Circularly polarized (CP) antennas are a type of antenna with circular polarization. Due to the features of circular polarization, CP antennas have several important advantages compared to antennas using linear polarizations, and are becoming a key technology for various wireless systems including satellite communications, mobile communications, global navigation satellite systems (GNSS), wireless sensors, radio frequency identification (RFID), wireless power transmission, wireless local area networks (WLAN), wireless personal area networks (WPAN), Worldwide Interoperability for Microwave Access (WiMAX) and Direct Broadcasting Service (DBS) television reception systems. Lots of progress in research and development has been made during recent years.The CP antenna is very effective in combating multi-path interferences or fading [1,2]. The reflected radio signal from the ground or other objects will result in a reversal of polarization, that is, right-hand circular polarization (RHCP) reflections show left-hand circular polarization (LHCP). A RHCP antenna will have a rejection of a reflected signal which is LHCP, thus reducing the multi-path interferences from the reflected signals.The second advantage is that CP antenna is able to reduce the 'Faraday rotation' effect due to the ionosphere [3,4]. The Faraday rotation effect causes a significant signal loss (about 3 dB or more) if linearly polarized signals are employed. The CP antenna is immune to this problem, thus the CP antenna is widely used for space telemetry applications of satellites, space probes and ballistic missiles to transmit or receive signals that have undergone Faraday rotation by travelling through the ionosphere.Another advantage of using CP antennas is that no strict orientation between transmitting and receiving antennas is required. This is different from linearly polarized antennas which are subject to polarization mismatch losses if arbitrary polarization misalignment occurs between transmitting and receiving antennas. This is useful for mobile satellite communications where it is difficult to maintain a constant antenna orientation. With CP, the strength of the received signals is fairly constant regardless of the antenna orientation. These advantages make CP antennas very attractive for many wireless systems.

In the emerging Internet of Things, stretchable antennas can facilitate wireless communication between wearable and mobile electronic devices around the body. The proliferation of wireless devices transmitting near the human body also raises interference and safety concerns that demand stretchable materials capable of shielding electromagnetic interference (EMI). Here, an ultrastretchable conductor is fabricated by depositing a crumple-textured coating composed of 2D Ti 3 C 2 T x nanosheets (MXene) and single-walled carbon nanotubes (SWNTs) onto latex, which can be fashioned into high-performance wearable antennas and EMI shields. The resulting MXene-SWNT (S-MXene)/latex devices are able to sustain up to an 800% areal strain and exhibit strain-insensitive resistance profiles during a 500-cycle fatigue test. A single layer of stretchable S-MXene conductors demonstrate a strain-invariant EMI shielding performance of ≈30 dB up to 800% areal strain, and the shielding performance is further improved to ≈47 and ≈52 dB by stacking 5 and 10 layers of S-MXene conductors, respectively. Additionally, a stretchable S-MXene dipole antenna is fabricated, which can be uniaxially stretched to 150% with unaffected reflected power <0.1%. By integrating S-MXene EMI shields with stretchable S-MXene antennas, a wearable wireless system is finally demonstrated that provides mechanically stable wireless transmission while attenuating EM absorption by the human body.existing mobile devices. [1] To enable highperformance wireless communication between wearable sensors, displays, and data processing devices around the body, new routes to fabricating for stretchable antennas that exhibit mechanically stable performance are needed. Furthermore, the proliferation of mobile and wearable devices based on various wireless technologies, including GPS, Bluetooth, Wi-Fi, and near-field communication, is increasing the frequency and duration of the human body exposed to electromagnetic (EM) fields, which raises interference and safety concerns that may require certain suitable materials for EM protection. [2] Therefore, in addition to the growing demand for stretchable antennas, electromagnetic interference (EMI) shielding materials that are stretchable, durable, and can be integrated closely with wearable wireless technologies are needed to reduce the exposure of the human body to EM fields. Integrating such stretchable antennas with on-site EMI shields not only provides protection against EM fields, but also prevents unauthorized wireless transmission between wearable electronics and mobile devices for enhanced wireless privacy.Both wearable antennas and stretchable EMI shields face similar technological challenges, where the key materials awaiting to be developed are the stretchable conductors with high strain tolerance and strain-invariant electrical conductivities.Metals (e.g., Cu and Al) are the conventionally used materials for EMI shields and antennas on many occasions. As the trend in today's electronic devices becomes faster, lighter, and...

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attraction due to their synergistic functions of hydrophilic characteristics, [1,2] superior electrical conductivities, [3] high surface area, [4] efficient electrochemical activities, [5,6] and tunable surface functional groups. [7] Ti 3 C 2 T x MXene nanosheets have been utilized as easy-to-assemble building block units for the fabrication of micro-and nanoarchitectures with multifunctionality, which have been applied to energy storage devices, [1,5,8] optoelectronics, [9,10] electromagnetic interference (EMI) shielding, [11][12][13] wireless communication, [14] and water desalination. [15][16][17] However, during the self-assembly processes, MXene nanosheets are prone to aggregate or restack due to strong van der Waals forces, which largely decreases the accessible surface area and active sites of functional MXene structures. [13,18,19] To scale the synergistic properties of MXene nanosheets to the macroscopic level, one promising strategy is through the construction of foam-like 3D structures, such as aerogels with hierarchical pores. [19,20] To date, various fabrication strategies have been adopted by incorporating external spacers/binders, [8,21] inducing crosslinking reaction between MXene nanosheets, [13,22] and utilizing supporter materials as templates. [23,24] Although these approaches have demonstrated the successful creation of MXene-based aerogels with high porosity, their electrical conductivities and electrochemical Scaling the synergistic properties of MXene nanosheets to microporous aerogel architectures requires effective strategies to overcome the nanosheet restacking without compromising MXene's advantageous properties. Traditional assembly approaches of 3D MXene aerogels normally involve external binders/templates and/or additional functionalization, which sacrifice the electrical conductivities and electrochemical activities of MXene aerogels. Herein, inspired by the hierarchal scale textures of Phrynosoma cornutum, a crumple-textured Ti 3 C 2 T x MXene platform is engineered to facilitate Mg 2+ -induced assembly, enabling conformal formation of large-area Mg 2+ -MXene aerogels without polymeric binders. Through a doctor blading technique and freeze drying, the Mg 2+ -MXene aerogels are produced with customized shapes/dimensions, featuring high surface area (140.5 m 2 g −1 ), superior electrical conductivity (758.4 S m −1 ), and high robustness in water. The highly conductive MXene aerogels show their versatile applications from macroscale technologies (e.g., electromagnetic interference shielding and capacitive deionization (CDI)) to on-chip electronics (e.g., quasi-solidstate microsupercapacitors (QMSCs)). As CDI electrodes, the Mg 2+ -MXene aerogels exhibit high salt adsorption capacity (33.3 mg g −1 ) and long-term operation reliability (over 30 cycles), showing a superb comparison with the literature. Also, the QMSCs with interdigitated Mg 2+ -MXene aerogel electrodes demonstrate high areal capacitances (409.3 mF cm −2 ) with superior power density and energy density compared with ...

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