A comprehensive survey of the reconfigurable leaky-wave antennas (LWAs) is made in this paper. Beam-steering and unique radiating features of LWA are highlighted particularly. Therefore, the radiation mechanism of different types of LWA, including uniform, quasi-uniform, periodic, and metamaterial LWAs are discussed in detail. The guiding structures for realizing LWAs, namely microstrip, waveguide, substrate integrated waveguide (SIW), and half-mode SIW (HMSIW) are investigated as well. Basic concepts of electronic beam-scanning LWAs are also introduced, and several state-of-the-art reconfigurable LWAs are studied thoroughly. The investigated reconfigurable LWAs are suitable for beam-scanning applications due to their compactness, ease of implementation, reasonably high gain, and relatively wide beam-scanning range, as will be demonstrated through this comprehensive review.

In this paper, a comprehensive review of portable microwave sensors for monitoring moisture content (MC) is presented. MC monitoring is crucial in different industries, particularly food and farming. Microwave-based approaches for measuring the MC of the grains and mineral materials are studied. These approaches are categorized into three groups: S-parameters, dielectric constant, and impedance measurements. While these methods are interrelated, they have differences. The investigated methods use different microwave antenna sensors for MC monitoring, such as coaxial probes, horn antennas, loop antennas, microstrip patch antennas, and frequency selective surface (FSS) antenna. State-of-the-art microwave sensors were investigated thoroughly to clarify the current challenges and possible solutions of MC monitoring. A comparison between the investigated sensors was made to determine their advantages and disadvantages. According to the comparison, sensors operating above 10 GHz suffer from cross-interference. Moreover, microstrip patches can monitor a wide MC range as extensive as 60%. At the same time, the FSS sensor has the highest sensitivity with an error as low as 0.023% at X-band. Microstrip patch and FSS antennas can be printed directly on a flexible, low-loss, and lightweight material to monitor the grain MC. The flexibility, compactness, portability, ease of environment-friendly fabrication, and high sensitivity are among the criteria determining the most suitable microwave sensors for industrial and consumer MC monitoring applications.

A novel portable, lightweight, and compact microwave sensor for monitoring the grain moisture content (MC) is proposed in this letter. The proposed sensor is a frequency-selective surface (FSS) antenna consisting of 21 square-shaped complementary split-ring resonators (SCSRRs). The proposed sensor was printed directly on a lightweight and flexible container made of PF-4 with the size of 60 mm × 28 mm × 50 mm. Each SCSRR consists of two cocentered square-shaped rings with 0.3-mm spacing. The length, width, and thickness of the conductive section are 52 mm, 20 mm, and 0.017 mm, respectively. The operating frequency band was chosen as 1−6 GHz due to the insensitivity of the water's dielectric constant to the temperature variation. Monitoring variations in S -parameters, notably the resonance frequency, indicate the MC variations. The proposed sensor detects a 160-MHz shift in the resonance frequency for barley MC ranging from 10 to 25%. The high sensitivity, compactness, flexibility, low weight, portability, small cross-interference sensitivity, simplicity of operation, environment-friendly fabrication, and low cost of the proposed sensor make it attractive for monitoring grain MC.

In this communication, we introduce a novel design for mitigating the side-lobe level (SLL) of the half-mode substrate integrated waveguide (HMSIW) based leaky-wave antenna (LWA). Applying a novel approach through modification of the side aperture of HMSIW, we achieved SLL of -13.8 dB and -11.2 dB in the upper hemisphere and full space, respectively. The key novelty of this manuscript is the reduction of SLL in full space while the state-of-the-art antennas only mitigated the SLL in the upper hemisphere. Furthermore, tapering the open side aperture in a thin trapezoid shape led to a significant reduction of beam squint. The operating frequency band of the antenna matches the allocated 5G wireless network mm-wave bands from 26 to 30 GHz. The measured peak realized gain of the antenna is 10.6 dB at 28.5 GHz. The length, width, and height of the HMSIW antenna are 70 mm, 15 mm, and 0.5 mm, respectively. The antenna was fabricated on a Rogers RT/Duroid 5880 substrate. Excellent agreement between the measurements and simulated results was observed. The discrepancies between measured and simulated results were analyzed by a complete and thorough sensitivity analysis that included the effects of the fabrication tolerances, connectors misalignment, and bending due to the mechanical stress. The high gain, low SLL, and compactness are among the advantages of the proposed antenna making it a suitable candidate for the miniaturization of 5G communication systems.

A new reconfigurable slotted antenna is presented in this manuscript. The proposed antenna is realized on a half-mode substrate integrated waveguide (HMSIW). The beam-steering is achieved using embedded varactor diodes in circular cells. Sweeping the bias voltage causes variations of the phase constant, which leads to the fixed frequency beam-scanning. Each reconfigurable cell includes a circular slot at the top plate and varactor switch, DC block, and RF block at the backside of the structure. The operating frequency is chosen as 28.5 GHz in the support of the upcoming 5G mm-wave communications systems. The proposed antenna beam scans 29° of space by switching among different states at 28.5 GHz. The length, width, and height of the antenna are 67 mm, 48 mm, and 0.32 mm, respectively. The proposed antenna’s main novelties are compactness and electronic beam-scanning capability using a single switch per cell. The measured peak realized gain and sidelobe level (SLL) at 28.5 GHz are 8.2 ± 0.6 dBi and 5 dB, respectively. While the impedance bandwidth of the proposed antenna is 1.5 GHz. A good agreement between measured and simulated results is observed. The discrepancies are investigated through a sensitivity analysis. Fixed frequency beam-scanning capability, compactness, simplicity of the assembly, backward radiation, and slight gain variation of the proposed antenna make it a suitable candidate for blind-spot monitoring and 5G vehicle to everything (V2X) communications.

A novel fixed frequency beam-steering leaky-wave antenna (LWA) is presented in this manuscript. Radiation occurs through thin offset longitudinal slots etched on a substrate integrated waveguide (SIW). The beam-steering is achieved by implementing GaAs varactor diodes on the backside of the antenna. Sweeping the bias voltage from 1V to 15V causes the phase constant variation, leading to the 25° beam-steering at 28.5 GHz. The key novelties of the proposed reconfigurable antenna are the low-profile and electronic beam-steering capability using a single varactor diode per cell located on the antenna's backside. Moreover, placing the components on the backside reduces the unwanted blockage effects on the radiation. The antenna's thickness, length, and width are 0.32 mm, 95 mm, 24 mm, respectively. The measured peak realized gain of the proposed antenna at 28.5 GHz is 9 ± 0.8 dBi. Low-profile, small gain variation, ease of fabrication, and electronic beam-steering capability make the proposed LWA suitable for the 5G beam-steering applications. A detailed sensitivity analysis along with the experimental validations were performed to prove the validity of the proposed novel design.