Matti Kuosmanen

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.

Integral operator-based theory of characteristic modes (CMs) for conducting, material, and lossy structures is reviewed. CMs are defined as solutions of a generalized eigenvalue equation (GEE). These GEEs are presented for various surface and volume integral operators and material structures. Interpretation of the characteristic eigenvalues in terms of electromagnetic power is studied based on the Mie expansion and integral operator formalism. Orthogonality and diagonalizing properties of the CMs are summarized. Challenges related to dielectric-magnetic bodies, lossy structures and spurious modes are discussed, as well as differences between the surface and volume operator approaches.

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.