A 3D-printed polarization-independent low-cost lightweight and flexible frequency selective surface based dual-band microwave absorber is presented in this paper. Two concentric square loops fabricated at different heights using 3D printing technology are responsible for exhibiting dual-band responses at 3.32 GHz (S-band) and 5.46 GHz (C-band) with more than 97% absorptivities. The corresponding full widths at half maximum bandwidths are observed as 230 MHz (3.21–3.44 GHz) and 450 MHz (5.27–5.72 GHz). The proposed topology is polarization-insensitive owing to the four-fold symmetry. The absorption phenomenon is explained with the analysis of current distributions at the surface and impedance curves at the frequencies of resonance. Further, the performance has been evaluated for both planar and curved surfaces with different angles of curvature, and the good agreement between the measured and simulated responses confirms the flexible behavior of the proposed structure.

We introduce a systematic approach for designing ultrathin, flexible, and polarization-insensitive metasurface absorbers (MSAs), suitable for aviation applications, such as radar cross-section reduction of unmanned aerial vehicles. Metal-backed resistive patches are arranged on a flexible polyethylene terephthalate substrate of thickness about 1/100 of the operating wavelength, classifying the absorbers as ultrathin. The ultralow weight of the proposed MSAs is crucial for the targeted aviation applications, to ensure airworthiness. A narrowband uniform MSA is designed to achieve maximum absorption and serves as a starting point to synthesize a broadband and polarization-insensitive $3 \times 3$ absorber supercell. The non-uniform absorber is systematically designed by a fast semi-analytical method. The proposed absorbers have been fabricated and experimentally tested both on flat and cylindrical curved surfaces, with measurements being in very good agreement with the corresponding simulations, and corroborate the high absorption and broadband behavior of the proposed non-uniform ultrathin and flexible absorber.

This study intends to provide a comprehensive review of the basic concepts, types, applications, and experimental studies of frequency selective rasorber (FSR) presented in the literature. Analyzing the characteristics of FSR became crucial for future adaptability when taking into consideration of immense development in RADAR, military, stealth, and electromagnetic interference applications. The rasorber was initially conceived as a radome for antennas and it was developed expeditiously in recent years. This survey is focused on evaluating the unit cell design (2D, 2.5D, and 3D), equivalent circuit model, polarization characteristics, fractional bandwidth, insertion loss, absorptivity, bandwidth enhancement for absorption, transmission, and their applications based on the FSR. Various techniques like exploiting lumped elements, magnetic materials, lumped components, dual/triple layer structures, varactor diodes, PIN diodes, and distributed elements (slots and stubs) are used to improve the novelty and performance of the FSR that are discussed in the works of literature. At last, these techniques, bandwidth, structures, and performances are compared based on their relative positions to feature the benefits and limitations.

This work presents a passive intelligent surface designed at 2.45 GHz that has the capability of transmitting and reflecting electromagnetic waves that are incident upon it. The proposed surface does not require any circuitry or power source to function. Therefore, it makes a cost-effective and simple intelligent surface. It is a simple metallic structure that has embedded waveguide slots on its surface, allowing the waves to couple for transmission. A prototype of the proposed surface is designed using an aluminum foil and analyzed for both transmission and reflection of the wave. Further, the designed surface is investigated for tuning the directionality of the radiated field from the antenna. For this purpose, a coplanar patch antenna is first designed and then combined with the surface to tune the directionality of the radiated field of this antenna. The outcome of the measured performance validates that the proposed surface has the potential capability of field reconfiguration in wireless communication for Wi-Fi, WLAN, and Bluetooth applications.

In this paper, a capsule endoscopy system with a sensing function is proposed for medical devices. A single-arm spiral antenna is designed for data transmission and is combined with the voltage controlled oscillator to achieve sensing capabilities. The designed antenna operates at a 900 MHz industrial scientific medical band. By establishing a three-layer cylindrical model of the stomach, it was concluded that the antenna in the stomach has a high peak gain of −1.1 dBi. Additionally, the antenna achieved a −10 dB impedance bandwidth of 5%. The capsule endoscopy was experimentally measured in both actual stomach and simulated environments. The maximum working distance of the capsule endoscope was measured to be 6.8 m. Additionally, the proposed capsule endoscope was tested for its sensing function using solutions with different dielectric constants. Finally, it was confirmed through link analysis that it has good communication capabilities. The results and analysis confirm that the proposed capsule endoscope can be used for examining gastric diseases.

In this paper, a wearable antenna array based on a 9 × 3 artificial magnetic conductor (AMC) array is proposed with the characteristics of compact, low profile, low specific absorption rate (SAR), high front-to-back ratio (FBR) and high gain for wireless body area network (WBAN) bands. The proposed wearable antenna consists of a four-element array and an AMC array. The size of antenna array loaded AMC is 137.7 × 45.9 mm2. The dielectric substrate of the antenna and the AMC structure are made of 0.1 mm liquid crystal polymer material, which is flexible and low profiled. The antenna operates from 5.62 to 6 GHz after the AMC structure is loaded. The gain increases by 3.23 dB, reaching 12.03 dB at 5.8 GHz. And the FBR value is raised by 26.04 dB. The highest SAR value of the simulated antenna on the human model is 0.0496 W/kg, far less than the US federal or EU requirements. After constructing and testing the antenna, the outcomes of the tests agreed with the results of the simulation. The flexible antenna array with AMC structure has good prospect in WBAN applications.

The rapid expansion of digital media platforms and their growing user base in the wireless industry necessitate communication systems to provide information at high speeds with reliable connections. Therefore, wireless communication systems with a single antenna cannot accomplish these requirements. Consequently, the access and utilization of multi-input multi-output (MIMO) antennas are becoming more common in contemporary high-speed transmission systems. This article covers the fundamentals of MIMO antenna operation, the metrics for MIMO antenna performance parameters, and the design methodologies for specifying the three most commonly used antennas (two-port, quad-port, and eight-port). Additionally, it discusses their ability to improve channel capacity significantly. It focuses on designing MIMO antennas with ultra-wideband (UWB) for 5G systems operating between 1 and 27 GHz and millimeter-wave (mmWave) bands from 30 to 100 GHz. This article is valuable for researchers interested in developing MIMO antennas for diverse applications. It compiles advanced methods related to materials, advancements, challenges, and state-of-the-art technologies used in the design of high-performance MIMO antennas. We concluded that antennas that operate at mmWave frequencies have small dimensions and suffer from isolation problems in the MIMO formation. In contrast, antennas operating below 6 GHz are large and do not suffer from isolation problems.

This paper presents a substrate-integrated waveguide (SIW) differential antenna for full duplex applications. The proposed antenna consists of two square SIW cavities named as outer and inner. The inner cavity is nested into the outer cavity. The outer cavity is differentially excited with a pair of coaxial feed lines, while the inner square patch is orthogonally excited with another pair of differential coaxial feed lines. This orthogonal feeding arrangement results in high isolation between the differential ports. The modified hybrid TE130/310 mode of the outer cavity radiates through a pair of arc-shaped slots at 9.35 GHz, while the TM01 mode of the inner square patch is responsible for the radiations at 8.65 GHz. The proposed antenna prototype is fabricated and measured for validation. Moreover, the designed antenna has a front-to-back ratio better than 22 dB and measured maximum gain values of 6.1 dBi and 7.6 dBi at 8.65 GHz (Port 2 ON) and 9.35 GHz (Port 1 ON), respectively.

3D printed orthomode transducer (OMT) integrated with a 3D printed lens antenna is presented in this work. The OMT integrated with the lens antenna covers the range of 54–80 GHz, the radiator can handle a fractional bandwidth of 38%. Fused filament fabrication printing process is used for the domed elliptical profile lens antenna and polyjet printing process is used for fabrication of the OMT. The simulated radiation efficiency of the antenna remains above 90% for the entire bandwidth and the structure shows a gain of above 16 dBi.

A novel metasurface offering polarization conversion characteristic in five bands is studied and developed in this paper. To provide the anisotropic feature to the structure, a diagonally placed tapered rod is combined with two semicircular stubs. Due to the controlling ability to convert horizontal to vertical polarization and vice versa, and linear to circular polarization (CP), it serves as a multifunctional polarization converter. Simulation results suggest that the proposed polarizer functions as cross polarizer over 4.74−5.12 and 9.12−13.48 GHz. Additionally, it exhibits a distinct type of rotational sense across 4.24−4.68, 5.24−8.64, and 13.72–15.14 GHz in its linear to CP conversion behavior. The axial ratio of the polarizer is well below 3 dB throughout overall CP bands due to the minimum tolerance level in reflection phases with respect to acceptable limits. Moreover, it is noticed that the sense of CP is left-handed in the first band while right-handed in the remaining two bands. Thus, the suggested polarizer has potential to be integrated with antennas for satellite, defense, industry applications for getting the desired type of polarization in the distinguished bands.

This paper presents a novel compact self-quintuplexing antenna using a half-mode substrate-integrated waveguide cavity to implement multi-operation wireless services. The proposed antenna design incorporates five triangular protrusions of different dimensions, assembled with SIW to function as the radiating elements. Each radiator supports the one-eighth mode of the SIW cavity. The resonance frequencies of radiators are 3.63, 4.44, 5.23, 6.21, and 7.05 GHz. Each radiator operates at a distinct frequency due to the differing dimensions and is independently driven by 50 Ω microstrip lines. The measured reflection coefficients and isolation among any two ports are less than −10 dB and better than 23.6 dB, respectively. The measured gains at their respective resonant frequencies are 5.66, 4.84, 5.03, 7.08, and 6.59 dBi. The front-to-back ratio is better than 8.7 dB in each band. The difference of co-to-cross-polarization is greater than 19.3 dB.

In this paper, a high-order-mode (HOM) (TE330) cavity-fed 45° linear polarized 6×6 slot array antenna is proposed. The 45° linear polarization is achieved by introducing asymmetric cross slots on the HOM cavity, resulting in low profile and wide bandwidth. The antenna array was verified using standard printed circuit board technology. Measured results show that the impedance bandwidth ( $|S_{11}|\le$ −10 dB) is 13.9% (36.98–42.92 GHz), and the peak gain is 19.3 dBi with a 3-dB gain bandwidth of 13.6%. Attributed to its simple structure, low profile, and wide bandwidth, the presented antenna is a good candidate for 5G applications.

This paper presents a pattern reconfigurable antenna that accomplishes wideband and multibeam characteristics. The antenna design comprises a cross-slot radiator as the primary element and reconfigurable partially reflecting surface (PRS) layer placed above and below the cross-slot radiator. This configuration allows the antenna to adapt its radiation patterns effectively. The choice of the cross-slot radiator is based on its capability to offer wideband characteristics. The PRS layer consists of a precisely arranged array of 4×4 unit cells, incorporating PIN diodes into both the upper and lower PRS layers. The direction of radiation pattern can be changed by altering operating states of the PIN diodes on the PRS layer. The antenna operates in three distinct states, each exhibiting a unique radiation pattern. The antenna produces broadside, backward, and bidirectional radiation patterns. It demonstrates effective pattern reconfigurability across the frequency range of 3.10–3.86 GHz (21.71%), with a peak gain of 9.60 dBi. The simulated and measured results of the antenna are found to be in good agreement.

This study presents the design and realization of a dual-port multiple-input–multiple-output (MIMO) filtering antenna system. The device shows good response in the frequency rage 1 (4.1 to 7.125 GHZ) frequency band, which handles most of the cellular mobile communication traffic. First, the single element ultra-wideband (UWB) filtenna is designed by combining a UWB antenna and a band-reject filter (BRF) where the antenna works in the frequency range from 1 to 11 GHz and the BRF is rejecting the frequency range of 3–3.42 GHz with 3.2 GHz as its resonant frequency. Finally, the proposed two-port MIMO filtenna combines two single element UWB filtennas in antiparallel manner which shows impedance bandwidth from 2.59 to 7.1 GHz with a band notch from 3 to 3.42 GHz. The structure is built on cost-efficient FR4 substrate (εr = 4.4, tanδ = 0.02) of dimensions 0.68${{\boldsymbol{\lambda }}_{\boldsymbol{c}}}$×0.27${{\boldsymbol{\lambda }}_{\boldsymbol{c}}}$×0.01${{\boldsymbol{\lambda }}_{\boldsymbol{c}}}$ (mm3), which is compact in size and utilizes a defected ground structure for further miniaturization. The proposed design is simulated using Ansys HFSS software, and after fabrication, it is measured, and the output shows good results for the proposed application. The designed antenna system is suitable for fifth-generation (5G) wideband systems.

This paper presents a new topology for a microwave active mixer using compact microstrip diplexer constituted from two dual-mode open loop resonators. The resonators are tuned to the radio frequency (RF) and local-oscillator (LO) frequencies, respectively, to effectively isolate the two input signals, while the diplexer output port is connected to the gate circuit of a commercial low cost GaAs pHEMT transistor. The drain circuit of the active device is connected to the load via an intermediate-frequency (IF)-modified Chebyshev low-pass filter to remove the unwanted RF and LO signals and reduce other spurious frequencies at the output of the mixer circuit. The circuit has been designed and constructed for an RF of 850 MHz, LO frequency of 1050 MHz, and an IF of 200 MHz. Experimental measurements show that the circuit provides a conversion gain of 18 dB at LO drive power of +2 dBm. It also operates satisfactorily over the RF range from 800 to 900 MHz with good image frequency rejection and stable operation.

In this paper, on–off switching digitization of a W-band variable gain power amplifier (VGPA) with above 60 dB dynamic range is introduced for large-scale phased array. Digitization techniques of on–off switching modified stacking transistors with partition are proposed to optimize configuration of control sub-cells. By the proposed techniques, gain control of a radio frequency variable gain amplifier (VGA) could be highly customized for both coarse and fine switching requirements instead of using additional digital-to-analog converters to tune the overall amplifier bias. The designed VGA in 130 nm SiGe has achieved switchable gain range from −46.4 to 20.6 dB and power range from −25.0 to 15.7 dBm at W band. The chip size of the fabricated VGPA is about 0.31 mm × 0.1 mm.

An ultra-wideband current-reused low-noise amplifier (LNA) monolithic microwave integrated circuit design is presented in this letter. Negative feedback networks are employed at both stages of the proposed LNA to expand bandwidth. Furthermore, source adaptive bias networks is designed in the first stage and combined with a current-reused construction to acquire a compact chip size and maintain low power consumption. Then, the validation of design theory is implemented by employing a 0.15-µm gallium arsenide pseudomorphic high-electron-mobility transistor process. The measured results show that the proposed LNA achieves a small signal gain of 15.5–17.8 dB, a noise figure of 3–3.65 dB, and an output 1 dB compression point (OP1 dB) of 14.5–15.5 dBm from the target bandwidth of 2–18 GHz. In addition, the fabricated LNA consumes 220 mW from a 5 V supply and occupies a chip area of 1.2 × 1.5 mm2.