Jari Holopainen

Utilizing scatterers in communication engineering, such as reconfigurable intelligent surfaces (RISs) and backscatter systems, requires physically consistent models for accurate performance prediction. A multiport model, which also accounts for structural scattering, has been developed for non-periodic scatterers. However, many emerging systems operate at multiple frequencies or generate intermodulation harmonics, particularly when incorporating space-time modulation (STM) or dynamic load control. These functionalities demand advanced modeling approaches capable of capturing scattering behavior across several frequencies and directions simultaneously. To address these challenges, this article extends a multiport S-parameters-based model for predicting the scattering properties of multifrequency operating structures. The proposed model broadens the applicability of convenient S-matrix models to time-modulated multiport scatterers. Unlike known approaches, this model incorporates structural scattering, mutual coupling, the possibility of non-digital modulation, and spatially non-periodic configurations, enabling physically consistent analysis and optimization across a wide range of communication and sensing systems. Validation against experimental results for a space-time modulated scattering structure demonstrates the accuracy and practical applicability of the proposed model.

Wireless capsule endoscopy (WCE) is the only option for non-invasive small intestine disease diagnostics. However, the small size of the capsules poses a major design challenge for engineers since multiple optical and electrical components must reside inside the capsules. This article proposes a space-saving dual-purpose component utilized in both optical image capturing and microwave wireless data transmission of a WCE capsule at the 915 MHz industrial-scientific-medical band. The optical performance is verified through ray-tracing simulations, whereas the microwave operation is demonstrated by full-wave impedance simulations and by conducting a radio link simulation and measurement with an up-scaled prototype. The results show that the proposed component can be utilized in capsule optics to achieve an acceptable imaging quality. Furthermore, the component operates as an antenna, providing adequate link gains for WCE wireless communications.

Plant-based data from southern Finland were used to reconstruct late Holocene warm-season temperature variability on inter-annual to longer scales. Temperature-sensitive records representing maximum latewood density of Pinus sylvestris tree rings (since AD 760) and phenological stages of several plant species (since AD 1750) explained ~60% and ~70% of instrumentally observed temperature variance, respectively. The value of a multi-proxy approach was demonstrated by statistical models including both variables, which explained ~80% of the temperature variance. Temperatures from the CRUTEM5 and Berkeley datasets had slight variations in their correlativity with proxy data, possibly resulting from their differing spatial representativeness over the proxy sites. Temperature history inferred from maximum latewood densities extended over the past millennium and correlated with previously published data from similar proxy records in Fennoscandia and adjacent areas. These data indicate that the region cooled since the Medieval Climate Anomaly and warmed markedly since the Little Ice Age/Maunder Minimum. In the study region, the magnitude of this long-term warming was 2.1 °C and 2.8 °C, calculated between the coldest and warmest 100-year and 30-year intervals, respectively. Collectively, our results display the potential of plant-based data from low-lying and mild boreal sites to extend our understanding of preindustrial and recent climatic changes.

This paper investigates a thin low-pass filtering antenna array based on dual-polarized Vivaldi elements. The low-pass filtering in the antenna elements reduces the requirement for the front-end filtering between the antenna and the microwave electronics, resulting in improved overall out-of-band suppression, size reduction, and lower cost. The array employs a novel stacked-PCB structure, where simple two-sided PCBs are stacked on top of each other. The via-connected metal layers of all PCBs form a tapered slotline along the surface normal of the PCBs. The filtering effect is realized by corrugating the tapered slotlines, which provides effective, space-saving integration of the filters that fit into a half-wavelength lattice. According to unit-cell simulations, the proposed antenna array operates at 6–18.5 GHz, and the stopband extends from 21 GHz to 37 GHz. The antenna array provides a 10-dB active reflection coefficient (ARC) with beam-steering angles within ±60∘ in E-and D-planes, and 6 dB within ±55∘ in the H-plane. At stopband frequencies, the attenuation with respect to simulated total efficiency is at least 20 dB. The operation of the proposed antenna array is confirmed by measurements of an 11×12 antenna array prototype, which show that the gain suppression level in the stopband is more than 30 dB up to 37 GHz, and more than 20 dB up to 40 sGHz.

Various applications, including reconfigurable intelligent surfaces, radio frequency identification, and ambient backscatter devices, are based on scattering. Predicting the scattering properties of these systems accurately and universally in a computationally efficient manner is crucial. In this paper, we propose a model for predicting the scattering properties of an electromagnetic structure controlled by loads terminated to multiple ports. This model is based on 1) S-parameters describing the coupling between the ports, 2) embedded element radiation patterns associated with each port, and 3) structural scattering under multiple incident wave directions. To construct the model, one set of electromagnetic descriptions (e.g., simulations or measurements) needs to be done for a structure before computing the scattering properties of the structure for arbitrary tunable load values. Unlike many other methods, the proposed method fully takes into account structural scattering in different directions simultaneously and requires no simplifications or approximations to the scattering structure, such as the assumption of local periodicity or element identity. This method facilitates characterizing the scattering ability of the structure in terms of bistatic cross section, also known as bistatic radar cross section (bRCS), and can be beneficial, for instance, in designing reconfigurable intelligent surfaces and backscatter systems. Simulations and experiments at different frequencies verified the proposed model.

This paper presents a band-pass filtering antenna array based on corrugated Vivaldi antenna elements and finline waveguides. The low-pass section of the filtering antenna element is realized by periodically corrugating the inner edges of the Vivaldi elements. The structure can be considered as a periodic transmission line that inherently acts as a low-pass filter. The high-pass section of the filtering element is realized by surrounding the corrugated slotline with a waveguide. The resulting antenna structure, a finline-fed Vivaldi element, exhibits a wide 1.2–3.1 GHz band-pass response with the upper band-stop rejection exceeding 20 dB up to 6 GHz. The filtering properties are almost independent of the scan angle. The active reflection coefficient is <-10 dB with beam-steering angles within θ = ±60° in the E- and D-planes, and <-6 dB in the H-plane. The operation of the filtering antenna element is confirmed by periodic unit-cell simulations and measuring a manufactured 6×6 array prototype.

This article investigates a dielectric-loaded Vivaldi-type antenna array that is formed inside a plastic block. It utilizes the inverted body-of-revolution (BoR) structure, where cone-shaped cavities are formed in a dielectric material and metalized from the inside. The metalized cavities form an array of inverted BoR elements that are fed by a single printed circuit board. In contrast to previous work, this paper uses a solid, shaped plastic block instead of foam as a dielectric material and exploits its considerably higher permittivity to reduce the height of the elements. The reduced profile provides an excellent polarization purity in the class of tapered slotline (TSL) antennas while a 3:1 bandwidth is maintained with an active reflection coefficient (ARC) ≤ -10 dB. The shaping of the dielectric block decreases the scan loss of the array and prevents detrimental surface waves. The proposed array can steer the beam up to ±50∘ in all planes without exceeding -10 dB ARC. The proposed dielectric loading may be applied also to other antenna types, such as conventional BoR antennas. The excellent electrical performance and manufacturability of the array are confirmed by fabricating and measuring a prototype.

Orthogonality of the characteristic modes with respect to the weight operator of the generalized eigenvalue equation, and in the far field, is investigated in the case of lossy conducting and dielectric objects. Linking the weight operator to radiated power is shown to provide orthogonal far fields in the lossless case. In the lossy case, both the orthogonality of the characteristic far fields and the weight operator orthogonality of the modal currents are satisfied with respect to the Hermitian inner products only for sufficiently symmetric geometries, such as a sphere. For irregular lossy shapes, independently of the symmetry of the formulation, the far-field orthogonality can be obtained only with respect to the symmetric (non-Hermitian) product. The weight operator orthogonality can be satisfied with (complex) symmetric formulations, but again only with respect to the symmetric product. Since the symmetric products are not related to any physical power quantity, the modes do not form a (radiated) power orthogonal set in the lossy case. Hence, for lossy structures CMs do not satisfy their classical definition and they need to be redefined.

This article presents a wideband, dual-polarized antenna array with inverted BoR elements. The antenna structure is based on the conventional body-of-revolution (BoR) element, but the structure is inverted; i.e., instead of metallic cones, the elements are metalized cavities inside a dielectric block. This structure allows a more lightweight and low-cost design since all-metal parts are not needed. In addition to the inverted BoR structure, this paper proposes a feeding system that is fully integrated into a 5-mm-thick standard printed circuit board. The designed antenna operates at 2–6 GHz, providing 3:1 impedance bandwidth with an active reflection coefficient (ARC) < −10 dB. The ARC<−10 dB criterion is fulfilled for all beam-steering angles up to ±50° in all planes. The operation of the simulated antenna array was verified by measuring a built prototype. The measurement results are in good agreement with the simulations. However, the radiation efficiency of the prototype was approximately 11% inferior than that of the conventional all-metal BoR array due to conduction losses caused by the metalization of the used dielectric.