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Abstract Terahertz (THz) spectral region from 0.1 to 3 THz is envisaged to hold immense potential in the next generation of wireless technologies. Recently, research has focused on this terahertz gap, because of its unprecedented channel capacities. At the physical layer, the design complexities and fabrication of THz devices, especially antennas are the prime bottlenecks to realize its full potential. This article introduces a cost-effective, easy-to-fabricate, and reproducible sub-THz antenna design based on a single-layer planar printed circuit board technology. The antenna incorporates carefully designed quasi-cross slots and applied machine learning-assisted global optimization techniques to achieve the desired performance metrics. The antenna performance is elucidated through numerical simulations and verified through a rigorous in-house THz experimental framework around 100–110 GHz. The proposed antenna offers a peak gain of 13.90 dBi with less than 1 dB variation within the entire band of 100–110 GHz. The antenna holds the potential to achieve terabits per second data rates and futuristic high-resolution short-range THz imaging applications.

Abstract A multifunctional beam splitting frequency selective surface (FSS) is modeled, analyzed, and tested in transmission and reflection modes. The proposed FSS comprises a C-shaped split-ring resonator designed and fabricated on an ultrathin, flexible polyimide material. When a linearly polarized incident wave interacts with the unit cell of the proposed FSS, half of the wave is reflected, and the other half is transmitted at two frequency bands from 5.8–6.2 GHz and 18.5–22 GHz. Moreover, the proposed FSS is angularly stable upto 40° and also performs simultaneous beam splitting and quarter-wave operation within one of its two bands of operation i.e., from 16.5–18.2 GHz. Such flexible beam splitting FSSs with polarization transformation operation and having angular stability, size miniaturization and multi-band operation is a specialized component having potential to be used for electromagnetic wave manipulation in antenna systems, radar technology, stealth technology, wireless communication, satellite communication, medical imaging, security and surveillance, aerospace and defense, and automotive radar.

Abstract This work presents an optically transparent and flexible MIMO antenna that features two square patch elements placed in close proximity, aiming to meet the demands of compactness, flexibility, optical transparency, and visual appeal for IoT applications and future 5G wireless communication. The design includes a simple offset fed configuration to achieve the required isolation and impedance matching. It simplifies the process of creating closely spaced transparent MIMO antenna configurations. By optimizing and analyzing this structure, the antenna achieves better isolation and diversity gain performance, even when the patch elements are positioned very close to each other. To achieve optical transparency and flexibility, the antenna uses thin polyethylene terephthalate (PET) material as a substrate, which is a thermoplastic polymer resin from the polyester family. The wired metal mesh parameters for conducting parts of the MIMO antenna and offset position of the feed are carefully optimized to achieve required optical transparency, isolation, impedance matching and radiation performance without any complex decoupling or impedance matching network.

Abstract In this letter, a compact, planar circularly polarized (CP) sub-GHz slot-based multiple-input-multiple-output (MIMO) antenna with dual sense CP along with polarization bandwidth reconfigurability is presented. The pentagonal reactively loaded slot is fed by two folded tapered feedlines to achieve CP. The antenna offers left-hand-circular polarization (RHCP) with the as well as right hand circular polarization (LHCP). The antenna exhibit linearly polarization (LP) by exciting two ports simultaneously. Moreover, the antenna CP resonance can be reconfigured by varying the capacitance of the varactor diode. The antenna has a wide −10 dB operating frequency band from 578–929 MHz. while the axial ratio (AR) bandwidth ranges from 490–810 MHz. Moreover, the two elements MIMO are optimized and placed on compact dimensions 100 × 100 × 0.76 mm3 to realize pattern diversity. The antenna’s key characteristics are compact size, wide-band sub-GHz operation, dual sense CP, polarization bandwidth reconfigurability and good MIMO performance. Thus, it is a suitable candidate to be utilized in CubeSats applications in sub-GHz bands.

Abstract A miniaturized folded dipole patch antenna (FDPA) design for biomedical applications operating at sub 1 GHz (434 MHz) band is presented. Antenna is fabricated on FR-4 substrate material having dimensions of 16.40 mm $$\times$$ × 8.60 mm $$\times$$ × 1.52 mm (0.023 $$\lambda$$ λ $$\times$$ × 0.012 $$\lambda$$ λ $$\times$$ × 0.002 $$\lambda$$ λ ). Indirect feed coupling is applied through two parallel strips at bottom layer of the substrate. The antenna size is reduced by 83% through lumped inductor placed at the center path of the radiating FDPA, suitable for biomedical (implantable) applications and hyperthermia. Moreover, Impedance matching is achieved without using any Balun transformer or any other complex matching network. The proposed antenna provides an impedance bandwidth of 6 MHz (431–437 MHz) below − 10 dB and a gain of − 31 dB at 434 MHz. The designed antenna is also placed on a human body model to evaluate its performance for hyperthermia through Specific Absorption Rate (SAR), Effective Field Size (EFS), and penetration depth (PD).

In this paper, a unique structural design methodology for chipless radio-frequency identification (RFID) tags in frequency domain is presented. The tag geometry is developed by loading an open-loop resonator with micro-metallic-cells (MMC). The realization give rise to a checkerboard resonator type with electromagnetic signatures in its radar cross section that are extremely efficient to manipulate. The resonators layout is distributed on either side of a Rogers substrate to double its coding density. The proposed chipless RFID tag has a memory of 8-bits in total. The operating band of the tag is 6.5-10.5 GHz. The tag has a high bit coding density of 10.94 bits/cm2 and spectral efficiency of 2 bits/GHz. The tag has a very compact size of 17.4 × 4.2 mm2. The simple structuring methodology and efficient resonators layout will give RFID system designers the flexibility to apply the proposed tag in a wide range of modern applications.

Abstract An impedance matched metasurface can efficiently channel the electromagnetic fields for maximum power transfer. The thin film based impedance matching techniques often utilize highly dissipative materials and destructive interference of reflection components from multiple subwavelength layers. Here, we propose a novel method to achieve anti reflection characteristics through destructive interference of antiparallel electromagnetic scattering emerging from chiral metasurface. The supercell structure of metasurface consists of four adjacent multi split-rings on FR-4 substrate. The split-rings are arranged to induce anti-parallel surface currents leading to destructive interference for scattered fields. The antireflection characteristics results in near perfect broadband absorption at dual frequency bands. A broadband absorption of 983 MHz is achieved between 12.687 and 13.669 GHz. Similarly, a narrow band absorption of 108 MHz is achieved in frequency range of 15.307–15.415 GHz. The impedance matched with unique symmetric design of supercell results in identical absorption for both x- and y-polarized incident fields. The numerical and experimental results verify broadband absorption characteristics at Ku band frequencies. The proposed metasurface absorber can be used for microwave energy harvesting applications.

Abstract This work presents the design of a low profile Fabry-Perot cavity-based multiple-input-multiple-output antenna with low correlation coefficient. The fields of closely placed antenna elements are tilted by using a phase-gradient partially reflective surface (PRS), thereby decreasing the correlation coefficient. The PRS is designed in such a way that its reflection phase is complementary to that of the ground plane. The PRS decorrelates the fields of the two radiating elements when placed at a height of λ/10 above them resulting in a reduction of the correlation coefficient by almost 95% for an isotropic environment. This height is considerably less than λ/4, which has been reported previously.

Abstract We present the design and practical implementation of a microstrip diplexer based on the wave discrimination property associated with the electromagnetically induced transparency (EIT)-like effect. The EIT is a quantum interference phenomenon which happens between two atomic transition pathways and allows wave propagation within a medium’s absorption spectrum. Here, we exploit an analogous interference mechanism in a three-port microstrip structure to demonstrate a diplexer based on the EIT-like effect in the microwave regime. Since the transparency is accompanied by a high transmission and strong dispersion characteristics, compact frequency discriminating structures that can resolve nearby frequencies with high isolation can be devised. Our proposed C-band diplexer consists of pairs of unequal open-circuit stubs, which resonate at detuned frequencies and interfere to form the EIT-like passbands for diplexer action. The design is highly compact and scalable in frequency for both PCB and on-chip applications. A prototype of diplexer is fabricated for the center frequencies of lower and upper passbands at 4.6 GHz and 5.5 GHz respectively. The transmission zeros are designed at the complementary channels so that the two passbands are highly isolated presenting the isolation of about 40 dB. The measured insertion loss of lower and upper passband is 0.59 dB and 0.61 dB respectively. Measured input return loss is better than − 15 dB, while the output return losses are well below − 12 dB. Moreover, a decent value of about 200 is achieved for the group refractive index around the EIT-like passbands, which reveals the slow wave characteristics of the proposed EIT-based diplexer.

A wideband snowflake antenna for 28 GHz millimeter-wave communications is proposed. The antenna has a size of 8×5 mm2, fabricated on ultra-thin 0.254 mm substrate. The antenna consists of four small hexagons surrounding a bigger hexagon in order to attain broadband characteristics. Bandwidth response of the antenna is between 25.28-29.04 GHz (13.43% @ 28 GHz), having 3.12 dBi gain and more than 80% of radiation as well as total efficiency. The antenna specific structure is designed such that it gives dual beam over its entire bandwidth without a noticeable shift in its beam directions. For enhanced gain performance, its 4-element array (of size 32×12 mm2) has been fabricated separately to increase its gain upto 10.12 dBi at 28 GHz. The proposed dual-beam antenna and its array has been modelled on thin Rogers substrate to show that its application in millimeter-wave communication equipment is viable.

A wideband antenna with a high gain and narrow beam-width for future 5G communication systems is presented in this research. The antenna operates in 28 GHz 5G band with a large 35.53% bandwidth ranging from 23.41-33.92 GHz. The array has 4-elements arranged in a linear fashion to attain a high gain of 10.7 dBi. It radiates along its end-fire direction and provides a very narrow beam-width of 14.6° in its H-plane. A corporate feed network specifically designed for thin substrates was used in order to excite the array elements. It is built upon thin 0.254 mm Rogers substrate to minimize transmission losses and attain high radiation efficiencies of more than 90% throughout its operating frequency range. It has a compact structure bearing low cost and is easy to fabricate. This antenna fulfills necessary requirements of 5G communication and is therefore a good candidate to be used in the millimeter-wave range.

We present a single-layer metasurface, which performs both cross and circular polarization conversion at multiple frequency bands. The unit cell of the proposed metasurface consists of periodic array of a uniquely designed hexagonal split ring resonator (SRR). The proposed metasurface behaves as an efficient 90° polarization rotator at dual frequency bands of 6.36-6.59 GHz and 10.54-13.56 GHz. The polarization conversion efficiency approaches 90% within these two operating bands. Moreover, linear-to-circular polarization conversion is also achieved in other three wide frequency bands from 6.10-6.20 GHz, 6.84-9.02 GHz and 14.10-15.48 GHz. The novel features of the subject hexagonal metasurface, which qualify it for numerous practical applications are high cross-polarization conversion (CPC) efficiency, wideband operation and angular stability for oblique incidence up to 45°.

A very compact and small sized 2-element planar Ultra Wide band (UWB) antenna having high isolation with band rejection characteristics designed on a FR4 substrate (23 × 26 × 0.8 mm3) is presented in this paper. Each antenna element have a rectangular stepped-slot and a z-shaped slot, both etched in ground plane. The rectangular slot, proximity fed by a 50 Ω micro-strip transmission line, acts as a main radiator while z-shaped slot acts as a filter for rejecting the full WLAN band (5.15-5.85 GHz). Use of an inverted T-shaped slot near top end of ground plane improves isolation more than 24.5 dB in lower UWB band i.e. 3-5 GHz, while the rectangular vertical slot etched at center of the ground plane near the bottom edge reduces the mutual coupling at lower as well as at higher UWB band i.e. 5 - 10.6 GHz. Both measured and simulated results are presented which are found generally in good agreement.

A 32-bit chipless RFID tag operating in the 4.5–10.9 GHz band is presented in this paper. The tag has a unique multiple-arc-type shape consisting of closely packed 0.2 mm wide arcs of different radii and lengths. The specific tag geometry provides multiple resonances in frequency domain of an RCS plot. A frequency domain coding technique has also been proposed to encode the tag’s RCS signature into a 32-bit digital identification code. The tag has an overall dimension of 17.9 × 17.9 mm2, resulting in a high code density of 9.98 bits/cm2 and spectral efficiency of 5 bits/GHz. The proposed tag is built on a low loss substrate bearing a very small footprint, thereby making it extremely suitable for large-scale product identification purposes in future chipless RFID tag systems.

An inkjet-printed, fully passive sensor capable of either humidity or gas sensing is presented herein. The sensor is composed of an interdigitated electrode, a customized printable gas sensitive ink and a specialized dipole antenna for wireless sensing. The interdigitated electrode printed on a paper substrate provides the base conductivity that varies during the sensing process. Aided by the porous nature of the substrate, a change in relative humidity from 18% to 88% decreases the electrode resistance from a few Mega-ohms to the kilo-ohm range. For gas sensing, an additional copper acetate-based customized ink is printed on top of the electrode, which, upon reaction with hydrogen sulphide gas (H2S) changes, both the optical and the electrical properties of the electrode. A fast response time of 3 min is achieved at room temperature for a H2S concentration of 10 ppm at a relative humidity (RH) of 45%. The passive wireless sensing is enabled through an antenna in which the inner loop takes care of conductivity changes in the 4–5 GHz band, whereas the outer-dipole arm is used for chipless identification in the 2–3 GHz band.