  Theory of a high impedance coil (HIC) - a cable loop antenna with a modified
shield - is discussed comprehensively for both in transmitting and receiving
regimes. Understanding a weakness of the previously reported HIC in
transmitting regime, we suggest another HIC which is advantageous in both
transmitting and receiving regimes compared to a conventional loop antenna. In
contrast with a claim of previous works, only this HIC is a practical
transceiver HIC. Using the perturbation approach and adding gaps to both shield
and inner wire of the cable, we tune the resonance frequency to be suitable for
ultra-high field (UHF) magnetic resonance imaging (MRI). Our theoretical model
is verified by simulations. Designing the HIC theoretically, we have fabricated
an array of three HICs operating at 298 MHz. The operation of the array has
been experimentally studied in presence of different phantoms used in UHF MRI
and the results compared with those obtained for a conventional array.
