Internet of Things (IoT) devices require orientation insensitive communication and a small footprint. Wireless communication should be maintained across the whole operation band, hence electrically small antennas (ESAs) with wideband radiation isotropy are also desired. This paper proposes a theoretical model based on annular ring currents to synthesize quasi-isotropic antenna radiation patterns. The theoretical model shows that wideband radiation isotropy can be achieved by optimizing the combination of azimuthal currents. We subsequently present spherical and cubical ESA designs that achieve measured wide impedance and radiation isotropy bandwidths exceeding 10% for the GSM900 band. The ESAs were fully printed, with substrates 3D printed and metallization applied using screen printing. Despite the electrically small sizes and low cost fully printed fabrication, the antennas achieved approximately 90% radiation efficiencies. The proposed designs are low cost because of additive manufacturing, can have embedded electronics because of their 3D structure and have the largest radiation isotropy bandwidth for an electrically small antenna in published literature.INDEX TERMS Azimuthal current rings, electrically small antennas, Internet of Things, antenna miniaturization, printing, quasi-isotropic radiation patterns, antenna synthesis theory, wideband antennas.
This work presents a cross-polar dual-layer chipless radio-frequency identification (RFID) tag based on a laddershaped resonator design. An integrated ground plane enables direct attachment to human skin without performance deterioration. Simulations show that the ladder-shaped resonator provides several advantages over traditional L-shaped and straight resonators, including a strong cross-polar radar cross section (−23.4 dBsm), third-order harmonics, orientation insensitivity, and compact size (0.062 λ 2 ). The effects of the ground plane shape on the surface current distribution are investigated, and a circular tag of 20 mm radius is designed using ladder resonator groups and frequency shift encoding to provide an active area of 96.45 bits/λ 2 and a unit frequency of 6.03 bits/GHz. The tag substrate is three-dimensionally (3D) printed with metallic resonator patterns that are subsequently screen-printed on the substrate. The maximum read range is measured at 40 mm using a cross-shaped, dual-polarized Vivaldi antenna connected to a network analyzer. The measured characteristics in free space are in good agreement with the simulation results, and practical onbody performance tests for the manufactured prototype using simulation and direct measurements indicate that the tag performance remains stable for both free space and on-body cases. The fully printed fabrication process makes the proposed tag design suitable for mass production at a low cost.
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