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.

If the SWR reading is high

SWR measures the antenna match. It does not measure audio quality or how far a station can be heard.

1. Confirm the frequency and antenna length belong to the same setup.
2. Check whether you are looking at a local estimate or a selected running session.
3. Adjust the antenna length and watch the estimate. A length of zero is a nominal template, not a guarantee of a good match.
4. Recheck after moving to another frequency. Do not compensate for a badly detuned antenna only by increasing power.

Around 1 to 2 is a useful practical match. The transmitter reduces power above the protection threshold described below. Antenna System gives the tuning workflow.

What SWR means

• SWR 1.0 is a perfect match: no power is reflected by the mismatch. Feedline and antenna losses can still reduce radiated power.
• 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

SWR in SimTX behaves as it does in reality:

• Operating far from the antenna's resonant frequency raises SWR quickly. A wire length of 0 selects the type's nominal template. It does not solve for a perfect impedance match, and the EFHW template remains 32.5 m.
• 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 and confirm the match in the estimate, see Antenna Calculations.
• A modest SWR up to about 2 is normal and costs little. Chasing a perfect 1.0 is rarely worth it.