In this paper, edge-broad-side coupled spiral resonator (EBC-SR) is proposed to enhance the miniaturization of the metamaterial resonators. After a comparative analysis, it is found that the proposed strategy can drastically reduce the electrical size of metamaterial resonators. This is due to the use, at the same time, advantages of edge coupling, broad-side coupling, and self-coupling. A lumped element equivalent circuit model is proposed for the EBC-SR-loaded transmission line. This model is validated by comparing the results of electromagnetic simulations with the circuit simulations using the extracted parameter values. To validate the proposed strategy, a prototype of the EBC-SR embedded inside a drive loop is fabricated. The experimental results are presented with the simulation results.

A miniature dual-band bandpass filter with interlocked stepped-impedance resonators (SIRs) is presented in this paper, which was designed for the student design competition held in European Microwave Week 2019. This bandpass filter is required to have two concurrent passbands, namely, the first passband at 900–1000 MHz and the second passband at 1427–1518 MHz bands, which cover six designated bands in sub-6 GHz range of fifth generation (5G) New Radio Access Technology. Three stopbands are required at 500–850, 1050–1350, and 1600–2000 MHz, respectively. To achieve the best figure of merit, an interlocked configuration of two SIRs is proposed. One advantage is that the impedance ratio of the inter-locked SIR can be controlled to have two passbands at the required frequencies. Second, the coupling section of the interlocked SIR gives three transmission zeros distributed to every stopbands such that the stopband suppression are dramatically enhanced. The measured results show that the passband insertion losses are 2.16 dB at the first passband and 1.33 dB at the second passband, and the return losses are greater than 10 dB. The stopband suppression at the transmission zeros are greater than 38 dB. The circuit is very compact as 41.40 × 19.96 mm2, equivalent to $0.25 \times 0.12\,\lambda _g^2$.

This paper investigates the generation of passive intermodulation (PIM) in coaxial connectors during vibration. A series of experiments were designed and the simulation model and method were proposed for understanding these phenomena. We found that PIM is mainly influenced by the contact stress and contact surface roughness during vibration. Thus, a power spectral density method is presented to identify the roughness parameter of contact surface based on the Weierstrass–Mandelbrot model, and the simulation model and method were verified by the relative experiments. Eventually, some suggestions for engineering application were provided.

This paper presents a novel GaN-based digital outphasing power amplifier (PA) for the 800 MHz range. The PA reaches a maximum output power of 5.8 W at 30 V final-stage (FS) drain supply voltage. A novel output combiner circuit is used and efficiency is improved by resonant commutation of the FSs and optimized driver circuits for the two GaN push-pull FSs. 3D electromagnetic simulation of output network has been conducted to extract an equivalent circuit model and to access full information in terms of functionality and broadband impedance characteristics for optimized outphasing operation in the final design. Measured total efficiencies (ηtot) of 59 and 25% at 0 and 10 dB power back-off are achieved, respectively, fitting the simulation quite well. The proposed digital outphasing module is a promising candidate for fully digitized base-station architectures in future wireless communications.

This paper presents a novel design of a low profile circularly polarized (CP) metasurface (MTS) antenna with in-band radar cross-section (RCS) reduction property. The MTS is loaded as a superstrate on slot antenna and it can be viewed as a polarization-dependent MTS (PDMTS). The rectangular patch-based PDMTS is analyzed using characteristic mode analysis to find two orthogonal degenerate modes, which produces CP waves. Linearly polarized slot antenna is used to excite the PDMTS. The performance of PDMTS loaded slot antenna is analyzed numerically using full-wave analysis method. The PDMTS CP antenna is fabricated and its performance is tested experimentally. The proposed antenna has a compact structure and it has an overall size of $0.52{\lambda _0}\times 0.52{\lambda _0} \times 0.078{\lambda _0}$ (where ${\lambda _0}$ is the free space wavelength). The measured results show that the PDMTS antenna achieves $-10\,{\rm dB}$ impedance bandwidth of 29.41$\%$, 3-dB axial ratio bandwidth of 9.05$\%$, broadside gain of 6.34 dB, and monostatic RCS reduction of $-30.2\,{\rm dBsm}$ at the resonant frequency of 5.86 GHz. The simulated results are in well agreement with the measured results and it is well suited for C-band Radar and Satellite communication.

In this paper, a novel miniaturized implantable circular antenna is presented. It supports both wireless information communication and wireless energy transmission at the Medical Device Radiocommunication band (MedRadio 402–405 MHz) and the industrial, scientific, and medical bands (ISM 902.8–928 MHz). The antenna is circular to avoid sharp edges while miniaturization is achieved by adding two circular slots to the patch. The main scenario includes embedding into the muscle layer of a cylindrical three-layer model of a human arm for which several parameters are analyzed (resonance, radiation pattern, and specific absorption rate). Power transmission efficiency and interaction distance limits to ensure connections are also evaluated. Finally, the design is validated by an experimental measurement in an anechoic chamber, and some new improvements are proposed.

The dynamic characteristic of rain fade slope is one important factor in determining the availability of a communication system, and it is very useful in the design of fade countermeasures. In the literature, many models were proposed for rain fade slope for earth-to-satellite links. However, there are no models available for rain fade point to point terrestrial microwave links. This paper proposes a new model for the estimation of rain fade slope statistics for terrestrial microwave links in tropical regions. First, the ITU-R model for rain fade slope for earth-to-satellite link was compared with the corresponding statistics obtained from rain attenuation data measured from three terrestrial links in Malaysia. It is found that the expression of its distribution and its standard deviation should be modified. This leads to the derivation of the proposed rain fade slope model based on the statistics of one link. Then, it is tested using the remaining data and its results were very close to the measured statistics for all attenuation levels higher than 1 dB. Moreover, the model was validated using the chi-square goodness-of-fit test.

The plane wave diffraction by a planar junction consisting of a thick metallic sheet and a lossy double-negative metamaterial slab is studied by using the Uniform Asymptotic Physical Optics approach. This approach assumes the radiation integral as a starting point and uses the physical optics surface currents as sources to be integrated. The integral is manipulated by taking advantage of useful approximations and evaluations, and re-formulated in order to apply an asymptotic procedure able to generate a closed-form approximate solution in the framework of the Uniform Geometrical Theory of Diffraction. Accordingly, advantages and drawbacks result from the application of the proposed solution. The jumps of the geometrical optics field are compensated. Implementation and handling of the computer code are facilitated by the evaluation of well-known functions and parameters. No differential/integral equations or special functions must be computed.

This paper presents a simple topology of 180° hybrid coupler with non-interspersed inputs and outputs, as opposed to the conventional rat-race topology. Such hybrid coupler topologies with inputs located on one side and outputs at the opposite side simplify the design of signal routings and module packaging when integrated with other circuits and components. It also relaxes the necessity of using auxiliary compensating components such as tapers, jumpers, and crossovers. The proposed coupler topology is theoretically analyzed through a T-matrix approach. The analysis comes up with design equations and diagrams that help choose the structural parameters for the desired specification. The simplicity and also the immunity against intrinsic parasitic effects within the proposed topology make it an excellent candidate for operation over any RF and millimeter-wave frequency bands, for applications such as automotive radar or E-band backhaul radio. To this end, a coupler based on the proposed scheme is realized for operation over 77 GHz and assessed through on-wafer measurements. Good agreement of the measured results with the simulated and analytical counterparts demonstrates the superior performance of the proposed 180° hybrid coupler.

Hyperthermia (i.e. heating of tumor tissue to 40–43°C) is used in clinical oncology to enhance the therapeutic effect of chemotherapy and radiotherapy. Many tumor sites are heated either by a single RF or MW antenna positioned on the tumor location, or by a phased array positioned around the patient. Superficial tumors are generally heated with MW antennas (434–2450 MHz) and deep-seated tumors with RF antennas (70–150 MHz). These devices cover the major, more common tumor sites, but more rare locations require more dedicated applicators. We discuss dedicated RF systems aiming for heating semi-deep-seated tumors in the leg, breast, and upper thorax. Clinical results show that adequate heating is possible with these systems, with achieved temperatures in the therapeutic range.

A novel planar dual-band bow-tie slotted patch antenna backed by high-impedance surface (HIS) is designed at 2.5 and 3.5 GHz for wireless application. The antenna employs coplanar waveguide fed patch and bow-tie slot as radiating elements. The bow-tie slot enables dual-band operation for the antenna. The HIS is made asymmetric in design to make it polarization dependent. This polarization-dependent HIS is eventually designed to reflect circularly polarized waves from linearly polarized incident waves.

Early detection of breast cancer is required to increase the chances of a successful treatment. However, current breast-imaging systems such as X-Ray mammography, breast ultrasound, and magnetic resonance imaging have technological limitations so that novel solutions are needed to address this major societal problem. The current paper considers ultra-wideband (UWB) microwave radiation in the frequency band from 1 to 9 GHz. Given by the non-ionizing nature of microwaves frequent check-ups are more feasible. In this work, we propose algorithms for qualitative and quantitative microwave breast imaging for a transmission-based UWB system. Based on numerical and experimental data, the performance of the algorithms has been investigated and compared. Finally, microwave images obtained during an initial patient study are discussed relative to corresponding X-ray images.

The design of proper antenna element (AE) for microwave-based head imaging or brain stroke detection is a crucial challenge in the development process of microwave imaging (MWI) systems. The main purpose of this paper was to design, fabricate, and experimentally verify the compact and dimensions-reduced H-slot antenna suitable for the new generation of multichannel MWI system for brain stroke detection. The slot antenna type was chosen based on the numerical study of three AEs available in the literature, i.e. bow tie, slot, and waveguide-based. The study was focused on the sensitivity of the antennae (change of magnitude and phase of S21) due to dielectric parameters change or type and diameter of inclusion in a head phantom representing a hemorrhagic (HEM) or ischemic (ISCH) stroke phantom, respectively. Further, the analysis of antenna radiation to lossy medium/air and its immunity against plane wave exposure was carried out. The H-slot antenna was fabricated and experimentally verified (measurements of reflection as well as transmission coefficients) using a liquid head phantom with inserted HEM stroke phantom (both prepared as a mixture of propylene glycol, water, and salt). The phantoms were filled inside the designed two-port test system. Numerical models were validated by comparing calculated and measured S-parameters. The sensitivity of the H-slot antenna to the presence of the HEM stroke phenomenon within the phantom of the head was also demonstrated. The main advantage of the proposed H-slot antenna is its small dimensions, easy, inexpensive, and repeatable fabrication as well as mechanical stability.

This paper presents a novel concept named meta-waveguide structures to design slot array antennas based on metasurface. The proposed method investigates the geometrical parameters of the embedded metasurface and their relations to build up the engineered electric and magnetic fields to introduce efficient slot array antennas. This approach eliminates the necessity of using the well-known offsets in typical slot arrays, which can be benefited from linear array theorem. This idea is realized by employing successive filed curvature along the propagation direction, utilizing periodic metasurfaces. Thereafter, a step-by-step algorithm is recommended to design an efficient slot array antenna. Finally, a prototype is fabricated and tested experimentally validating the theoretical calculations.

This paper presents a novel assessment method that minimizes test-fixture-induced errors in non-coaxial power combiner measurement by extending the port reduction method. This method involves terminating certain ports to acquire the scattering matrix of an N-port network from the scattering matrix measured at a reduced port order. The entire DUT scattering matrix is obtained from multiple scanning measurements, which are taken from partial coaxial accessible ports, based on a set of configurable terminating states. This advantage is leveraged to exclude a major portion of coaxial launch structures that would otherwise be incorporated in the conventional multiport test fixture. An analogous concept here is applied to measure a waveguide traveling-wave power combiner. A sandwiched twin structure, containing a divider/combiner pair with certain auxiliary through-type components cushioned between them, is utilized to assess the combiner characteristics. A theoretical framework of the proposed method was established to test its potential precision. Thereafter, an in-situ implementation was conducted to test its practical application on a traveling-wave combined amplifier prototype operating at the Q-band (33–39 GHz).

The airborne radomes have to cater superior electromagnetic (EM) performance with bandpass characteristics of stealth application. In this regard, a hybrid A-sandwich radome is proposed in this paper. The proposed radome consists of a novel strongly coupled frequency selective surface (FSS) core sandwiched between two dielectric layers (acts as skin) to form an A-sandwich structure. The dielectric layers are cascaded in such a way that the middle layer has less dielectric parameters than the skin dielectric. The core layer comprises a modified FSS array using strongly coupled FSS layers through a series of metallic vias. This strongly-coupled FSS element will have the advantage of eliminating inter-element interference and improves the EM performance characteristics of the structure. The structure exhibits very good band-pass characteristics (>90%) at a normal impinging angle with sharp roll-off characteristics. To show the efficacy of the proposed structure, the transmission loss has been compared with that of conventional A-sandwich radomes at 0°, 50° incidence angle for both TE and TM polarization. Conformal analysis of the unit cell has been carried out, and sector-wise thickness optimization was performed to analyze the structure for the conformal shaped radome application. Finally, a physical prototype has been fabricated and measured its scattering parameters, radiation characteristics in a fully shielded anechoic chamber. The results are encouraging and prove its suitability for radome application.

A superheterodyne transmission scheme is adopted and analyzed in a 300 GHz wireless point-to-point link. This was realized using two different intermediate frequency (IF) systems. The first uses fast digital synthesis which provides an IF signal centered around a carrier frequency of 10 GHz. The second involves the usage of commercially available mixers, which work as direct up- and down-converters, to generate the IF input and output. The radio frequency components are based on millimeterwave monolithic integrated circuits at a center frequency of 300 GHz. Transmission experiments over distances up to 10 m are carried out. Data rates of up to 60 Gbps using the first IF option and up to 24 Gbps using the second IF option are achieved. Modulation formats up to 32QAM are successfully transmitted. The linearity of this link and of its components is analyzed in detail. Two local oscillators (LOs), a photonics-based source and a commercially available electronic source are employed and compared. This work validates the concept of superheterodyne architecture for integration in a beyond-5G network, supplying important guidelines that have to be taken into account in the design steps of a future wireless system.

We consider the scattering and absorption of the H-polarized plane wave by finite and semi-infinite systems of layers in the THz range. Every layer consists of an infinite graphene strip grating embedded into a dielectric slab. The solution of the problem we obtain in several steps. First, with the use of the method of singular integral equations we obtain scattering matrices of a single layer. Then, we present equations for the finite and semi-infinite systems of layers relatively the Fourier amplitudes of the scattered field. The frequency dependences of the reflection, transmission, and absorption coefficients demonstrate the variety of resonances: plasmon, slab-mode, grating-mode resonances, and resonances of the multilayer structure.

This paper deals with a near-field to far-field transformation able to predict the radiation of UHF antennas located over a lossy ground. From in-situ near-field measurements, an equivalent set of dipole sources is obtained as a model of the characterized antenna. The paper details the main steps of the transformation and describes the specific experimental set-up designed for the application. Simple directional antennas (monopoles array) as well as more complex omnidirectional antennas (like a biconical antenna as a scaled-down model of a HF antenna) have been tested in realistic environments. This approach is very efficient for separating the contributions of the radiated waves: the sky wave and the surface wave.

This work examines the impact of the inverse chirp z-transform (ICZT) for frequency-to-time-domain conversion during image reconstruction of a pre-clinical radar-based breast microwave imaging system operating over 1–8 GHz. Two anthropomorphic breast phantoms were scanned with this system, and the delay-multiply-and-sum beamformer was used to reconstruct images of the phantoms, after using either the ICZT or the inverse discrete Fourier transform (IDFT) for frequency-to-time domain conversion. The contrast, localization error, and presence of artifacts in the reconstructions were compared. The use of the IDFT resulted in prominent ring artifacts that were not present when using the ICZT, and the use of the ICZT resulted in higher contrast between the tumor and clutter responses. In one of the phantoms, the tumor response was only visible in reconstructions that used the ICZT. The use of the ICZT evaluated with a time-step size of 11 ps resulted in the reduction of prominent artifacts present when using the IDFT and the successful identification of the tumor response in the reconstructed images.