SWR Calculator
Compute SWR, reflection coefficient, return loss and mismatch loss from forward and reflected power — or work backwards from a measured SWR. Live, in your browser. 实时计算驻波比、反射系数、回波损耗与失配损耗，纯本机运算。
SWR = —（仪表最大刻度 3.0，超出按 3.0+ 显示）
Γ = √(Pr / Pf) · SWR = (1 + Γ) / (1 − Γ) · Return loss = −20·log₁₀(Γ) · Mismatch loss = −10·log₁₀(1 − Γ²)公式：反射系数 Γ=√(反射功率/前向功率)；驻波比=(1+Γ)/(1−Γ)；回波损耗=−20·log₁₀(Γ)；失配损耗=−10·log₁₀(1−Γ²)。前向功率为 0 或反射大于前向时显示 — 。
SWR — standing wave ratio — is the single number every ham checks before trusting an antenna system. It tells you how well your antenna and feed line accept the power your transmitter produces. A perfect 1.0:1 match sends every watt out into the air; as the SWR climbs, a growing share of your power is reflected back down the coax instead of being radiated.
This calculator works in both directions. If your power meter or radio shows forward and reflected power, use Power to SWR: enter both readings and you get the SWR, the reflection coefficient Γ, the return loss and the mismatch loss, plus the percentage of power being reflected. A classic sanity check: 100 W forward and 4 W reflected is an SWR of exactly 1.5:1, with 4% of your power coming back.
If you only know the SWR — from an antenna analyser, a club meter or a friend’s report — use SWR to Loss to see what that number costs in decibels. The results surprise many operators: an SWR of 2:1 reflects 11% of power, but the mismatch loss is only about 0.5 dB. Even 3:1 loses roughly 1.25 dB to mismatch. Your signal report will barely notice, which is why chasing a perfect 1.0 across every band is usually wasted effort.
High SWR still matters. Reflected power heats the feed line, adds to coax loss on the return trip, and can make modern transceivers fold back their output or shut down entirely. Most radios are comfortable at 2:1 and below; above 3:1, stop and investigate — a corroded connector, water in the coax, or an antenna that has detuned in the wind is far more likely than a band that simply “doesn’t like” your wire. Use the colour gauge as a quick guide: green up to 1.5, amber to 2.0, red beyond.
Remember that the SWR you measure at the radio includes the effects of your feed line. A long run of lossy coax makes a mediocre antenna look better than it is, because the line absorbs the reflected power on its way back. For the truest reading, measure at the antenna feed point — or use the Coax Loss Calculator to estimate what your line is hiding.
Frequently asked questions
What SWR is acceptable for my antenna?
Below 1.5:1 is excellent, 1.5–2.0:1 is fine for almost every transceiver, and most modern radios work without complaint up to about 2.5–3:1 before their protection circuits reduce power. Above 3:1, stop transmitting and find the cause.
How much power do I actually lose at 2:1 SWR?
About 11% of your power is reflected, which is a mismatch loss of roughly 0.5 dB — less than one S-unit at the receiving end. The bigger risks of high SWR are extra feed-line loss and transmitter foldback, not the mismatch itself.
Why does my SWR change with frequency?
An antenna is only resonant at one frequency (per band). As you tune away from resonance, its impedance moves away from 50 Ω and the SWR rises. That is why multiband antennas quote a usable SWR bandwidth — the span where SWR stays under 2:1.
What's the difference between SWR and return loss?
They describe the same mismatch in different units. SWR is a ratio starting at 1.0; return loss is the reflected signal's attenuation in decibels, where higher is better. SWR 1.5:1 ≈ 14 dB return loss; SWR 2:1 ≈ 9.5 dB. This calculator gives you both.