is absorbed and radiated.

If the antenna presents something other than 50 ohms, some of that power cannot be absorbed. It reflects back down the line. The forward and reflected waves interfere, creating fixed patterns of high and low voltage along the cable — standing waves.

SWR is the ratio between the maximum and minimum voltage in that pattern.

• 1:1 — nothing reflected. Perfect match.
• 2:1 — about 11% of the power is reflected.
• 3:1 — about 25% reflected.
• Infinite — everything reflected, as with an open or short circuit.

What high SWR actually costs you

Three real consequences, in order of how much they should worry you:

1. Transmitter protection kicks in. Modern solid-state finals fold back their output power when SWR rises, to avoid destroying the output devices. You end up transmitting less power — often the first thing you notice.
2. Extra loss in the feedline. Reflected power travels back down the cable and gets partly absorbed as heat. On a short run of good coax at HF this is small. On a long run of lossy cable at UHF it is significant.
3. RF where you do not want it. A badly matched system is more likely to put RF on the outside of the coax braid and into your shack.

What high SWR does not do is change your operating frequency or damage the antenna itself.

What an antenna tuner really does

This trips people up. An antenna tuner (or ATU) does not fix the SWR on the feedline between the tuner and the antenna. Those standing waves are still there.

What it does is present a 50 ohm load to the transmitter, so the transmitter is happy and delivers full power. The mismatch — and its associated loss — still exists beyond the tuner.

A tuner makes your radio comfortable. It does not make your antenna correct.

If the feedline loss is low, that trade is fine. If the loss is high, you are just heating your coax more efficiently.

Baluns

A dipole is a balanced antenna: two equal legs. Coax is unbalanced: an inner conductor and a shield. Connect them directly and RF flows on the outside of the braid, which turns your feedline into part of the antenna. That distorts the radiation pattern and brings RF back into the shack.

A balun (BALanced to UNbalanced) stops this. It is not optional on a coax-fed dipole — it is part of doing it properly.

What to actually aim for

Under 2:1 across the part of the band you use is a good practical target. Chasing 1.0:1 exactly is usually wasted effort — the difference between 1.0 and 1.5 is a fraction of a decibel, which nobody on the other end will ever notice.