At the UK’s University of Sheffield, a cross bow-tie element has been created with a single-layer, dual-band, tunable high-impedance-surface (HIS) groundplane. To realize this independently tunable, low-profile antenna system in the ultra-high-frequency (UHF) band, the researchers—Hyung-Joo Lee, Kenneth Lee Ford, and Richard J. Langley—assembled it with a standard coplanar-waveguide (CPW) -fed, printed-circular-disc, wideband-monopole antenna. Yet a CPW-fed circular-patch monopole antenna is a poor radiator when positioned close to a conducting platform. It can be made to radiate more efficiently, however, with an HIS groundplane backing the antenna. By making that HIS groundplane tunable, the engineers surmised that they could also achieve a wider effective bandwidth.
The groundplane and wideband antenna were fabricated with 1SV245 varactor diodes from Toshiba. The antenna system showed an effective tunable bandwidth of roughly 0.9 to 2.8 GHz with dual-band capability. By performing simulations of the HIS groundplane, the researchers showed that either single- or dual-band reconfigurable reflection-phase resonances could be achieved between 0.95 and 2.45 GHz by controlling C1 (4.55 pF~0.6 pF) and C2 (4.55 pF~0.6 pF) independently. (These represent the two groups of varactor diodes mounted on the surface elements of the HIS.)
According to simulations performed with the wideband antenna 4 mm above the HIS groundplane, an effective, tunable operational bandwidth from 1.07 to 2.75 GHz is achievable with dual-band capability. Parametric studies show that the system’s radiation efficiency rises as the antenna height is increased and diode resistance is reduced. In addition, radiation efficiency rises with the general frequency over the tunable frequency range. With the optimum parameters, typical values from -4 to -2 dB can be achieved. Radiation efficiency can be further increased by using low-loss substrate materials. The proposed HIS groundplane features a unit cell measuring 19.71 by 19.71 mm and 1.74 mm thick. See “Independently Tunable Low-Profile Dual-Band High-Impedance Surface Antenna System for Applications in UHF Band,” IEEE Transactions On Antennas And Propagation, Sept. 2012, p. 4092.