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NanoVNA course – Phase and reflection

Red = forward voltage wave → travelling right. Blue = reflected voltage wave ← travelling left. At the reflection point, the phase difference is exactly ∠Γ.
Voltage ratio Vreflected / Vforward.
0° = open; ±180° = short (ideal examples).
Display speed only, not the RF frequency.
Red = forward, blue = reflected, yellow = Vforward + Vreflected.
1.00 ∠ 0°Γ (S11, linear)
100%reflected voltage
100%reflected power = |Γ|²
0.00 dBReturn Loss = −20 log₁₀|Γ|
∞SWR = (1+|Γ|)/(1−|Γ|)

Voltage along the transmission line

Phases as the NanoVNA sees them at the measurement plane. The cable shifts the measured phase of Γ.
Phases at the load. Only here is the angle between the two arrows exactly ∠Γ.
Forward →← ReflectedSum = Forward + Reflected

The red wave travels right and the blue wave travels back left. The NanoVNA measurement plane is on the left and the reflection point is on the right. Between these points, their local relative phase changes because they travel in opposite directions.

Interactive explanation of reflection and S11

Investigate how forward and reflected waves travel along a transmission line. Adjust the reflection coefficient and phase and compare an open circuit, short circuit and matched load. See how phase difference, standing waves, SWR and S11 are related and why the NanoVNA measurement plane matters.

View the NanoVNA course programme or read specifically about S21, filters and transmission lines in module 3.

Assumptions in this demonstration: the line length is fixed at 2.35 λ (846°). The transmission line is ideal and lossless, with no attenuation and a purely real characteristic impedance. A real cable has both attenuation and a small imaginary component in its characteristic impedance, so its behaviour differs from this simplified model.

From reflection point to measurement plane

In an S11 measurement, the signal traverses the cable twice: first as the forward wave to the load, then as the reflected wave back to the NanoVNA. For an ideal lossless line, this gives Γmeas = Γload · e−j2βL. For each chosen reflection coefficient, the animation below calculates how the electrical cable length shifts the measured phase.

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