The Mode of Operation of High-Impedance Coils and Shielded Coaxial Cable Coils: A Comparative Study

High-impedance coils (HICs) and shielded coaxial cable coils (SCCs) are of the same physical structure but with different matching networks and different input impedances (which depend on the mode of operation). This article explores the working and decoupling mechanisms of the HICs and the SCCs and establishes a design principle for highly decoupled elements. Decoupling properties, input impedances, and modes of operation of HICs and SCCs were analyzed and compared through a frequency range from 50 to 500 MHz. The coils were modeled in CST Microwave Studio, and full-wave electromagnetic simulations were performed to analyze coil parameters. Input impedance and S-parameters were examined for individual and two elements with various overlaps. The current distribution is computed along the outer surface of the conductor of the coils. It was shown that HICs have uniform current distribution, although SCCs have a combination of sinusoidal and constant current. Input impedances of SCCs designed for operation at the second resonant mode did not change, while two SCCs overlapped for varying amounts. Contrarily, HICs showed characteristic impedance splitting when various amounts of overlap were examined. $B_1^{+}$ maps of the SCC pairs were not affected when the overlap was changed from no overlap to 100% overlap. In contrast, the HICs showed degradation of the $B_1^{+}$ field (for the overlap increasing from 20% up to 100%) related to strong inter-element coupling of HIC coils when the amount of overlap increased. The stronger coupling of HICs operating at the first resonance region led to more vital degradation of their observed $B_1^{+}$ profile than SCCs. SCCs operating at the second resonance region exhibited stable $B_1^{+}$ profiles independent of the overlap amount. When operating in the correct mode, the coaxial cable resonators exhibit favorable decoupling properties in comparison to traditional coil designs.