Characteristics of an electronically tunable surface-coil-type resonator for L-band electron paramagnetic resonance spectroscopy

This note describes considerable additional information on the key parameters of an electronically tunable surface-coil-type resonator for L-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy [Hirata , J. Magn. Reson. 142, 159 (2000)]. For the 1.1 GHz prototype tunable surface coil (10.6 mm in diameter), the measured efficiency for generating the rf magnetic field is 77 muT/W-1/2. The unloaded quality factor of the tunable surface coil is in the range of 260-310. In absence of an EPR sample, 57% of the magnetic energy in the system is stored in the parallel coaxial line, 16% in the surface coil, and a half of the energy is dissipated in the parallel coaxial line. The energy dissipation for the surface coil occurs mainly through radiation (25% of the total energy dissipation). The level of the second harmonic in the reflected microwaves generated by the tunable surface coil was measured. The varactor diodes used in the matching and tuning circuits are the dominant source of the second harmonic. (C) 2001 American Institute of Physics.