Noise-Immune Coil for Unshielded Magnetic Resonance Measurements

A modified split-loop resonator that is electrically balanced and that has no magnetic dipole moment is shown to be relatively immune to environmental noise. Using a magnetic resonance surface coil of this design for14N NQR at 3.4 MHz, it is demonstrated that magnetic resonance measurements can be made in the laboratory without additional RF shielding and with less than a 2 dB increase in the rms noise. Compared to more traditional designs, the modified split-loop resonator showed a net 17-dB gain in sensitivity for unshielded measurements.