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This letter presents a $4 \times 4$ Butler matrix realized using stereolithography 3-D printing technology and liquid metal filling. The matrix is in coaxial technology and relies on circular branch lines and curved $50~\Omega $ lines. Input and output ports are aligned and placed on the same plane, while branch lines are organized using all three dimensions to minimize the length of the connecting lines. A prototype, designed at an operating frequency of 12 GHz, is manufactured and tested, showing a footprint of only $1.7\times 1.3\,\,\lambda _{0}^{2}$ . A good agreement between simulated and measured transmission coefficients is observed regardless of the selected input port. These encouraging results open the door to the realization of compact low-cost and high-performing RF beam-steering networks in coaxial technology, based on curing-free additive manufacturing processes.

This paper presents a novel passive Schottky-diode frequency doubler equipped with an on-off keying (OOK) modulation port to be used in harmonic transponders for both identification and sensing applications. The amplitude modulation of the second-harmonic output signal is achieved by driving a low-frequency MOSFET, which modifies the dc impedance termination of the doubler. Since the modulation signal is applied to the gate port of the transistor, no static current is drained. A proof-of-concept prototype was manufactured and tested, operating at 1.04 GHz. An on/off ratio of 23 dB was observed in the conversion loss of the doubler for an available input power of −10 dBm. The modulation port of the circuit was excited with a square wave (fm up to 15 MHz), and the measured sidebands in the spectrum featured a good agreement with the theory. Then, the doubler was connected to a harmonic antenna system and tested in a wireless experiment for fm up to 1 MHz, showing an excellent performance. Finally, an experiment was conducted where the output signal of the doubler was modulated by a reed switch used to measure the rotational speed of an electrical motor. This work opens the door to a new class of frequency doublers, suitable for ultra low-power harmonic transponders for identification and sensing applications.

River discharge is an important variable to measure in order to predict droughts and flood occurrences. Once the cross-sectional geometry of the river is known, discharge can be inferred from water level and surface flow velocity measurements. Since river discharges are of particular interest during extreme weather events, when river sites cannot be safely accessed, noncontact sensing technologies are particularly appealing. To this purpose, this work proposes a prototype of a low-cost continuous wave (CW) Doppler radar sensor, which is able to monitor the surface flow velocity of rivers. The prototype is tested at two gauged sites in central Italy, along the Tiber River. The surface flow velocity distribution across the river is monitored by means of the analysis of received Doppler signal. The surface velocity statistics are then extracted using a novel algorithm that is optimized to run on a microprocessor platform with minimal computing power (ArduinoUNO). In particular, the radar measurements are used to initialize a 2-D entropy-based velocity model (EVM) that is able to estimate river discharges in any flow condition. Finally, the results concerning the observed discharge provided by the EVM prove to be comparable with those obtained with more expensive commercial solutions. The results are important since the described methodology can be extended to small-size Doppler radar sensors onboard unmanned aerial vehicles (UAVs), the latter providing a method for mapping surface velocity of rivers.

This paper presents a beamforming network used to synthesize steerable nulls. The antenna topology consists of four patch antennas placed on the lateral sides of a cube. From the analysis of the array factor and the radiation pattern of the single radiating elements, the theory to realize fully steerable nulls on the azimuth plane is presented. The array, targeted for an operating frequency of 3.6 GHz, is fed by a reconfigurable radio-frequency network. The network, which bases its reconfigurability on eight phase shifters, makes it possible to excite couples of adjacent output ports with arbitrary amplitudes and phases. The radiation pattern of the topology is measured for different excitations, confirming the theory. This approach opens the way to a new beamforming approach, that can be applied to interference removal and radio localization applications.

This contribution describes the feasibility of a wireless sensor that uses Micro-Electro-Mechanical Systems (MEMS) on cellulose materials. In particular, the focus is on a paper-aluminum (paper-Al) bimorph cantilever formed by an Al foil glued on top of a paper layer. It is well known that paper relaxes its fibers in the presence of humidity. This means that the length of the paper layer increases (respect to the Al layer) when humidity is absorbed, thus determining the cantilever deflection. Such a mechanism is used to control a capacitor formed by the cantilever itself (Al layer) and by a metal plate printed on a PCB. The capacitor is then applied to a planar notch filter whose resonant frequency is hence determined by the humidity. The preliminary experiments show that the developed circuit features a 2.45 GHz center frequency and a sensitivity of about 3.7 MHz ner each nercent of relative humidity variation.

This paper presents a novel displacement transducer, based on a 3-dimensional coupled-line directional coupler, suitable for wireless passive vibration sensing. The 3-D printed transducer is based on two quarter-wave microstrip lines, which are aligned one on top of the other so that part of the signal flowing in one line is coupled to the other. The four ports of the directional coupler are realized on the same layer by adopting two transmission-line transitions. The circuit is manufactured with stereolithography 3-D printing. Selective metallization is achieved by embossing the desired patterns on the 3-D printed dielectric and then applying silver nanoparticle ink to the protruding areas. Vibrations change the distance between the lines, thereby modulating the coupling coefficient of the directional coupler. This way, the sensed vibration can be encoded in the amplitude modulation (AM) of a backscattered RF carrier. This information can be transmitted in real time and recovered by the reader using a simple AM demodulator without the need for any additional electronic component on the tag, that, thus, results in an ultra-low-cost chipless sensor.

This paper introduces a novel additive-manufacturing technique to obtain high-resolution selective-ink-deposition on complex 3-D objects, packages, and modules for 5G applications. The technique consists of embossing the desired pattern directly on the 3-D printed dielectric surface and then applying ink with a suitable tool. This approach is tested in combination with stereolithography 3-D printing technology to obtain selectively metallized 3-D circuits. In particular, the “clear” resin from FormLab is utilized for the 3-D printed dielectric, while the metallization is performed with silver nanoparticle ink from Suntronic. As a preliminary study, test samples containing lines with different widths are manufactured, demonstrating a pitch down to $135~\mu \text {m}$ and satisfactory sheet resistance of $0.011~\Omega /\text {sq.}$ (the electromagnetic characterization of the dielectric resin is reported in the Appendix). Then, two broadband multiport RF structures are developed to show the versatility of the proposed technology. First, an ultrawideband 3-D crossover, operating in the range 100 MHz–5 GHz, is conceived to test the suitability of the proposed technology to perform selective metallization on curved semienclosed areas. Then, the technology is applied to a multiple-input–multiple-output (MIMO) antenna system, based on four proximity-fed annular slot antennas, arranged on the lateral sides of a cube and decoupled by introducing a cross-shaped structure in the interior of the cube. This circuit offers a broad range of metallization challenges, as it features embossed and engraved parts, high-resolution patterns (line widths down to 0.7 mm) and sharp edges. Each slot radiates unidirectionally with the same polarization and uses the cube and its internal cross-shaped structure as a resonant cavity. The antenna system is designed to operate in the band 3.4–3.8 GHz, which is one of the sub-6-GHz 5G bands in Europe, and it is thought for hotspot and access-point applications. The final antenna topology is composed of only two blocks, weighs 21.29 g, and occupies a volume of $44.4\times 45.8\times 45.8\,\,\text {mm}^{3}$ , featuring an envelope correlation coefficient (ECC) lower than 0.005 and a total active reflection coefficient (TARC) lower than −6 dB in all the bands of interests.

This article presents a new circuit topology for a highly reconfigurable $1\times 4$ radio frequency network that, for a given input, is able to modify magnitude and phase relations among its output signals with great flexibility while preserving matching at all ports. The circuit, based on a four-way power divider, four hybrid couplers, and eight variable phase shifters, is first theoretically analyzed. It is shown that magnitude and phase relations among signals at the output ports can be modified just by changing the state of the variable phase shifters, while the sum of the squared magnitude of the transmission coefficients of the network is always equal to one. For the specific purpose of the experimental validation of the developed theory, a particular version of the network, working at 3.6 GHz, is then designed and tested. All configurations are affected by the same insertion loss, depending on the adopted phase-shifting technology. The obtained results show the potential of the conceived circuit and testify its unprecedented degree of reconfigurability.

This letter describes a single-transistor oscillator implemented on a paper substrate at the operating frequency of 24 GHz, based on a design procedure that makes it possible to overcome the limitations caused by thin high-loss substrates and package parasitics of commercial components. The commercial transistor adopted in the oscillator is led to instability by exploiting its parasitic gate inductance. The drain network compensates for the parasitic inductance of the transistor package and connects the oscillator circuit to a 50- $\Omega $ load resistance. Finally, an external inductor at the source terminal is used to resonate out the capacitive contribution of the circuit at the oscillation frequency. Overall, the active device appears in a quasi-common-drain stage. Experimental results on prototype show that the oscillator exhibits an output power of +6.2 dBm at 24 GHz, with a dc power consumption of 72.5 mW.

This paper describes a reconfigurable beamforming network that can be connected to a four-element circular array to perform continuous beam-steering on the azimuthal plane of the array. The network, which consists of a fourway power divider, four 90° hybridjunctions, two 90° fixed phase shifters and four variable phase shifters, is able to feed each couple of adjacent radiating elements with arbitrary power ratios, without spoiling the matching conditions of the other ports. That way, the maximum of the radiation pattern can be steered arbitrarily on the azimuthal plane only by varying the phases of the variable phase shifters of the network.

This paper proposes a Ka-band receiver front-end for future CubeSats Low-Earth Orbit (LEO) to Geostationary (GEO) inter-satellite links. The receiver is able to support very high data rates (up to 100 Mbit/s) in Quadrature Phase-Shift Keying (QPSK) when in the line of sight of a GEO satellite that is equipped with a steerable 70-cm antenna and transmitting a 25-W signal. The originality of the proposed approach is twofold. First we will demonstrate the receiver feasibility based on a class of miniaturized and low-cost microwave integrated circuits, currently available on the market. In particular, our receiver is based on a novel combination of integrated Low-Noise Amplifiers (LNA) with an image rejection filter, the latter exploiting the Substrate Integrated Waveguide (SIW) technology. An optimization of the via placement proved to be able to reduce the need for shielding apparatuses, thus simplifying the mechanics and reducing mass, volume and hardware costs. Secondly, we will propose a noise injection circuit capable of measuring and calibrating the receiver gain, also during in-orbit operation. Self testing capabilities are particularly relevant for CubeSats because the usage of commercial components poses serious reliability issues.

In this contribution, the authors perform the design and show the experimental results relative to a prototype of a combined wireless power transfer (WPT)–power line communications (PLC) system, in which the WPT channel is interfaced to a PLC environment to allow data transfer when the cabled connection is no longer available. The main rationale behind this idea stays in the fact that PLC communication is now a popular choice to enable communications, for instance, in smart grids and in home automation, while WPT devices start to be available in the market (i.e. for mobile phones) and soon they will be a reality also for higher power (i.e. vehicle battery charging). In particular, theoretical insights about the requirements of the system are given; a two coils system has been implemented and a measurement campaign, together with simulations, show that the system is of great potentiality and could be used in applications where both wireless power and data transfer are needed (such as vehicles battery charging), achieving maximum power transfer and good data rate in order to transmit high-speed signals.

This paper describes the design, realization, and application of a custom temperature sensor devoted to the monitoring of the temperature differential between the leaf and the air. This difference is strictly related to the plant water stress and can be used as an input information for an intelligent and flexible irrigation system. A wireless temperature sensor network can be thought as a decision support system used to start irrigation when effectively needed by the cultivation, thus saving water, pump fuel oil, and preventing plant illness caused by over-watering.