Electromagnetic wave compression and radio frequency homogeneity in NMR solenoidal coils: Computational approach

The homogeneity of the radio frequency (RF) magnetic field in the region of the NMR sample is prerequisite for good performance in many NMR experiments. When calculating such fields, the quasistationary approximation is valid for wavelengths that are large compared to the physical dimensions of the NMR coil, but it no longer applies for the geometrical dimensions of NMR coils at high frequencies. The present study investigates electromagnetic fields in solenoidal coils. Considering the electromagnetic field derivable from first principles allows us to systematically study the effects that originate from the finiteness of the wavelength compared to the physical size of solenoidal coils employed in NMR probes. There are three major effects that occur: the wavelength in the coil is generally different from the wavelength in free space, even for an absent sample; the dielectric properties of the NMR sample have an additional influence on the electromagnetic field distribution inside the coil; and the circuit surrounding the coil leads to a wave reflection that affects the RF magnetic field distribution originating from the superposition of forward and backward traveling waves. The results allow the derivation of principal conclusions for the characterization of NMR solenoidal coils. (C) 2002 Wiley Periodicals, Inc.