You send a 2 V peak, 1 GHz sinusoidal RF signal into a
50 Ω coaxial cable connected to an NMR probe.
If the probe impedance ZL is not exactly 50 Ω, part of the wave
reflects back. This wastes power and creates standing waves that
distort your experiment.
Reflection coefficient: Γ = (ZL − 50) / (ZL + 50)
Reflected voltage = Γ × 2 V
Reflected power (%) = |Γ|² × 100 %
Adjust the probe impedance ZL:
At ZL=50Ω: perfect matching → no reflections, maximum power transfer, and a clean peak voltage (~2 V) at the probe.
Move away from 50Ω: reflections appear. The probe voltage may rise or fall, but delivered power always decreases.
Key insight:
ZL>50Ω: reflected wave adds → higher voltage
ZL<50Ω: reflected wave subtracts → lower voltage
When there is an impedance mismatch, part of the wave reflects back.
At the probe, the reflected and incident waves combine, so the voltage can increase or decrease depending on phase. This can be misleading.
Key point: mismatch always reduces the real power delivered, even if voltage appears higher.
Tuning we did in the previous page minimizes reflections, ensuring maximum power transfer and the strongest B₁ field.