SWR and Reflected Power Calculator

SWR (Standing Wave Ratio) measures how well your antenna is matched to your transmission line and transmitter.
A perfect match is 1:1.
High SWR means power is being reflected back toward your transmitter, reducing efficiency and potentially damaging the final amplifier stage.

The Formulas:

Reflection Coefficient: Γ = (Z_L - Z_0) / (Z_L + Z_0)

SWR = (1 + |Γ|) / (1 - |Γ|)

Where:

• Z_L = load (antenna) impedance in ohms
• Z_0 = characteristic impedance of the feedline (typically 50Ω)
• |Γ| = magnitude of reflection coefficient (0 to 1)

Antennas Are Not Pure Resistances

An antenna off resonance has reactance as well as resistance, and an analyzer reports it as R + jX. Reactance raises SWR even when the resistive part is a perfect 50 Ω, which is why an antenna that reads “50 Ω” on a cheap resistance-only meter can still show 2:1 on the rig. With reactance in the picture the magnitude becomes:

|Γ| = √((R - Z_0)² + X²) / √((R + Z_0)² + X²)

Enter X = 0, or leave the box empty, for the plain resistive case. A negative X means the antenna is short of resonance (capacitive, so the element is too short); a positive X means it is long.

Percent Power Loss from SWR:

Power reflected (%) = |Γ|² × 100

Power to antenna (%) = 100 - Power reflected (%)

Worked Example:

Antenna impedance: 75Ω. Feedline: 50Ω coax.

Γ = (75 - 50) / (75 + 50) = 25 / 125 = 0.2

SWR = (1 + 0.2) / (1 - 0.2) = 1.2 / 0.8 = 1.5:1

Power reflected: 0.2² × 100 = 4% Power delivered to antenna: 96%

A 1.5:1 SWR is excellent. Most transceivers operate at full power up to 1.5:1 or 2.0:1.

SWR Reference Table:

Mismatch loss is -10 × log10(1 - |Γ|²) and it is smaller than most operators expect. Even a 3:1 SWR throws away only 1.25 dB, which is a fifth of an S-unit and inaudible at the far end. The reason to chase SWR down is not the reflected power. It is that the radio folds back its output to protect itself, and that a long lossy feedline turns the extra round trip into heat.

Practical Tips:

• Most modern HF transceivers fold back power above 3.0:1 SWR to protect the PA (Power Amplifier)
• SWR changes with frequency: always measure at your operating frequency
• Coax length can affect the SWR reading at the radio but not at the antenna: use an antenna analyzer at the feedpoint
• High SWR in a long coax run causes additional heat loss in the cable itself, and the worse the coax, the more the SWR reading at the shack end flatters you