Adam Lamecki

In this letter, a novel design for a 3-D-printed, self-supported coaxial-line X-band filter is presented. The filter is intended for Earth observation (EO) data downlink systems, where it must effectively reject signals in a wide frequency range. The filter design incorporates a 15th-order low-pass filter structure with a smooth profile, integrated with a short bandpass section with four $\lambda /4$ short-circuited stubs. The optimization of the low-pass section is attained by means of shape deformation, including the inner and outer coaxial conductors, and leads to a wide rejection band up to around 40 GHz, to suppress the third harmonic and other undesired out-of-band frequencies. A prototype was fabricated in one piece in an aluminum alloy using selective laser melting (SLM) and measured, exhibiting excellent agreement with simulations. In terms of out-of-band performance, the proposed coaxial-line filter is superior to other related state-of-the-art solutions.

The objective of this article is to demonstrate the applicability of frequency-dependent couplings (FDCs) to the design of self-equalized, generalized Chebyshev microwave bandpass filters (BPFs) in inline coupled-resonator circuit topologies. To this aim, a family of frequency-variant reactive coupling (FVRC) networks with double-zero single-pole (DZSP) characteristics is exploited, where the zeros can be either positioned at the imaginary axis or as a pair of real zeros. Thus, flattened-group-delay sharp-rejection microwave BPFs with in-band equi-ripple-type response and close-to-passband transmission zeros (TZs) can be realized while avoiding more-complex cross-coupling structures. The theoretical foundations of the proposed class of DZSP FVRC networks for the flexible allocation of the two zeros in the complex plane, as well as different circuit variants for their implementation, are presented. Two design examples of self-equalized fifth-order microwave BPFs in lumped-element/transmission-line and 3-D technologies centered at 1.5 and 9.98 GHz, respectively, are also shown, in which different structures for the DZSP FVRC networks are adopted. In both BPF designs, their coupling-matrix-based-synthesis responses obtained from solving an inverse structured nonlinear eigenvalue problem and electromagnetically (EM)-simulated results are provided. Furthermore, for practical validation purposes, a proof-of-concept microstrip prototype of the first BPF design example is built and tested. To the best of the authors’ knowledge, this is the first time that self-equalized, generalized Chebyshev microwave BPFs in inline schemes—i.e., without cross couplings—are experimentally verified.

The theoretical design and practical development of a family of high-order bandpass filtering dispersive-delay structures (DDSs) with radiofrequency (RF)-input-quasi-absorptive capabilities are presented. They consist of RF complementary-diplexer-based circuits, in which the main channel is a reflective-type bandpass filtering DDS and the resistively terminated auxiliary channel exhibits a bandstop filtering response with a quasi-complementary frequency profile with regard to that of the main channel. In this manner, the out-of-band/non-transmitted RF-input-signal power echoed by the main channel is mostly dissipated by the loading resistor of the auxiliary channel. As a fundamental constituent circuit block arranged in in-series-cascaded configurations for high-order reflective-type bandpass filtering DDS realizations, a triplet coupled-resonator network with one frequency-dependent coupling (FDC) is used. Its transmission properties are nearly transferred to the total RF-input-quasi-absorptive bandpass filtering DDS, which preserves the patterned group-delay/phase-variation response of the main channel. The theoretical design method of the devised RF-input-quasi-reflectionless bandpass filtering DDSs is detailed and exemplified with fifth-and seventh-order synthesis examples. It is based on a two-step design process, as follows: 1) separate polynomial/coupling-matrix synthesis of the main and auxiliary channels for the desired bandpass filtering DDS specifications in transmission and the complementary bandstop filtering functionality, respectively, and 2) coupling-matrix optimization of the overall RF-input-quasi-absorptive bandpass filtering DDS. Furthermore, for experimental demonstration purposes, a 2-GHz microstrip prototype of the previously designed RF-input-quasi-reflectionless fifth-order filtering DDS that features a stepped-type group-delay response is manufactured and characterized.

This paper presents a novel, very low-profile transmitarray antenna (TA) designed specifically for applications in the 24 GHz Industrial, Scientific, and Medical (ISM) band. The design innovation lies in embedding the switchable feed antenna into the beam-focusing surface and adding a reflector, which effectively halves the antenna's size in the boresight direction. This compact antenna allows for easy beam switching through the use of microswitches, making it well-suited for vehicular radar applications. A TA consisting of 829 units with an F/D ratio of 0.5 has been successfully engineered and fabricated for operation at 24.125 GHz. Notably, the antenna exhibits beam-switching capabilities at various angles, including 8, 16, and 24 degrees in the negative X-axis direction and 8, 16, 24, and 32 degrees in the positive X-axis direction, in addition to the boresight direction. The maximum measured gain of the antenna is 19.85 dBi, and it can achieve a tilt of 32 degrees with a gain reduction of approximately 3.52 dB compared to the boresight direction.

This work presents a design methodology for synthesizing a category of compact inline sharp-rejection waveguide cavity bandpass filters based on novel frequency-variant coupling (FVC) structures. These FVCs consist of a rectangular slot placed in the top broadwall of the WR-90 rectangular waveguide and loaded with an additional top cavity above the slot coupled through the slot. Both the slot and the top cavity resonate in the vicinity of the filter passband and can produce TZs at their resonance frequencies. The top cavity could be arranged either symmetrically or asymmetrically along the reference plane to excite or suppress the cavity TE ₁₀₁ mode, respectively. A double-zero-single-pole (DZSP) response can be achieved by either placing two cavity-loaded slots on opposite broadwalls of the main waveguide or by asymmetrically placing a single cavity-loaded slot on one broadwall. The asymmetry perturbs the field-pattern distributions of the TE ₁₀₀ slot and TE ₁₀₁ cavity modes with regard to the symmetrical case so as to convert them into hybrid slot-and-cavity modes—referred to as S&CM 1 and S&CM 2. This approach excites both S&CM resonances to obtain the DZSP filtering transfer function with size-compactness advantage. Two fifth-order inline waveguide bandpass filter prototypes are designed at the electromagnetic (EM) simulation level based on DZSP-slot-mode (DZSP-SM) and DZSP-slot-and-cavity-mode (DZSP-S&CM) FVCs, respectively. A proof-of-concept prototype of fifth-order sharp-rejection waveguide bandpass filter using a DZSP-S&CM FVC, which is centered at 10 GHz and exhibits a pair of close-to-passband TZs, is manufactured and characterized. The measured results are in good agreement with the EM-simulated results, thus fairly verifying the viability of the devised filter design concept.

This paper introduces a new type of waveguide filter with smooth profile, based on specially designed dual-mode (DM) cavity resonators. The DM cavity design is achieved by applying a shape deformation scheme. The coupling between the two orthogonal cavity modes is implemented by breaking the symmetry of the structure, thus eliminating the need for additional coupling elements. The modes operating in the cavity are carefully analyzed and a scheme for managing the spurious modes is discussed. Two filter prototypes employing the designed DM cavities are developed and described in detail. The first design is a fourth-order bandpass filter (BPF) with a 90° rotated output and a transmission zero (TZ), whereas the second design is an eighth-order filter with four TZs. Both designs are developed, taking into account the limitations of 3D printing technology to enable their single-piece fabrication without internal supports. The structures benefit from additive manufacturing (AM) by having a smooth surface profile and reduced weight, which is often highly desirable for high-power and low-loss applications. Filter prototypes were manufactured using selective laser melting (SLM) from aluminum alloy and tested to validate the designs. Measurement results are consistent with the simulation and prove the validity of the proposed solutions. Both measured BPF prototypes demonstrate low insertion loss, i.e., 0.11 dB and 0.25 dB for the fourth-order and eighth-order designs, respectively. The estimated Q-factors reach 3500 and 4500, which is a very good result for 3D-printed parts.

In this study, a novel low-cost polishing method for internal surface roughness reduction of additively manufactured components, developed for waveguide (WG) circuits operating in the millimeter frequency range is proposed. WG components fabricated using powder bed fusion (PBF) generally feature roughness of ten to fifty microns, which influences the increase of roughness-related conductor power losses having a major effect on the electrical performance of additively manufactured all-metal WGs. To improve and decrease the surface roughness of circuits fabricated using PBF, glass microbeads as an abrasive medium are proposed to be used in combination with a rotary tumbler. This technique allows the abrasive medium to efficiently penetrate internal long channels and cavities, having cross section dimensions in the range of sub-to a few millimeters. An experimental study was carried out on an example of WG sections and bandpass filters fabricated using PBF through selective laser melting (SLM), operating within the 8.2 to 40 GHz range. Polishing impact on both mechanical and electrical properties was studied showing surface roughness reduction by 18% and sixth order filter’s insertion loss reduction at 23 GHz by 40% after 24 h of tumbling with 300–400 $\mu$ m large glass microbeads.

This article presents an optimized dual-polarized transmitarray antenna (TA) designed for MIMO applications at the Ka-band, capable of switching beams in two directions. The antenna aperture uses a small unit cell with three layers of Taconic RF-35 dielectric substrates, which can be easily fabricated using PCB technology. The unit cell achieved a 360-degree phase shift and a transmission magnitude exceeding–0.4 dB at 28 GHz. We used nine dual-polarized patch antennas in a cross shape, each with a 10.5 dBi gain at 28 GHz, to switch the beams in two directions without changing the feed location. We optimized the phase distribution in the TA aperture and adjusted the feed antenna’s $F/D$ and tilt to achieve a high-gain antenna with low-gain roll-off during beam switching. The fabricated TA exhibited excellent agreement with the full-wave simulation results. It achieved ±15 degrees and ±30 degrees beam tilts in the x- and y- directions, with less than 0.8 dB gain roll-off for both polarizations.

This work presents a design technique to implement miniaturized cross-coupled bandpass filters in inline physical configurations based on triple-mode resonators. Triple-mode resonances are obtained by using integrated coaxial-waveguide cavity resonators. They consist of two coaxial conducting posts placed in the sidewalls of a rectangular waveguide cavity. In the proposed triplet, a transmission zero (TZ) can be positioned at any of the two sides of the passband by simply locating the coaxial posts either on the same wall or on opposite walls of the rectangular waveguide cavity. Coaxial-to-waveguide mode coupling is implemented by means of coupling screws, which adds flexibility in terms of tuning easiness. Three 9.9-GHz proof-of-concept prototypes corresponding to third- and sixth-order integrated coaxial-waveguide bandpass filters are designed at the electromagnetic-simulation level. Furthermore, the triplet with a TZ below the passband is manufactured and characterized to experimentally validate the engineered 3-D RF filter principle and its underlying design theory.

In this work, a family of frequency-variant reactive coupling (FVRCAFVC) networks is introduced and discussed as new building blocks for the synthesis of coupled-resonator bandpass filters with real or complex transmission zeros (TZs). The FVRCAFVC is a type of nonideal frequency-dependent inverter that has nonzero elements on the diagonal of the impedance matrix, along with a nonlinear frequency-variation profile of its transimpedance parameter. The distinctive feature of these new FVRCsAFVC is that they can be modeled with a simple bridged-T network consisting of four reactive elements. Such FVRCsAFVCs can introduce one pole and up to two TZs—so that they are referred to as double-zero single-pole (DZSP) networks. DZSP networks allow the pole to be located above, below, or in-between the TZs. Depending on the choice of the elements in the bridged-T circuit, the position of the TZs and the pole can be controlled independently. The coupling matrix for filters with DZSP networks can be found by solving an inverse-structured nonlinear eigenvalue problem (ISNEVP). Examples of possible implementations of DZSP couplingAFVC networks are provided for both lumped and quasi-lumped-element circuits, as well as for transmission-line-based and waveguide technologies. The application of DZSP coupling networks to the design of microwave bandpass filters with generalized Chebyshev-type characteristics is illustrated by means of the synthesis of three microstrips and two waveguide coupled-resonator bandpass filters. The waveguide filters are verified through electromagnetic (EM) simulations and the microstrip filters are manufactured and characterized for proof-of-concept demonstration purposes, showing a fairly close agreement between EM simulations and measurements.