Transformer-coupled NMR probe

► We propose the use of a transformer with a ferromagnetic core for impedance matching. ► The ferromagnetic core provides a strong but confined coupling field. ► We built a probe that operates at 500 kHz-5 MHz without matching adjustments. ► The loss compared to a capacitor-coupled probe was ∼1 dB at 2 MHz. ► Such a probe could be useful for mobile NMR application of multi-frequency operation.