Fluid antenna systems (FAS) have recently emerged as a promising solution for sixth-generation (6G) ultra-dense connectivity. These systems utilize dynamic radiating and/or shaping techniques to mitigate interference and improve spectral efficiency without relying on channel state information (CSI). The reported improvements achieved by employing a single dynamically activated radiating position in fluid antenna multiple access (FAMA) are significant. To fully realize the potential of FAMA in multi-user multiplexing, we propose leveraging the unique fast-switching capabilities of a single radio-frequency (RF)-chain meta-fluid antenna structure to achieve multi-activation. This allows for a significantly larger set of independent radiating states without requiring additional signal processing. Simulations demonstrate that multi-activation FAMA enables robust multi-user multiplexing with a higher signal-to-interference ratio (SIR) under various Rayleigh-fading environments compared to other single RF-chain technologies. We further show that the SIR can be optimized within a 15~$μs$ timeframe under a multi-user Rayleigh-fading channel, making the proposed scheme highly suitable for fast-changing wireless environments. Verified through the theoretical Jakes' model, full three-dimensional (3D) electromagnetic (EM) simulations and experimental validation, multi-activation FAMA enables effective CSI-free, multi-user communication, offering a scalable solution for high-capacity wireless networks.

Smart cities demand versatile antenna systems supporting heterogeneous wireless applications across diverse propagation environments. This paper presents a wideband pixel-based fluid antenna system (PB-FAS) designed as a general-purpose antenna solution for smart city infrastructures, addressing fundamental challenges in wideband operation, spatial adaptability, interference mitigation, and scalable deployment. The proposed PB-FAS integrates parasitic elements for enhanced bandwidth (6.0-7.0 GHz) and a compact 6-PIN-diode pixel surface enabling 64 distinct fluid states, achieving optimal cost-performance balance. An integrated FPGA-based control system provides microsecond-level reconfiguration for real-time channel adaptation. We establish a rigorous exact spatial geometry (ESG) channel model capturing state-dependent antenna responses across near-field and far-field regions, providing a unified theoretical foundation for interference mitigation analysis. Comprehensive validation through full-wave electromagnetic simulations, anechoic chamber measurements, and experimental two-source 16-QAM communication tests demonstrates up to 11 dB SINR improvement and 13.2% EVM reduction through hardware-level spatial diversity, confirming the system’s effectiveness as a scalable, cost-effective solution for next-generation smart city wireless infrastructures ranging from IoT sensor networks to high-capacity backhaul links.

Mobile smart devices, such as smart glasses, smartwatch, smartphone, and unmanned aerial vehicle (UAV), have rich capabilities of Internet of Things (IoT). They are so handy and ready-to-use that interfacing with the ubiquitous IoT nodes becomes much easier. Mobile smart devices capability of multi-protocol massive IoT relies on enough bandwidth of their antennas. However, the small size results in a higher quality factor (Q-factor) of the antennas, which severely constrains the bandwidth. In this paper, we proposed a novel feeding mechanism that can excite and detune dual-degenerate modes for multiple orders of a loop antenna, compared with present loop antennas one single mode/resonance for each order. This loop antenna significantly increases the number of the excited modes/resonances up to 12, thus covering 0.73–7.10 GHz, including the whole sub-6 GHz band, which is several times that of the present loop antennas. Further, we demonstrated its size-compressing method. An even smaller folded multi-order-dual-degenerate-modes loop antenna was applied to smartwatch-size devices. It achieves a bandwidth of 1.74–8.87 GHz, which is several times that of the present smartwatch antennas. The proposed method requires no impedance-matching networks, multi-ports, switches, or tuners, offering ultra-broadband, low-loss, and cost-effective advantages. Because of its ultra-broadband property, it provides a ready-to-use antenna solution to massive IoT, ultra-wideband (UWB) energy harvesting, and multi-band cellular network communication for mobile smart devices.

Quasi-isotropic antennas are essential to the massive Internet of Things (IoT) of 6G for mobile smart devices, such as uncrewed aerial vehicle (UAV)/uncrewed ground vehicle (UGV) and smart glasses. However, until now, present quasi-isotropic antennas can achieve at most two discrete frequency bands. It is because of the substantially increased structural complexity with the increased number of frequency bands. Since dual-band property is much far from the requirement of massive IoT, we investigated a novel method for multiband (>2) quasi-isotropic antennas. This method combines two inductor-embedded U-radiators, manipulates and purifies the four isotropically radiated modes, and excites all of them simultaneously by wideband-matching capacitive coupling feeding. Consequently, four isotropically radiated frequency bands are produced with a highest-to-lowest frequency ratio up to 3. More importantly, this method has great potential to achieve even more frequency bands for future massive IoT applications. This research releases quasi-isotropic antennas from the constraint of dual frequency bands, which impedes the advances of 6G massive IoT.

This work introduces a novel metasurface (MS) antenna for the network of Internet of Things (IoT) gateway. The suggested antenna technology combines the MS with a dielectric resonator antenna (DRA) to provide a stable conical radiation pattern over the entire operating bandwidth in its compact size. This invention aims at increasing the structural suitability while maintaining good radiation characteristics, including wide bandwidth, high efficiency, omnidirectional radiation, and low-cross-polarized level. First, a compact DRA with a small ground is studied. Then, the MS loaded on the DRA with a pair of shorting walls is investigated. The proposed antenna is reduced by 96.5% of volume in comparison to a commercial monopole base-station antenna as well as a conical-radiation indoor antenna. This devoted antenna design is a good candidate applied to large-scale IoT networks of airports, train stations, logistic centers, hospitals, libraries, supermarkets, and super stores.

Millimeter wave (mm-wave) antenna modules with switchable beam radiation provide a potential solution for enabling flexible wireless links and high data rate in Internet of Things (IoT) networks. Aiming to simultaneously fulfill a wide coverage and follow the cost-effective principle of IoT devices, a novel mm-wave wideband antenna module with switchable fan-beam radiation is proposed based on an air-filled planar Cassegrain beam-former. A comprehensive design procedure combing the theoretical calculation and differential evolution (DE) algorithm is first introduced to construct the beam-former with wide-angle scanning capability. The desirable aperture-shared switchable fan-beam radiation with high crossover is then obtained with the aid of a proposed two-feed-per-beam (2FPB) scheme based on a low-loss integrated switch network. A prototype of the antenna in the Ka-band is fabricated by simultaneously adopting the metallic 3-D printing and printed circuit board (PCB) technology. Excellent operating characteristics, including a wide bandwidth of more than 32%, twenty stable fan-shaped radiation beams scanning over a wide angular range of ±45° in the H-plane, a high beam crossover of about −3 dB, a gain of up to 19.6 dBi, and a radiation efficiency of greater than 80% are demonstrated experimentally. With the ability to fulfill the radio coverage within a wide conical area, the presented antenna module would be an attractive candidate for the fifth generation (5G) IoT applications.

Smartwatch is a potential candidate for the Internet-of-Things (IoT) hub. However, the performance of smartwatch antennas is severely restricted by the smartwatch structure, especially when the antennas are designed by traditional methods. For adapting smartwatches to the role of IoT hub, a novel method of designing the multiband smartwatch antenna is presented in this article, aiming at increasing the number of frequency bands, omnidirectivity, and structural suitability. First, the fundamental structure (including the full screen and the system PCB) of the smartwatch is analyzed as a whole by characteristic mode analysis (CMA). Thus, abundant resources of characteristic modes are introduced. The fundamental structure is then modified as the radiator of a multiband antenna. Then, a nonradiating capacitive coupling element (CCE) excites the desired four 0.5${\lambda }$modes from this structure. This method could fully utilize the intrinsic modes of the smartwatch structure itself, thus exhibiting multiple advantages: significantly small size, smaller ground, omnidirectional radiation, and fitting to the full-screen smartwatch structure.

We introduce a dual-polarized (DP) Fabry—Pérot cavity (FPC) antenna operating at the millimeter-wave (mmWave) frequency band with high-gain and wideband characteristics. A DP feeding source and a partially reflective surface (PRS) integrated with a Fresnel zone lens are suggested to realize dual-polarization wave radiation over a wide impedance bandwidth. The feeding source provides vertical and horizontal polarizations while keeping high isolation between the two polarizations. PRS is used to realize Fabry cavity to produce a directive beam radiation. The integrated Fresnel zone rings are introduced for phase correction, leading to a significant gain enhancement for the antenna. For verification, a 60-GHz FPC antenna prototype with DP radiation is designed and fabricated with measurement results. It consists of a feeding source, a PRS integrated with a Fresnel zone lens, a quasi-curved reflector, and four three-dimensional printed supporters. The results illustrate that the peak gains of vertical and horizontal polarizations are 18.4 and 17.6 dBi, respectively. The impedance matching bandwidth for the two polarizations is 14%. The performance ensures that the proposed DP FPC antenna is a promising candidate for the fifth-generation wireless communication systems in the mmWave band.

This work introduces a wideband printed patch antenna design for 60 GHz radio applications. The proposed antenna is a single-layer structure with J-shaped slots loaded on the patch for the impedance bandwidth enhancement. The function of the J-shaped slots provides an additional current path for generating the second resonance of the patch. With this additional resonance, the antenna could find double resonances near the centre frequency of the patch. The antenna is excited by the grounded-coplanar waveguide (G-CPW) to microstrip line. This proposed antenna has an impedance bandwidth of 14.3% (8.6 GHz, 57-64 GHz) and the peak gain of 7.2 dBi. Its array of 4×1 arrangement yields impedance bandwidth of 19.2% (11.5 GHz, 55-65.5 GHz) with respect to the center frequency of 60 GHz, and rewards the maximum gain of 13.5 dBi at the broadside direction. This antenna has simple structure and is easy to fabricate by a conventional PCB technology. The proposed design finds a potential application of microstrip patch antenna in millimeter-wave wireless communications.

The objective of this paper is to provide the context, physical insight, and perspective on antennas in wireless communications. Although it does not mean to be comprehensive, the four key technologies related to small antenna designs to be reviewed and discussed, including multiband planar inverted-F antennas, broadband folded patch antennas, compact differentially fed antennas, and miniature circularly polarized patch antennas, would cover a wide range of topical interests and practical applications. A brief overview of computer software for analyzing these antennas is also provided. Hopefully, this paper will be beneficial for the diverse engineering readership of the IEEE.