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We propose a general method for designing wideband matching tapers in inhomogeneous media where phase velocity is coupled to the taper impedance profile. Such tapers are used to match wideband gradient index (GRIN) lens antennas. To simplify fabrication tapers are often constrained to physically uniform layers wherein commensurate line theory cannot predict the frequency response. Therefore, we present a new design algorithm which derives an effective permittivity $\epsilon_{eff}$ to equalize the electrical length of commensurate and non-commensurate line tapers. The algorithm provides a systematic design method with predictable frequency response for non-commensurate line tapers. Nevertheless, there are several unavoidable nonidealities present in such discretized tapers which we discuss and provide recommendations for mitigation. The algorithm is used to design a Klopfenstein taper with return loss better than 15 dB from 8 to 78 GHz. The design is fabricated and measurements agree with simulation across the WR90, WR28, and WR12 bands. An approximate efficiency formula is proposed which predicts aperture efficiency of taper-matched lenses without the need for time-consuming full-wave simulations. Various lenses are designed and compared to highlight the advantages of Klopfenstein tapers in GRIN lens design. The results demonstrate the usefulness of the proposed design method., Comment: 11 pages, 9 figures. Accepted June 5, 2021 for publication in IEEE Trans. Antennas Propag

In this paper, a review of frequency-coded (FC) passive chipless RFID sensors is presented. The prime aim of this review paper is to discuss the key concept of a passive chipless RFID sensor and its classification according to fundamental sensing parameters. The fundamental sensing parameter refers to the intrinsic characteristics of a sensor that is sensitive to an environmental parameter to be sensed. The widely used intrinsic parameters are effective permittivity, effective electrical length, and electromagnetic coupling. The categorization of frequency-coded passive chipless RFID sensors based on fundamental sensing parameters facilitates the future researcher to better understand the operating principle of the diverse RFID sensors. The improved understanding helps in the advanced design and implementation of RFID sensors. In addition, the key performance measures and recent areas of applications are discussed. This paper also focuses on existing challenges and the scope of the further developments of frequency-coded passive chipless RFID sensors.

This paper presents the design and testing of a novel triple-layer frequency selective surface (FSS) with ultra-miniaturization for electromagnetic shielding applications. The proposed unit cell is constructed using meandered lines running along the three layers and they are interconnected using metalized vias to increase the electrical length of FSS. The FSS is designed to offer band stop resonance at L-band frequency with a foot-print of 0.022o0.022owith a thickness of 0.006o and it provides at least 38% of miniaturization on comparing with similar literature. To validate the transmission performance, a prototype FSS with a periodicity of 6mm is fabricated. The FSS consists of 2500 elements arranged in a 5050 square lattice. The transmission performance of the proposed FSS is verified using simulation and experiment. The proposed FSS has rotational symmetry and hence it is polarization insensitive. Furthermore, the FSS offers excellent angular stability up to the maximum incidence angle of 60o under both TE and TM modes of operation. The proposed FSS can be used for shielding applications in global positioning systems (GPS) tracking and aircraft surveillance systems.

In this paper, we propose a novel circularly polarized (CP) dipole antenna fed by modified quadrature hybrid couplers (QHCs). In a conventional CP antenna, such as a sequentially rotated structure, the electrical length of the line connecting the radiating elements changes as the frequency changes. This characteristic produces a narrow axial ratio (AR) bandwidth and changes the radiation patterns of the array antenna at different frequencies. A modified QHC with minimal variation in the phase and amplitude difference between the output ports over a wide frequency range is used to achieve wide AR bandwidth and stable radiation patterns. The -10 dB impedance bandwidth of the proposed antenna is 4.1-7.6 GHz (58.3%), and the 3-dB AR bandwidth is 4.8-7.4 GHz (43.3%). The maximum gain is 4.4 dBic, and the 3-dB gain bandwidth is 5.0-7.5 GHz (40.0%). The antenna size is 21.0 × 21.0 × 0.2032 mm3 (0.42 × 0.42 × 0.004 λo 3 at 6 GHz).

A five-band polarization-insensitive perfect metamaterial absorber (PMA) is reported in this paper for THz detection and sensing applications. The proposed absorber is constructed using interconnected circular ring elements enclosed by a square loop. The ring elements are interconnected using short strip lines which increases the electrical length to offer resonance at the lower frequencies of the THz regime without increasing the electrical length. The proposed absorber has a footprint of 0.12 λeff × 0.12 λeff where λeff is the effective wavelength calculated at the lowest operating frequency. The absorber provides 92%, 84%, 90%, 100%, and 100% absorption at 0.24, 0.56, 0.65, 0.82, and 0.95 THz, respectively. The proposed structure offers structural symmetry, and hence, it is polarization-insensitive. The proposed five-band absorber has good angular stability consistent with many research works reported in the literature and has a small frequency ratio of 1:2.3:2.7:3.4:3.9. The proposed absorber can be used as a permittivity sensor and its sensitivity is estimated to vary from 5.8 GHz/permittivity unit (PU) to 23.56 GHz/PU.

This paper presents a highly sensitive phase-variation sensor for dielectric constant measurements based on a capacitively-loaded periodic slow-wave transmission line. Such line is implemented in microstrip technology, and the loading capacitors are formed by closely spaced rectangular patches. The sensing area of the device is delimited by the region occupied by the capacitive patches, where the material under test (MUT) must be located. The presence of the MUT modifies the coupling capacitance between adjacent patches, thereby producing a variation in the electrical length, or phase of the transmission line, which is the output variable. A detailed analysis of the equivalent circuit model of the unit cell, useful for design purposes, is carried out in the paper. Based on such analysis, a prototype device sensor is designed and fabricated. It is demonstrated in the paper that the sensitivity of the proposed sensor is by far superior to the one achieved in an ordinary meandered line with similar sensing area. Thus, the proposed slow-wave structure constitutes a good alternative to meandered lines for the implementation of phase-variation sensors with simultaneously high sensitivity and compact size.

In this paper, a new compact design of frequency reconfigurable slot antenna is presented. The proposed radiating patch consists of two symmetrical D shape slots and incorporates four PIN diodes with two stubs which cover a small dimension area of 8.6 × 6.0 × 1.6 mm3. The ground plane is modified by etching two parallel rectangular slots to enhance the bandwidth and design parameter of the antenna. RF pin diodes are used as a switching mode in the slotted D section of the patch which affects the electrical length of the slot antenna, therefore switchable frequency response is obtained. The antenna produces four dual-band states and twelve triple-band states among the sixteen states of diode configuration. The designed antenna resonates at six different resonating frequencies such as 17.4 GHz, 17.5 GHz, 18.9 GHz, 19.0 GHz, 24.9 GHz, and 25 GHz and covers the wide range of frequency from 17.2 to 25.5 GHz which makes it applicable to function at the Ku and K band applications. Simulation of an antenna is done by Ansys HFSS 17.0 software. The results of the simulation show that the antenna has achieved a stable radiation pattern with an appreciable gain of 10 dB, peak directivity 4.26 dBi, peak gain 5.66 dBi, and a maximum bandwidth of 1 GHz with 98% radiation efficiency at these operating frequency bands.

A phase-variation microwave sensor operating in transmission and implemented by means of a high-impedance half-wavelength sensing line is reported in this paper. The sensor is useful for dielectric constant measurements and dielectric characterization of materials. By forcing the electrical length of the sensing line to be a half-wavelength when it is loaded with the so-called reference (REF) material, perfect matching is obtained regardless of the characteristic impedance of the line. This fact can be used to enhance the sensitivity for small perturbations, by merely increasing the characteristic impedance of the sensing line. An exhaustive analysis that supports such conclusion is reported in the paper. Then, two prototype sensors are designed and fabricated for validation purposes. As compared to the ordinary phase-variation permittivity sensor implemented by means of a matched ( $50-\Omega $ ) line with identical length, the sensitivity for small perturbations in the proposed sensor is 2.1 times larger. Further advantages of these sensors are low-cost, small size, implementation in planar technology, and very simple design and fabrication, derived from the fact that the sensing region is a half-wavelength transmission line.

A novel concept of tandem coupled-line directional couplers has been proposed. The proposed couplers consist of coupled-line sections having different electrical lengths and fixed identical coupling coefficients regardless of the required nominal coupling of the entire tandem coupler. It is proved that the available coupling in coupled-line sections constituting tandem directional couplers can be used for overall electrical length reduction of the entire network. As shown, this allows not only for reduction of the circuit’s size but also enables flexible design of such couplers, since tuning of the coupling characteristics can be easily achieved by change in solely electrical lengths of the coupled sections with no change in their coupling coefficients. The thorough theoretical investigation has been presented of such tandem couplers composed of two and three coupled-line sections having fixed coupling coefficients followed by measurements of four manufactured networks operating at a center frequency of 2 GHz.

In this letter, a new method for decreasing the loading effect of the transient voltage suppressor (TVS) diode has been presented and its technical details are studied under the immunity standard of IEC61000-4-2. In our approach a transmission line with a specific electrical length is employed to convert the TVS diode impedance to a significant impedance at the operating frequency. As a result, the TVS diode does not have any destructive effect on the RF specifications. In addition to the simulation using ADS software, a test circuit is fabricated on a RO4003 substrate. The simulated and measured S11 and S21 show appropriate frequency responses. However, by applying the proposed method, a parasitic inductance in the discharge path is created that can cause significant overshoot voltages during an electrostatic discharge event. In order to handle this shortcoming, the impact of this inductance is well studied using ADS and CST software and two techniques are introduced to minimize the overshoot voltages.

This paper presents the development of a compact monopole antenna bioinspired in the Opuntia ficus-indica plant-shape, generated by Gielis formula, built in fiberglass substrate, with hybrid feed by transmission line and probe, for wireless local area network band at 5 GHz. By using the plan-shape, it is possible to increase the perimeter of the design and thus reduce its total area, which can be employed in the development of compact antennas with larger electrical length. Results of the bioinspired plant-shape antenna is compared to the square-shape antenna, and the proposed antenna presents a more compact structure, with a reduction of 62.5% in the total area, with a value of 108.4 mm2, narrower bandwidth, and a directive radiation pattern, with a bandwidth of 1.44 GHz (4.41–5.85 GHz), which covers the 5 GHz WLAN band (5.15–5.85 GHz) and measured gain of 2.6 dBi, at the resonance frequency.

The benefits of empty substrate integrated waveguide (ESIW) are already well-known and contrasted in the literature. Different variations of transitions are available to maximize the power injection in that structure. Besides, several topological variations of the ESIW have also been proposed, such as the ridge ESIW, which enlarges the monomode bandwidth at the cost of higher losses and larger cross-section area. This paper presents a new variation of topology: the folded-ESIW, that halves the width of the original ESIW waveguide and maintains the cross-section area with good performance indexes and a similar bandwidth. A discussion is made about the feasibility of this novel topology and a new transition is presented with a high degree of robustness and a minimum electrical length. To validate and prove this new proposal, a back-to-back prototype has been designed, manufactured and measured, with results so promising that give access to new design strategies in the ESIW field.

In this paper, we describe the development of a new beam-steerable antenna array using a reconfigurable waveguide with tunable impedance side walls. The array has 36 radiating elements placed on the top wall of the waveguide and a common biasing voltage for tuning the impedance walls. The antenna operates at 9.3 GHz and its beam can be steered from 22° to 65° by changing the bias voltage from 16 to 7 volts. The gain of the antenna remains within the 3 dB limit of the maximum gain of 11.8 dB throughout the steering range. The effective radiating length of the antenna is $13\lambda $ and its half-power beamwidth is 5°. For this frequency range and electrical length, this antenna has a high radiation efficiency. This is due to the phase-shifting mechanism used which results in the varactor loss being less than all other sources of loss throughout most of the steering range. This makes the antenna suitable for applications where the varactors should operate at high frequencies with relatively low quality factors.

Altres ajuts: Universidad Politécnica de Madrid Young Researchers (Grant Number: VJIDOCUPM18MGB) i ICREA This article presents an exhaustive study of the sensitivity in reflective-mode phase-variation sensors based on an open-ended transmission line with a step-impedance discontinuity. Such discontinuity delimits the sensing region (which extends up to the open end of the so-called sensing line), from the transmission line section connected to the input port (design line), which is used to enhance the sensitivity. The theoretical analysis provides the design guidelines to achieve a sensor with high sensitivity compared with the one based on an ordinary (uniform) line with a similar length. In particular, it is shown that for sensitivity optimization, the electrical length of the design line must be set to 90° (or an odd multiple), whereas either a 90° (or an odd multiple) or a 180° (or an even or odd multiple) sensing line can be alternatively used in order to maximize the sensitivity. It is shown that the impedance contrast, defined as the ratio between the characteristic impedances of the design and sensing line, is a key parameter for sensitivity enhancement, and it must be as low or as high as possible for the 90° or 180° sensing lines, respectively. For validation purposes, two prototype devices (one with a 90° and the other one with a 180° sensing line) have been designed and fabricated following the design guidelines. Such devices have been tested by loading the sensing region with several materials with different dielectric constants. Compared with the ordinary line-based sensors, it is found that the maximum sensitivity is enhanced by a factor of 19.7 and 11.4 in the phase-variation sensor based on a 90° and 180° sensing line, respectively. Finally, the sensor concept is generalized to a multisection step-impedance transmission line as a means of further increasing the sensitivity, and a prototype device exhibiting 528.7° maximum sensitivity is implemented.

In this brief, a compact ultra-wideband monolithic microwave integrated circuit single pole double throw (SPDT) switch with high isolation level and low insertion loss is presented. A ${\pi }$ -type topology, consisted of a transmission line with an electrical length of less than 90° and two parallel grounded metal-insulator-metal capacitors, is employed to replace the conventional $\lambda $ /4 transmission line between the adjacent parallel field effect transistors of the serial parallel SPDT switch. In comparison with the traditional method, an ultra-wideband high isolation behavior and the reduced dimension is achieved simultaneously with the proposed SPDT switch. Furthermore, the equivalent circuit model is built to interpret the mechanism of the improved technique. A chip sample is fabricated with Gallium Arsenide processing with a size of $1.25\times 1.05$ mm2. The measured insertion loss is less than 1.9 dB and the isolation is better than 36 dB from DC to 30 GHz. Good agreements are observed between the simulated and measured results.

This brief presents a dual-band balanced filter with wide stopband, controllable transmission zeros and bandwidths, and compact size based on a newly designed electromagnetically coupled stepped impedance resonator (SIR) pair. The proposed SIR contains three different impedance lines, which can lead to more degrees of design freedom for resonant frequency control. Two electrical length ratios of the SIR can be individually tuned to obtain two desired differential-mode (DM) resonant frequencies for dual-band design, while keeping the first spurious frequency far away for wide-stopband design. Moreover, an inherent common-mode (CM) suppression can also be obtained without adding extra structures due to the discriminating CM resonant frequencies. The mechanism of transmission zeros (TZs) generation is also investigated by means of phases and admittance analysis. It reveals that four TZs around the two operating bands can be flexibly controlled via adjusting the ratio of magnetic and electric coupling coefficients of the SIR pair as well as the coupling space of the feeding lines, which have greatly enhanced the passband selectivities and the band-to-band isolation. Subsequently, the graphs of coupling coefficients and external quality factors for the filter design are extracted, showing sufficient degrees of design freedom for bandwidth control. The proposed dual-band balanced filter is finally fabricated and measured at 1.75 GHz and 3.64 GHz, respectively. Measured results are in good agreement with the simulated ones. Compared with the state-of-the-art works, the proposed circuit performs a deeper and wider stopband of $4.5{f}_{1}^{d}$ with 40dB attenuation (where ${f}_{1}^{d}$ is the fundamental frequency), and more than 50-/45-dB in-band CM suppressions with compact size ( $0.14\lambda _{g} \times 0.27{\lambda }_{g}$ , where ${\lambda }_{g}$ is the guided wavelength of the fundamental frequency).

This brief presents a self-packaged spiral shape power divider (SSPD) using substrate integrated suspended line (SISL). The two output paths are designed to form mutually coupled spiral shape, and the isolation resistor is placed at the center of the spiral to reduce the circuit area. By properly increasing the coupling, short total electrical length can be obtained. SISL is adopted for low loss and self-packaging. The suspended substrate is patterned to further reduce the dielectric loss. The even-/odd- mode method is used to analyze this circuit, and the design equations are derived. A SISL SSPD with patterned substrate works at 2.5 GHz is implemented for verification. In a wide fractional bandwidth (FBW) of 55%, the measured return loss and two output ports isolation are both better than 15 dB, while the SSPD with patterned substrate achieves 0.06 dB insertion loss at the center frequency.

In this paper, a Multiband Frequency Reconfigurable Substrate Integrated Waveguide (FR-SIW) cavity backed slot antenna for Cognitive Radio appropriate to Internet of Things (IoT) technology is presented. The proposed antenna is developed to address the specific design challenges posed by the IoT based Cognitive Radio (CR) networks. Reconfiguration of frequency bands is achieved using copper strips. The antenna resonates during the selected five switching states of the copper strips at 2.9 GHz, 2.6 GHz, 2.824 GHz, 2.792 GHz, 2.872 GHz, 4.488 GHz, 4.724 GHz, 4.712 GHz, 5.476 GHz, 5.336 GHz, 5.448 GHz, 5.392 GHz, and 5.42 GHz. The multiple frequency bands are controlled by altering the electrical length of designed slots. Overall, the simulated and measured results showed good agreement and the designed antenna is a potential candidate for cognitive radio applications, especially to IoT applications.

Resonance equations for two tunable symmetrical hairpin resonators with short-circuited ends are obtained for the first time. The case is considered when a parallel or series variable capacitance is connected to the middle of these resonators. It is established that odd resonance frequencies of such resonators do not depend on the variable capacitance value and they are constant. Two traditional symmetrical open-ends hairpin resonators, to which one or two variable capacitors are connected, are also considered. New resonance equations of the hairpin resonator with two capacitors have been obtained, where a segment of uniform transmission line is replaced by a segment of non-uniform transmission line. For all four hairpin resonators, the dependence of the resonant frequency ratio ${f_{i}}/{f}_{0}$ on the electrical length of the resonators was investigated. It was found that two hairpin resonators have the highest ${f_{i}}/{f}_{0}$ ratio: a resonator with two short-circuited ends and a parallel capacitance connected, and a loop resonator with two open ends connected by a variable capacitance. The capacitance of the first resonator is four times the capacitance of the second resonator, so it is advisable to use it at higher frequencies. The second resonator is most suitable for use at lower frequencies. Based on this resonator, a narrowband (2.4%) forth order microstrip bandpass filter (BPF) varactor-tuned in frequency range 225–400 MHz is designed. Due to the wide rejection band, the attenuation more than 40 dB at $3{f}_{0}$ at all tuning range is achieved.

Tunable transmission lines have a high demand in the 5G wireless communication applications. In this brief, a novel transmission line is presented to control both electrical length and characteristic impedance individually, creating flexible functions. For the first function, the proposed transmission line can perform as a tunable phase shifter with 90° phase tuning range and a low insertion loss variation. For the second function, the proposed transmission line can tune the characteristic impedance from 23 ${\Omega }$ to 145 ${\Omega }$ with a fixed phase delay. The ABCD -matrix method is applied to derive the design equations. For the practical implementation, the varactor equivalent parasitic resistances are considered, and the even-odd mode is applied to re-optimize the proposed two tuning functions. To verify the design theory, an experimental prototype is simulated, fabricated, and measured. Both simulation and measurement results match well with the design theory.

An experimental investigation of smooth matching transitions based on inhomogeneous sections of a microstrip transmission line with different wave impedances profiles was carried out in the present work. Comparative tests of smooth matching transitions prototypes are presented in order to confirm the theoretical data obtained by the optimal parametric synthesis method. The possibility of using synthesized piecewise linear transition to ensure good coordination in a fairly wide frequency range has been proven. It has been shown that fabricated inhomogeneous microstrip waveguide transitions provide matching in the frequency range from 1.2 to 2.7 GHz. It has been proven that the use of two-stage transitions allows a 1.5-times increase in the frequency matching band. Locally-optimized matching transitions near the lower boundary of the operating frequency range can give a significant gain in comparison with matching transitions with an exponential wave impedance profile for the same electrical length. The developed matching transitions can be used in the design of filters, matching transformers and connecting elements of the microwave frequencies transmission line.

A novel compact microstrip diplexer with tunable centre frequencies and reconfigurable filtering characteristics at both channels is presented. The designed diplexer is constructed by coupling two dual-mode square loop resonators (DMSLRs) having different electrical lengths. The DMSLRs are investigated by virtue of the coupled line analysis to evaluate the resonance frequencies, transmission zeros and out-band insertion loss level more conveniently. Centre frequencies of both channels can be independently tuned by the varactor diodes utilised instead of perturbation and reference capacitances. Depending on the specialised values of perturbation and reference capacitances, filtering characteristics of both channels can be reconfigured in terms of transmission zeros. The designed diplexer was fabricated and tested for the experimental studies and the measurements were obtained in a good agreement with the predicted results. Tuning ranges of the measured centre frequency in the first channel are between 1.23 and 1.43 and 1.35 and 1.64 GHz for the filtering characteristics with right- and left-side transmission zeros, respectively. In the second channel, they are 2.07–2.31 and 2.14–2.51 GHz for the filtering characteristics with real- and imaginary-axis transmission zeros, respectively. Isolation between two channels has been obtained better than 22 dB. © The Institution of Engineering and Technology 2020

A microwave pulse compressor (MPC) consisting of an iris, a straight waveguide section, and a T-junction can be used in order to convert a long pulse with lower power to a shorter pulse with a higher power. A significant compression gain can be achieved, provided that the system is designed precisely. The duration of the compressed pulse is proportional to the cavity length, thus imposing a constraint on the development of mobile systems. In this work, we present the circuit/transmission-line-based design methodology that we follow in order to fold the cavity (while retaining the same electrical length) by using T-junction turns. For example, we design a 1.3-GHz folded compressor and compare its performance to the corresponding one of a straight-cavity compressor. The 3-D full-wave simulations with CST Studio Suite and experimental measurements with the developed compressor confirm the accuracy of the proposed design methodology.

An “electromagnetic vector sensor” (EMVS) comprises three orthogonal dipoles and three orthogonal loops, all in spatial collocation. The former triplet aims to directly measure the $3 \times 1$ electric-field vector ${\mathbf{e}} $ , whereas the latter triplet aims to directly measure the $3 \times 1$ magnetic-field vector ${\mathbf{h}} $ . Their vector cross product $ {\mathbf{e}} \times {\mathbf{h}} $ would yield the incident source’s Poynting vector, which specifies the incident wavefield’s propagation direction. In reality, all these are only an idealization. Instead, a real-world dipole triad’s measurement could equal the incident ${\mathbf{e}} $ , only if the dipoles were electrically short (i.e., with an electrical length of $({L}/{\lambda }) ). Likewise, a practical loop triad’s measurement could equal the incident ${\mathbf{h}} $ , only if the loops were electrically small (i.e., with an electrical circumference of $2 \pi ({R}/{\lambda }) ). However, such short dipoles and small loops would be electromagnetically inefficient receivers. For a practical dipole that is electrically long , its measurement equals not the incident wavefield’s ${\mathbf{e}} $ but a vector dot product between: 1) the incident wave’s ${\mathbf{e}}$ and 2) that dipole antenna’s “effective length” vector (which depends on that dipole’s $({L}/{\lambda })$ and orientation). An analogous complexity exists for a practical loop that is electrically large . For such practical dipoles and loops, the aforementioned vector-cross-product would fail to yield the Poynting vector, hence it would inaccurately estimate the direction-of-arrival. Instead, this article will advance a new closed-form algorithm to simultaneously estimate an incident source’s direction-of-arrival and polarization, despite the practical dipoles’/ loops’ mathematically complicated gain/phase responses as described earlier, but without any prior knowledge of the dipoles’ electric length $({L}/{\lambda })$ nor the loops’ electric radius $({R}/{\lambda })$ .

Magnetic-free broadband circulator structure based on three switched reflection type delay lines with a total electrical length of 180-degree at the switching frequency is investigated. The nonreci...

This article introduces a new technique for filter design and tuning using the phase of the input impedance (PII) as the design parameter. This novel method is applicable to both narrow and wideband filters. Compared with conventional methods, this approach requires less computation time and provides a clear step-by-step procedure for identifying the proper interresonator coupling and the resonant frequencies of the resonators. In practice, the physical realization of the filter always has a nonideal I/O port, which can introduce an unexpected unknown transmission line between the physical reference plane and the port of the corresponding inverter in the circuit model. In this article, the PII response is used to determine the equivalent electrical length of this unknown transmission line. The technique is demonstrated through the design of a wideband planar filter with a fractional bandwidth of 72%, and the tuning of a 4–2 combline filter with two transmission zeros. Finally, a wideband diplexer is developed using the proposed technique.

In this brief, a tunable Wilkinson power divider (WPD) with port match and isolation is proposed based on a reconfigurable compact structure. The reconfigurable unit is composed of one varactor cascaded by a transmission line (TL) with specific characteristic impedance and electrical length. Each unit can be alternately connected with two cascaded varactors to constitute a T-type structure, where the operating frequency can be selected by providing proper bias voltages for the three varactors. By adjusting the characteristic impedance and electrical length of the transmission line in each unit, the continuous tuning range can be easily extended to exceed 120% tunability. The reconfigurable units can also be flexibly configured to work in designated discrete tuning range as well. An experimental reconfigurable WPD with wide continuous tuning range of 0.96-1.73 GHz (57.3% tunability) is designed and fabricated. The measured results show that more than −20dB return losses and isolations can be realized in the entire tuning range.

In this study, a single-layer balanced wideband phase shifter with a wide phase shift range of 45°–315° and acceptable common-mode (CM) suppression is proposed. The main branch has a simple ladder-like structure and the reference branch utilises a coupling structure. The main branch electrical length is constant for any desired differential-mode (DM) phase shift. Assuming a specific return loss (e.g. 16, 20, and 25 dB), the phase shifter is carefully analysed and a design procedure is given leading to an ultra-low phase error design. Then by making a trade-off between the CM and DM bandwidths at the cost of higher phase error, more design solutions for the phase shifter are given. To validate both the proposed structure as well as its design procedure, a balanced wideband phase shifter with the DM phase shift of 180°±4.2° (1.851–4.017 GHz) and the CM suppression of better than 15 dB (1.81–4 GHz) is fabricated, resulting in 73.82 and 75.25% fractional bandwidth, respectively. The fabricated balanced phase shifter has a return loss of and insertion loss of . The synthesised, simulated, and measured results are in good agreement with each other.

Producing passive printed circuit board (PCB) structures in the millimeter-wave range includes some fundamental challenges, such as low manufacturing costs and high fabrication accuracy. These aspects, in general, have to be combined with low signal attenuation, precise simulation models, and a reliable producibility. This article shows a new method to manufacture different kinds of passive RF waveguides with reduced attenuation and decreased electrical length on low-cost PCB substrates. First, the complex material parameters of the used substrate and the used copper foil are extracted from 10 to 100 GHz to have accurate simulation models in CST Microwave Studio. This is necessary because the parameters provided by the manufacturer are often no longer generally valid in this frequency range. In addition, PCB materials have a significantly higher surface roughness compared with conventional materials for millimeter-wave applications. Therefore, the correct determination and the correct modeling of surface roughness is particularly important. Next, laminate is removed partially on the outer layer of the PCB to increase the electromagnetic field ratio in air. The undesired substrate is removed by a CO2 laser drilling machine by setting narrow drills sequentially. The RF performance of these waveguides is compared with those manufactured in conventional PCB technology according to attenuation and producibility.

Designing Doherty power amplifiers (PAs) at high millimeter-wave (mm-Wave) range remains a major challenge because most Doherty combiners at this frequency band are inefficient. To resolve this bottleneck, we analyze balun response under load variation and subsequently propose the Doherty PA architecture with impedance inverting and scaling baluns that perform active load modulation on differential PAs and delivers combined power to a single-ended load. At the PA output/input stage, instead of utilizing the transformer-based balun commonly used at RF and low mm-Wave frequencies, we explore the coupler-based balun to resonate out the device parasitic capacitance and achieve well-balanced differential to single-ended conversion at high mm-Wave frequencies. Based on the coupled-line theory and our analysis, we derive the [Y] matrix of the coupler-based balun and originate various solutions for baluns that can invert or scale impedance. From this derivation, we conceive a canonical form of impedance inverting baluns when the electrical length of coupled lines equals 45°. We also propose a close-formed solution for the coupler-based impedance scaling balun that is electrically equivalent to an idealistic transformer in series with a small inductance. We then combine the proposed impedance inverting and scaling balun structures to construct a Doherty network that exhibits desirable Doherty active load modulation and occupies a single-transformer footprint. On the active circuit design, we employ the adaptive biasing circuit for both driver and PA stages of the auxiliary PA to enhance the main and auxiliary cooperation. Using a 45-nm GlobalFoundries CMOS SOI process, we fabricate a proof-of-concept Doherty PA that demonstrates 20.1-dBm $P_{\mathrm {sat}}$ , 19.3dBm OP1,dB, 26% peak power-added efficiency (PAE), and 16.6% PAE at 7-dB power back-off (PBO) from OP1 dB at 60 GHz. Our results indicate a substantial PAE enhancement at PBO and a state-of-the-art modulation efficiency achieved when transmitting digital pre-distortion (DPD)-free 64-QAM-modulated signals.

In this paper, a very small size (20 × 14.75 mm2) triple-band multiple-input multiple-output (MIMO) microstrip patch antenna for WLAN (2.4/5.2/5.8 GHz) and WiMAX (2.5/3.5/5.5 GHz) wireless applications. The goal of this paper is to merge these two wireless standards into a single antenna that can be easily accommodated inside the devices operating in three different frequency ranges. The proposed MIMO antenna consists of an asymmetric coplanar strip (ACS) feedline loaded with an inverted L-shaped slot and a meanderline. The inverted L-shaped slot and the meanderline both having an electrical length 0.5λ, are responsible for the achieving the operating band around 3.5 and 2.4 GHz, respectively whereas the ACS feedline is responsible for the generation of 5 GHz frequency band. The fabricated prototype of the proposed MIMO antenna when tested, shows three distinct frequency bands; I from 2.38 to 3.00 GHz, II from 3.21 to 3.61 GHz and III from 5.02 to 6.18 GHz that covers entire frequency bands of WLAN and WiMAX wireless applications along with different standards of IEEE 802.11 including a/b/j/p/g/n/ac/ax. The peak gain in I, II and III frequency bands is 7.93, 4.23 and 7.57 dBi, respectively. In addition, the diversity performance in terms of envelope correlation coefficient, diversity gain, multiplexing efficiency, total active reflection coefficient, and mean effective gain is also performed for the proposed MIMO antenna.

This brief proposes a novel octo-band bandpass filter (BPF) utilizing an unequal-width cruciform connected resonator (UCR) with open- and short-stub. In the UCR, each section featuring different characteristic admittances and electrical length helps obtain multi-poles and -zeros. The multi-mode resonator (MMR), based on two UCRs and one parallel short-stub connected in-between them, features 16 poles and 8 zeros with high tuning flexibility. In addition, the short-stub is constructed by ${Y}$ -shaped short-stubs with unequal characteristic admittances connected in parallel, which enhances pole and zero tunablity further. The octo-band BPF, utilizing a simple structure, obtains a high tuning flexibility of bandwidths and a low insertion-loss. To the best of the authors’ knowledge, it is the first time that the octo-band BPF is implemented. The overall size of the BPF is as small as $0.1 \,\, \lambda _{\mathrm{ g }}\times 0.24\,\,\lambda _{\mathrm{ g}}$ . The proposed filter also features sharp skirt and deep isolation between passbands.

In this article, a 1.7–2.6-GHz broadband power amplifier (PA) was proposed using the generalized quasi-Chebyshev lowpass matching approach, which was directly composed of step-impedance transmission lines (SITLs). First, the complete design theory for the introduced step-impedance matching network was provided with the detailed design equations. The impedance of the SITLs can be calculated directly based on the specified bandwidth, maximum allowable return loss, and impedance-transforming ratio. The electrical length of SITLs was utilized to adjust the harmonic impedances to perform a PA high-efficiency region. For demonstration, a PA using the 10-W gallium nitride high electron mobility transistor (GaN HEMT) with the measured drain efficiency (DE) of 65%–78% from 1.7 to 2.6 GHz was achieved. A good agreement between the simulated and measured results can be observed.

As the U.S. and Europe launched the very low frequency (VLF: 3-30 kHz) space-borne transmitting and propagation experiments during the past thirty years, space-borne antennas have been playing a more and more important role in contemporary VLF communication systems, which are very likely to become an indispensable approach for overwater/underwater communication and navigation in the future. In this paper, we propose a semianalytical method for evaluating the current distribution and input impedance of a VLF space-borne tubular antenna. By considering the effects of both the ordinary wave (O-wave) and the extraordinary wave (E-wave) in an anisotropic ionosphere, the analytical expression for the current distribution has a more complicated form and is derived via the method of moments (MoM) and the Gauss-Legendre quadrature (GLQ) algorithm. Computations show that the current distribution and input impedance under anisotropic conditions are very sensitive to parameter changes, but the overall trend for the input impedance will increase with the radius or electrical length of the antenna. Comparisons with linear models and numerical results obtained in FEKO verify the accuracy of this method. From simulations about the effect of the geomagnetic inclination angle to the input impedance, we advise that the parallel case can be preferred as an alternative for the best angle. Once all antenna parameters are determined, there is a possibility to find multiple optimal inclination angles. In addition, qualitative analyses of the impact of environmental changes to the antenna characteristics are also discussed.

A gap coupled hexagonal split ring resonator (GCHSRR) based metamaterial is presented in this paper for S-band and X-band microwave applications with absorptance. This gap coupled hexagonal split ring resonator is the amendment of the typical split-ring resonator (SRR). Three interconnected hexagonal split ring resonators are applied with a stripline to increase the electrical length and coupling effect of the GCHSRR. SRR has an impact on the extraction of effective parameters such as permittivity, permeability and refractive index. The dimension of the proposed GCHSRR unit cell is $10\times 10$ mm 2, which is printed on low-cost FR4 material. The transmission frequency of the proposed GCHSRR unit cell ranges from 3.42 GHz to 3.73 GHz and 11.27 GHz to 11.91 GHz, which makes the metamaterial applicable for S-band and X-band microwave applications. The GCHSRR unit cell has a double negative regime of 7.92 GHz to 9.78 GHz with an effective negative refractive index regime of 6.30 GHz to 10.22 GHz and 11.97 GHz to 12.61 GHz. The effective medium ratio is 8.4, which implies the novelty of the proposed design. Moreover, the GCHSRR has high absorption peaks of 99%, 98%, and 81% at 4.27 GHz, 5.42 GHz, and 12.40 GHz, respectively. An $18\times 20$ GCHSRR array structure is also designed and studied. The effective parameters and the effective medium ratio with a high absorptance make the proposed GCHSRR based metamaterial suitable for practical microwave applications.

A wearable electromagnetic belt system for the detection of hepatic steatosis (lipid accumulation within the major liver cells, hepatocytes), is proposed. To satisfy the requirements of the belt system, an array of body matched antennas is designed. The belt, which goes around the lower chest and over the liver, requires compact, wideband, unidirectional antennas that operate at low microwave frequencies. To avoid using conventional bulky reflector structures, the designed antenna utilizes the loop-dipole combination concept. To enhance electromagnetic wave penetration, the antenna is designed to match the human body. Thus, thanks to the high dielectric loading from the human body, the dipole element of the antenna is easily miniaturized. Since the same principle does not apply on the loop structure, meandered arc-shapes are employed to increase the effective electrical length of the loop. The final antenna design has a measured wide operating bandwidth of 0.58-1.6 GHz with a compact size of 0.096×0.048×0.048λ3. The proposed structure is effective in irradiating the torso, where the signal can reach center of the liver at a depth of 90 mm, with 64% of the peak radiated power. An electromagnetic belt is built using twelve elements of the designed antennas. The belt is then tested on a 3D printed torso phantom that includes models of the lungs and liver. Due to close dielectric properties of the other tissues inside the torso, these are represented using an average tissue mimicking mixture with permittivity of 46. Measured data are analyzed using multivariate energy statistics method. A peak measured dissimilarity of 15.1% between steatotic and healthy liver is attained. These initial tests and obtained results indicate the potential of the proposed system as a method to diagnose hepatic steatosis.