We present the design of a novel high-temperature superconductor double-sided racetrack resonator for a 13 C optimized nuclear magnetic resonance (NMR) transmitter/receiver coil. The coils operate in a 21.1 T magnet and accommodate a 3 mm × 6.2 mm cross-section rectangular sample tube. The design includes the incorporation of revised finger lengths to improve the homogeneity of current density across the fingers, a new laser trimming approach for adjusting the resonance frequency, and improved ability to shift higher-order modes for suitability in 1 H/ 13 C NMR probes. Resonator design methodology, simulations and experimental results are presented.