Calculate SWR from forward and reflected power readings. Works offline after your first visit.
Standing-wave ratio (SWR) is how well an antenna matches the radio. Enter forward and reflected watts from a directional meter; this calculator returns SWR so you know whether to keep transmitting or stop and fix the feedline.
1.0:1 is a perfect match. Under 1.5:1 is comfortable for most HF and VHF work. Between 1.5 and 2:1, many modern radios fold back power and still run. Above 2:1, stop high-power transmitting and check connectors, coax, antenna length, and tuner settings before you risk a final amplifier.
Field method: put a wattmeter in line at the radio, key a carrier or FM, read forward and reflected, then calculate. A 100 W forward / 4 W reflected reading is about 1.5:1. East Tennessee portable ops (POTA on the Appalachian Trail, NVIS dipoles in a valley) see SWR change with rain, tree proximity, and feedline length — re-check after you move the wire.
This tool does not replace an antenna analyzer. Use it when you only have a radio wattmeter. Related: coax loss, antenna length, and tuner advisor.
See also: Radio reference
1:1 is ideal; under 1.5:1 is fine for most HF/VHF work. Above 2:1 warrants investigation before high-power operation.
Use a directional wattmeter or built-in radio SWR meter with forward and reflected power readings.
Some radios will, at reduced power. Treat 3:1 as a fault: inspect the antenna and feedline before a long net or digital session.
A tuner can present 50 ohms to the radio so the transmitter is happy. It does not add radiated power that a matched antenna would have produced.
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