SWR and matching
Standing wave ratio describes how well the antenna system matches the feed line. Enter meter readings, a load impedance, or an SWR value.
- SWR
- 1.5 : 1
- Reflection coefficient
- 0.2
- Return loss
- 14 dB
- Mismatch loss
- 0.177 dB
96% of forward power reaches the load; 4% is reflected (ignoring feed line loss).
Most solid-state transmitters reduce output power as SWR rises beyond a certain level, to protect their output transistors. The threshold varies by model, so check your radio's manual.
How it works
- The reflection coefficient is the square root of reflected power divided by forward power.
- SWR = (1 + Γ) / (1 - Γ). For a purely resistive load, SWR is the larger impedance divided by the smaller: 100 Ω on 50 Ω coax gives 2:1.
- Return loss = -20 log10(Γ) dB. Mismatch loss is the power not delivered because of reflection; feed line loss adds to it in a real system.
- Most solid-state transmitters reduce output power as SWR rises beyond a certain level, to protect their output transistors. The threshold varies by model (one manufacturer states 1.5:1), so check your radio’s manual.
Learn more: Antennas and feed lines
Sources: Voltage standing wave ratio (VSWR) (Microwaves101); Feed Lines (ARRL); 2026-2030 Technician question pool (NCVEC Question Pool Committee); TS-590SG Instruction Manual (SWR protection above 1.5:1) (JVC KENWOOD)
Practice the exam questions: T7 Practical Circuits (Technician), T9 Antennas and Feed Lines (Technician), G4 Amateur Radio Practices (General), G9 Antennas and Feed Lines (General)