SWR and Transmitter Protection

The standing wave ratio (SWR) describes how well the antenna system is matched to the transmitter's 50 ohm output. It is shown live on the SWR meter of the Radio Deck, together with forward (FWD) and reflected (REF) power.

What SWR means

SWR 1.0 is a perfect match: all forward power reaches the antenna.
SWR 2.0 reflects about 11 percent of the power (about 0.5 dB loss).
SWR 3.0 reflects 25 percent (about 1.25 dB loss).
Very high SWR means the antenna barely accepts power at the operating frequency.

See Antenna Calculations for the arithmetic.

Where mismatch comes from

In SimTX the antenna is solved as physical wire geometry at the operating frequency, so SWR behaves as it does in reality:

Operating far from the antenna's resonant frequency raises SWR quickly. A wire length of 0 in the antenna form means "resonant at the operating frequency" and gives a good match on that frequency.
Antenna height, ground and geometry shift the feedpoint impedance.
Narrowband antennas (notably the full-wave loop) show a good match only over a small frequency range.
End-fed antennas are matched through a 49:1 transformer, which adds a small loss of its own, see End-Fed Half-Wave Antenna.

Transmitter protection

Like a real transceiver, the simulated transmitter folds back output power into a bad match to protect its finals:

Protection begins at about SWR 2.5.
Above that, output power is reduced progressively; a severe mismatch can cost several decibels on top of the mismatch loss itself.

The result is visible as reduced FWD power and reduced signal at receivers.

Practical advice

After changing frequency, check the SWR meter before a long transmission. The deck shows a local estimate even before the session starts.
Match the antenna to the band: set the wire length near resonance (or leave it at 0 for automatic resonant length), see Antenna Calculations.
A modest SWR up to about 2 is normal and costs little. Chasing a perfect 1.0 is rarely worth it.