A Variable Gap Shielded Loop with a Ground Plane for Surface Current Leakage Mitigation

Resonant antennas, such as monopoles, dipoles and patches, provide consistent radiation properties but are restricted by size and bandwidth, offering little flexibility for design tradeoffs. In this regard, the shielded loop is unique due to its construction. This loop, shown in Figure 1, consists of a terminated coaxial line with a gap. Different termination types, such as an open or short circuit, are used for various operation cases. For each termination, the outer conductor serves as a shield for the inner conductor, with a gap cut in the center of the shield to allow signal to reach the inner conductor (L.L. Libby, "Special Aspects of Balanced Shielded Loops," in Proceedings of the IRE, vol. 34, no. 9, pp. 641-646, Sept. 1946). As a result of this shield’s ability to lower electrical noise, it is mainly used as a magnetic loop in near-field operation, such as in MRI or near-field scanning. However, when operated as an electrical loop, the shield serves as the radiator and the inner conductor is the feedline. In this case, the gap serves as an additional degree of freedom and the gateway to flexibility in resonant antennas.