In this paper, we report the analysis, design, and implementation of stacked transistors for power amplifiers realized on InP Double Heterojunction Bipolar Transistors (DHBTs) technology. A theoretical analysis based on the interstage matching between all the single transistors has been developed starting from the small-signal equivalent circuit. The analysis has been extended by including large-signal effects and layout-related limitations. An evaluation of the maximum number of transistors for positive incremental power and gain is also carried out. To validate the analysis, E-band three- and four-stacked InP DHBT matched power cells have been realized for the first time as monolithic microwave integrated circuits (MMICs). For the three-stacked transistor, a small-signal gain of 8.3 dB, a saturated output power of 15 dBm, and a peak power added efficiency (PAE) of 5.2% have been obtained at 81 GHz. At the same frequency, the four-stacked transistor achieves a small-signal gain of 11.5 dB, a saturated output power of 14.9 dBm and a peak PAE of 3.8%. A four-way combined three-stacked MMIC power amplifier has been implemented as well. It exhibits a linear gain of 8.1 dB, a saturated output power higher than 18 dBm, and a PAE higher than 3% at 84 GHz.

The terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

The contribution of this paper is to propose a novel rat-race hybrid coupler of arbitrary coupling coefficient. Traditionally, the rat-race hybrid couplers are built by various loop-alike transmission-lines of multiple quarter-wavelength, and in this paper, we approach the coupler design by using a circular substrate integrated waveguide (SIW) cavity (SIWC). The employed SIWC supports two mutually orthogonal degenerate modes, and cavity field is formed by the two modes in an arbitrary weighting ratio which defines the proposed rat-race coupler's coupling coefficient. The cavity is excited by a microstrip combined coupling slot with the microstrip along a specifically chosen direction. The energy of each degenerate mode can be solely extracted by an associated subminiature version A (SMA) whose position is carefully determined. The isolation between the coupling slots is assured by their perpendicular layout, and the isolation between the SMA probes is obtained by the orthogonality of the two degenerate modes. Experiments are conducted on the 3- and 10-dB coupling coefficient samples to verify this novel rat-race coupler design. The measurements agree well with the simulations, and circuit's good performance is observed in terms of coupling precision, isolations, and small phase imbalances.

This paper presents a miniaturized 3-dB branch-line coupler based on slow-wave microstrip transmission lines. The miniaturized coupler operating at 2.45 GHz is designed and implemented on a double-layer printed circuit board substrate with blind metallic vias embedded in the lower substrate layer providing the slow-wave effect. Based on this concept, a 43% size miniaturization is achieved as compared with a classical microstrip branch-line coupler prototype. The measured S parameters present a return loss of 25.5 dB and an average insertion loss equal to 0.05 dB at the operating frequency.

An analytical method is proposed to reduce the memory effects and third-order intermodulation distortions for improving the linearity of wideband power amplifier (PA). An excellent linearity can be obtained by reducing the second-harmonic output power levels and reducing the envelope voltage components in the megahertz range. An improved wideband Chebyshev low-pass matching network including the bias network is analyzed and designed to validate the proposed method. The measured results indicate that a wideband high-efficiency linearized PA is realized from 1.35 to 2.45 GHz (fractional bandwidth = 58%) with power added efficiency of 60–78%, power gain of 10.8–12.3 dB, and output power of 40.0–41.2 dBm. For a 20 MHz LTE modulated signal, the adjacent channel leakage ratios (ACLRs) of the proposed PA with digital pre-distortion (DPD) linearization are −55.7 ~ −53.9 dBc across 1.5–2.4 GHz at an average output power of 32.4–33.6 dBm. For a 40 MHz two-carrier LTE modulated signal, the ACLRs of the proposed PA with DPD linearization are −51.1 ~ −48.2 dBc at an average output power of ~30.5 dBm in the frequency range from 1.5 GHz to 2.4 GHz.

The ultra-wideband bandpass-response power divider with high-frequency selectivity is presented in this paper. This power divider consists of an impedance transformer, a filter network, and two isolation resistors. In order to realize the ultra-wideband filtering performance, parallel coupling lines and parallel open-circuit branches are applied to the second impedance converter. A resistor is added to the ends of the coupling lines to achieve good isolation and output return loss. The equivalent-circuit method is employed to analyze the presented power divider. The power divider, working at 3.45–8.29 GHz, is designed and fabricated. Two transmission zeros are generated at 2.8 and 9 GHz, respectively, and the out-of-band suppression is >13 dB. The measured results are in good agreement with the simulation ones.

In this paper, we propose a novel energy detection (ED) receiver architecture combined with time-of-arrival (TOA) estimation algorithm, compliant to the IEEE 802.15.4a standard. The architecture is based on double overlapping integrators and a sliding correlator. It exploits a series of ternary preamble sequences with perfect autocorrelation property. This property ensures coding gain, which allows an accurate estimation of power delay profile (PDP). To improve TOA estimation, the interpolation of PDP samples is proposed and the architecture is validated by using an ultra-wideband signals measurements platform. These measurements are carried out in line-of-sight and non-line-of-sight multipath environments. The experimental results show that the ranging performances obtained by the proposed architecture are higher than those obtained by the conventional architecture based on a single-integrator in both LOS and NLOS environments.

The present paper presents a novel method to cope with the difficulty that an N-order filter can introduce a maximum number of transmission zeros (TZs) – no more than N. The method is described by a coupling topology and realized by a regular fully canonical filter structure to increase one more TZ for an N-order filter without any filter size increase. Besides, the TZ shift property, as another advantage of the proposed topology, is investigated. The coupling topology consists of a group of fully canonical coupling nodes with the harmonic node (non-resonating node (NRN)) excited by a λ/4 resonator. Owing to the unique coupling topology and coupling node distribution as well as the inclusion of an NRN, N + 1 TZs are generated by an N-order bandpass filter, while all the TZs are close to the passband and far away from the harmonic passband resulting the highly selective passbands. For demonstration, two-, three-, and four-order filters are designed and measured with the experimental results that confirm the proposed technique.

The detailed synthesis of a direct access ring filter topology fully controlled with the following targeted specification (center frequency, low transmission zero frequency, and matching level in the passband) is hereby presented. For this topology, the lowest achievable bandwidth is limited by technological constraints. Thereby a solution consisting in adding capacitive loads is proposed. The associated synthesis is also given and discussed. Both syntheses are illustrated with 60 GHz integrated planar filters implemented in the IHP 130 nm BiCMOS technology. Various 3 dB fractional bandwidths from 18 to 8% are targeted, some of them require the implementation of the capacitive loaded solution. The latter allows us to lower the bandwidth limit of the nominal topology as well as to get a high miniaturization, up to 3.4, depending on the capacitance value. Thanks to good measurement results, this implementation highlights the high efficiency, reliability, and versatility of the synthesis without the need of tuning simulations or post-simulations.

In this study, wideband bandpass power divider with good out-of-band performance is proposed. Two bandpass filters (BPFs) are utilized to substitute the quarter-wavelength transmission line in conventional Wilkinson power divider. A resistor is specially arranged between two BPFs for a good isolation. Four transmission zeros (TZs) are found to be distributed in the lower and upper stopband of the power divider. Moreover, the locations of two TZs can be shifted by tuning the impedance ratio of the center-loaded open stub, which is propitious to improve the frequency selectivity. Even- and odd-mode methods are applied to analyze the proposed power divider and closed-form design formulas are obtained. Finally, two prototype power dividers with measured rejection level in the upper stopband larger than 29.1 and 32 dB till to 2.7f0 and 2.69f0, respectively, are designed and fabricated to testify the proposed design concept. Good agreement between the simulated and measured results is observed, validating the validity of the proposed design principle.

Wireless sensor networks are an increasingly popular tool for monitoring various environmental parameters. They can also be used for monitoring the electromagnetic spectrum. Wireless sensors, due to their small size, typically have simplified radio receivers with reduced sensitivity and use small antennas. As a result, their effective performance area is similarly limited. This is especially important in urban areas where there are various kinds of adverse propagation phenomena related to area coverage. The aim of this paper is to present the phenomena in the wireless sensor networks and propose criteria and methods to optimize their deployment to ensure maximizing the probability of detection of emissions, minimization of unmonitored areas, and to provide the necessary hardware redundancy in the priority areas. Influence of detection parameters, number of sensors and range constraints between sensors on received outcomes are also presented.

This study examines K-band rectangular waveguide terminations with three-dimensional (3D)-printed loads, and proposes an Asymmetrical Tapered Wedge topology. This geometry shows a good tradeoff between microwave performance and 3D-printing issues (printing directions and support material requirements), thus improving noticeably the reproducibility of the devices. The effect of the density of the 3D-printed load on the reflection parameter of the termination was investigated. Even for a low density, reflection level remained below −27.5 dB between 18 and 26.5 GHz. Reproducibility was demonstrated by the characterization of six loads that were 3D printed under the same conditions. Measurements demonstrate that a maximum reflection parameter level of −33.5 dB can be ensured over the whole frequency band without any post-machining of the 3D-printed devices.

A selection of hybrid frequency- and time-domain approaches is evaluated with respect to their suitability for predicting the multi-tone behavior of a reference Ku-band helix traveling-wave tube. For this, a frequency-domain code is extended by a selection of frequency-dependent and frequency-independent envelope models and compared to a number of simulated and measured responses to two-tone signal excitation with varying frequency spacing. The selected envelope methods are investigated both in the hybrid approach and based on measured TWT characteristics. Output levels and classical multi-tone metrics, such as intermodulation products and phase transfer factors, are considered. By reducing a modulation scenario to a number of significant sequences, a comparison to full physics-based frequency-domain simulation is presented to show the potential of the proposed approach.

This paper presents an ultra-wideband coplanar waveguide (CPW)-to-asymmetric coplanar stripline (ACPS) transition based on aluminum nitride (AlN) substrate. The concepts of designing CPW, ACPS, and CPW-to-ACPS transition are explained. In order to suppress parasitic modes, vias going through AlN substrate are added along the ground traces. The signal trace is tapered out and chamfered to reduce the reflection caused by the termination of ground trace. The CPW-to-ACPS transition is designed, fabricated, and measured in a back-to-back configuration. The fabricated CPW-to-ACPS transition can provide a bandwidth of 165 GHz with an associated insertion loss of 3 dB.

A compact dual-wideband (DWB) bandpass filter utilizing open-shorted coupled lines (OSCL) is proposed in this paper. The introduction of the folded structure not only reduces the overall size but also contribute to the split of the transmission zero (TZ), which improves the selectivity effectively. Six transmission poles and five TZs are realized utilizing the shorted-circuit stub and OSCL. The characteristic of the flexible adjustment method of TZs guarantees an independently wide adjustable range of the bandwidth. For demonstration, a DWB filter (3 dB bandwidth 120 and 28%, respectively) is designed and fabricated. The filter is compact and has the characteristics of extraordinary wide bandwidth, low insertion loss, as well as high selectivity.

In this paper, a 3 dB H-plane substrate-integrated waveguide (SIW) power divider/combiner with improved isolation is reported. By adding two isolated ports into the Y-junction, it will perform like a multi-port coupler, so that the isolation between its dividing ports can be effectively improved as the newly-added ports are properly matched. To verify the availability and effectiveness of this concept, two prototypes, one is terminated by coaxial terminations and the other is loaded with lumped resistors, are developed. Their measured results are separately in good agreement with their corresponding simulations. Meanwhile, isolations better than 16 dB with fractional bandwidth (FBW) of 35 and 25% are achieved, respectively, as well as low phase and amplitude imbalances. Compared with some reported similar SIW power dividers, the proposed ones exhibit wider FBW with similar isolation, insertion loss, phase, and amplitude balance performance.