In this work, a multiband Electromagnetic Bandgap (EBG) unit-cell (UC), that provides surface-wave suppression in the $\mathbf{L}, \mathbf{S}, \mathbf{C}$ and X-bands, is presented. The proposed design makes use of a group of $2 \times 2$ dual-band slotted mushroom UC-EBG, that are resonant at the $\mathbf{C}$-band $(5.2 \text{GHz})$ and X-band $(9.6 \text{GHz})$, to create a tri-band UC-EBG that achieves an additional bandgap at S-band (3.2 GHz). Consequently, a geometrical arrangement of $2 \times 2$ tri-band UC-EBG are short-circuited to a single-band mushroom UC, that is resonant at L-band (1.5 GHz), leading to a multilayer UC-EBG structure with quad-band performance. The presented work is thoroughly validated making use of several simulation approaches, in which the different dual-band, tri-band and quad-band EBG configurations are analyzed. The proposed UC-EBG enables, with a single unit-cell design, and without using reconfigurable elements, the simultaneous suppression of surface waves in the $\mathrm{L}, \mathrm{S}, \mathrm{C}$, and X frequency bands, becoming particularly suitable for multifrequency applications.

This contribution presents an efficient analysis of microwave circuits, in rectangular waveguide technology, containing an unlimited number of arbitrarily shaped 3D conducting and lossy homogeneous dielectric and/or magnetic material objects. The proposed method uses the Electric Field Integral Equation (EFIE) combined with the Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT) formulation, employing frequency-domain Lorenz gauge Green’s functions and their spatial derivatives accelerated by the Ewald method. To enhance computational performance, the Method of Moments (MoM) matrices are divided into dynamic and static components. Unlike free-space problems, the rectangular waveguide environment presents the technical challenge of evaluating a series of specific terms for homogeneous objects. Furthermore, to reduce the computational cost, an adaptive numerical integration scheme is implemented that dynamically adjusts the number of integration points based on the distance between the source and observation points. A further novelty in the application of the PMCHWT formulation occurs when homogeneous objects are in contact with the waveguide boundaries. In such instances, the equivalent internal problem is addressed by applying image theory to the free-space Green’s functions relative to the waveguide walls. Finally, the proposed integral equation technique is validated against the commercial full-wave solver Ansys HFSS, demonstrating excellent agreement as well as a reduction in computation time and memory consumption.

This work presents the additive manufacturing of an X-Band diplexer composed of two filters: one centered at 9.5 GHz and another at 10.5 GHz, both with a bandwidth of 250 MHz, plus a power divider. The whole set was implemented in rectangular waveguide technology, manufactured in a single block. Good agreement between simulated and measured results is achieved, with return losses lower than 20 dB, insertion losses around 0.3 dB in both filters, and isolation of 30 dB between them. For demonstration purposes, the set was used for the characterization in an anechoic chamber of the directivity, gain, and radiation efficiency of an additive manufactured horn antenna. The diplexer was implemented using Powder Bed Fusion Laser Beam (PBF-L/M) technology on a metal alloy (AlSi10Mg). The horn antenna was manufactured via Vat Photopolymerization by exposure to Masked UV light (VPP-UVM) technology, using a polymeric resin (Gray from Applylabwork), subsequently metallized. Mechanical optimization for the additive manufacturing of the design allows the development of an ergonomic single-part device, enabling perfectly squared inner cavities. This reduces weight, volume, and manufacturing waste while maintaining the electromagnetic response of the diplexer.

In this contribution, an efficient integral equation (IE) formulation is proposed for the analysis of slot antennas. To solve the IE, the problem is divided into two equivalent subproblems: the first related to the rectangular waveguide component and the second related to the free-space medium. Subsequently, an equivalent surface magnetic current density defined at the discontinuity is used to couple the equivalent subproblems. In order to reduce the total number of unknowns before solving the IE using the method of moments (MoM), the Lorenz gauge Green's functions of the rectangular waveguide and a grounded half space is used respectively for both equivalent subproblems. Furthermore, due to the slow convergence behavior, the Ewald method has been used to accelerate the evaluation of the Green's functions of the rectangular waveguide. Finally, the proposed IE technique has been validated by comparing the results of the radiation pattern and reflection coefficient of a slot antenna with those provided by the commercial fullwave Ansys HFSS software, together with an example from the technical literature, showing good agreement and better numerical efficiency.

This article shows several practical improvements on impedance matching methods based on modern filter network theory. New conclusions about matching have been obtained using the Bode-Fano limit, adding frequency-independent reactances (FIRs) to the formulation for the first time. Moreover, a matching limit can be obtained analytically before building the network, thus providing a performance limit in advance. A lumped network is obtained analytically as a first attempt with the reflection coefficient bounded between two limits with equiripple behavior for a maximum bandwidth. Then, this network model is modified by adding frequency-independent reactances (FIRs) to provide a better match even for asymmetric responses. The lumped network is converted into a distributed network where the dispersion of the lines is accounted for. The next improvement is the conversion into an electromagnetic model, where dispersion is also considered in the inverters modeled with irises. Three examples with increasing bandwidths are considered, and a manufactured prototype is measured to test the good behavior of the proposed matching method.

This article describes a systematic design approach, based on the use of surrogate models in a low-accuracy space, to achieve smaller footprints and wider bandwidths for direct-coupled microstrip filter topologies. To reduce the computational cost of the design procedure, the frequency response of each resonant structure is matched (individually) to that of its equivalent circuit model, thus avoiding extensive optimization at electromagnetic (EM) level. The designed filter can then be translated into a high-accuracy space using the Aggressive Space Mapping (ASM) technique. In addition, an equivalent circuit based on a combination of inductive, capacitive and distributed elements is firstly synthesized. Results for the new filter topology are fully validated, through full-wave EM simulations from two commercial codes, and successfully compared (in terms of size and performance) with those of traditional coupled-line filters.

In this presentation we describe a practical and systematic procedure to design a new topology for wideband waveguide filters. The novel topology that we propose is obtained by replacing the conventional lambda(g)/2 resonators and inductive inverters, with resonant apertures (RAs) as resonators, and capacitive irises as inverters, respectively. The combined use of RAs and capacitive irises enables an efficient suppression of all spurious responses in the entire WR-75 waveguide operational frequency band, while, at the same time, significantly reducing the size of the filter. In addition to theory, a fifth-order prototype is designed, manufactured and measured. The measured response of the filter shows a high degree of correlation with both the in-band and out-of-band simulated responses, thereby fully validating both the novel topology and the design procedure.

In this contribution, an Integral Equation (IE) formulation is proposed for the analysis of microwave circuits, based on the junction of two different rectangular waveguides coupled by an arbitrarily shaped zero thickness discontinuity. These rectangular waveguides could include an unlimited number of conducting elements with arbitrary shapes inside them. To solve the IE, the problem is split into two equivalent subproblems, each of which is related to a rectangular waveguide. Subsequently, an equivalent surface magnetic current density ($\vec{\mathrm{\mathbf{M}}}_{\text{ap}}$) defined at the discontinuity is used to connect the equivalent problems of each rectangular waveguide. In order to reduce the number of unknowns, the Lorenz gauge Green's functions of rectangular waveguides and their spatial derivatives are used to model the boundary conditions. In addition, the Ewald method has been employed to significantly speed up the evaluation of these rectangular waveguide Green's functions. Therefore, the use of this surface magnetic current density can reduce in some configurations the number of unknowns compared to an alternative Electric Field Integral Equation (EFIE). In addition, it allows a simpler analysis of some kind of discontinuities with respect to an EFIE method. Finally, the proposed technique has been validated by comparison with the results provided by commercial full-wave software tools such as Ansys HFSS and CST Studio Suite, showing good agreement and a better numerical efficiency.

Multipactor under digitally modulated signals is a field of growing interest in the space industry. The need to operate with higher powers arises from the industry demand to increase data throughput, enabled by advances in space-born amplifiers. However, this often leads to signal distortion, which may alter the multipactor threshold measured in laboratory conditions. This work aims to characterize, through the combination of experimental tests and numerical simulations, how distortion affects the multipactor performance of microwave devices excited by modulated signals. To this end, several dual carrier modulated signals with different distortion levels are digitally created, simulated, and tested on the same multipactor sample. When the simulation configuration matches the laboratory conditions, an excellent degree of accuracy is reported. However, simulations also demonstrate that the nonzero time response of the sensors may play a crucial role in determining the measured threshold, suggesting that laboratory detectors may struggle to detect short-lived events. Numerical predictions also reveal how the thresholds linked to these events are affected by the degree of signal distortion.

In this work, the impact of the antenna carrier, an aerodynamic attachment structure that allows the antenna installment on the aircraft, is analyzed. The resulting polarization-dependent edge diffraction effects of a flight-model L-band phased array antenna, embedded in the antenna carrier, is experimentally validated. Measurements show a pattern distortion in the vertical polarization along with a gain reduction of 1 dB in the elevation plane, in which the beam steering is performed, that also corresponds to the E-plane, thus becoming this polarization more sensitive to edge diffraction effects. Further analysis is performed increasing the electrical size of the antenna carrier diameter up to 11.0. A low-profile and planar solution, based on Electromagnetic Bandgaps, is presented, by which the induced surface currents on the antenna carrier for the vertical polarization are mitigated, thus reducing the impact of edge diffraction effects without interfering in the performance of the horizontal polarization. Thereby, the desired radiation characteristics for both polarizations can be fulfilled regardless of the antenna carrier structure.

Multipactor is a key high-power effect limiting the system performance for onboard satellite hardware. Although modern particle simulators admit arbitrary geometries and signals as inputs, their practical use is often limited to continuous-wave (CW) excitations. Unfortunately, the multipactor analysis for input-modulated signals normally leads to prohibitively large CPU times, as signal lengths are very large compared to the electron population’s evolution time. The Coarse Method is an elegant way of overcoming this limitation, providing a good estimate of the multipactor threshold in reduced CPU times. However, if the input signal is not preprocessed before being analyzed, the method is unable to account for the frequency dependence as it operates with electron dynamics information extracted at a single frequency, leading to biased predictions for narrowband samples as filters. This article proposes an extension to the original Coarse Method implementation by considering the sample response and the modulated signal spectral distribution to account for the frequency dependence. The resulting method is suitable for estimating the multipactor threshold of narrowband samples excited by modulated signals, while keeping the benefits in terms of simplicity, efficiency, and generality of the Coarse Method. The proposed approach is benchmarked against laboratory measurement results, as well as particle simulators and legacy Coarse Method predictions, revealing the advantages of the novel technique and its range of applications.

This contribution presents an efficient and versatile Integral Equation (IE) formulation for the analysis of discontinuities in rectangular waveguides, with particular emphasis on T-junctions and bend-junctions as use example cases. The proposed methodology enables the modeling of zero-thickness discontinuities with arbitrary geometry, as well as the inclusion of arbitrarily shaped conducting elements within the rectangular waveguides. The original problem is decomposed into a set of equivalent subproblems, coupled through an equivalent surface magnetic current density ( M-ap) defined at the discontinuity. By employing Lorenz-gauge Green's functions of rectangular waveguides, a significant reduction in the number of unknowns is achieved. Furthermore, to enhance computational efficiency, the Ewald method is used for the evaluation of the Green's functions and their spatial derivatives. The accuracy and computational performance of the proposed approach are validated through comparison with results obtained from the widely used commercial full-wave Ansys HFSS software, demonstrating good agreement and better numerical efficiency.

In this contribution, an Integral Equation (IE) formulation is proposed for the analysis of horn antennas joined to a rectangular waveguide microwave filter as an input feeder. In order to solve the IE, the problem is split into two equivalent subproblems: the first one related to the rectangular waveguide component at the input, and the second one related to the horn antenna. An equivalent surface magnetic current density ((M) over right arrow (ap)) defined at the discontinuity is then employed to couple the equivalent problems. In order to reduce the overall number of unknowns after solving the IE by the Method of Moments (MoM), the Lorentz gauge Green's functions of the rectangular waveguide and a grounded half space is used respectively for both equivalent subproblems. In addition, due to their slow convergence behaviour, the Ewald method has been employed to speed up the evaluation of the rectangular waveguide Green's functions. Finally, the proposed IE technique has been validated by comparing radiation pattern and reflection coefficient results of a pyramidal horn filtenna with those provided by commercial full-wave Ansys HFSS software along with examples from technical literature, showing a good agreement and better numerical efficiency.

This article describes a practical method to design waveguide tapers with smooth profile. A tapered waveguide, in general, is used to match different waveguide sections with the lowest possible reflection level. This method, based on Bernstein approximations, is used to achieve a reflection levels lower than the typical 20 dB in waveguide designs. The taper is designed with a prescribed length for bandwidths beyond the common frequency band of the end waveguide sections. An example of waveguide taper is compared with previous works in terms of reflection and curvature parameters. Finally, a prototype has been simulated and measured with excellent results.

Multipactor is a harmful effect that may challenge the correct operation of satellite communication systems by limiting the system power-handling capability, and hence its overall performance. Therefore, the development of techniques for predicting the multipactor threshold is of great practical interest. For narrowband components, rough estimations can be obtained from experimental charts, normally resulting in conservative thresholds. More accurate predictions can be obtained with particle simulators, at the expense of a much higher computational effort. This study proposes an approach based on circuital models for swift and accurate multipactor threshold predictions, specifically addressing short-term discharges induced by multicarrier signals in narrowband samples. The use of reduced, but representative, electromagnetic (EM) models of the critical gap region is discussed in detail. Through the use of these models, it is possible to avoid simulating the complete structure, increasing the computational efficiency and enabling the fulfillment of the power requirements at early design stages. The proposed technique is validated through commercial particle simulators, showcasing its efficacy, efficiency, and key benefits.

Synthetic Aperture Radar (SAR) has become, nowadays, one of the most important techniques in remote-sensing, and the increasing interest in Earth monitoring reinforces this trend. Next-generation SAR sensors will enhance the radar resolution capabilities by means of digital beamforming (DBF) techniques along with multistatic systems. In order to support the technological development of future spaceborne SAR missions, airborne sensors become an essential scope of research. Thereby, future airborne SAR systems demand enhanced DBF capabilities that involve phased array antennas with a high density of array elements. However, the antenna aperture size is significantly limited in airborne applications, which leads to low-profile and highly integrated antenna solutions, becoming a more challenging task for lower frequency operations such as L-band. In this work, a compact L-band dual-polarized multilayer phased array antenna with beam steering in elevation, developed for the next-generation German Aerospace Center (DLR) airborne SAR system, is presented. The proposed design makes use of truncated cavities to improve the array element isolation and provides 66% more antenna elements than the previous L-band phased array of the current DLR airborne SAR sensor with the same antenna aperture size. Measurements of a manufactured prototype show an antenna bandwidth of almost 20%, matching levels better than 17 dB, up to 15 dB gain, and cross-polarization suppression values higher than 35 dB. Thus, the proposed work will allow the application of advanced DBF techniques in the upcoming first pulsed bistatic L-band airborne SAR sensor.

Passive intermodulation (PIM) measurements are typically conducted with a simplified test scenario composed of two radio frequency (RF) carrier tones operating in continuous wave (CW). However, satellite payloads tend to operate with a more complex scheme, where modulated signals are present at each transmission channel. This article aims at linking both PIM scenarios. A novel procedure is developed to obtain, from measured data obtained using two CW input tones, the spectral distribution and amplitude of a PIM contribution originated after exciting a nonlinear device with modulated signals. The technique determines the spectral distribution of the PIM term of interest from the spectrum of the input signals, and evaluates its amplitude from a recently formulated PIM power conservation rule. Moreover, an extensive PIM test campaign providing novel and valuable experimental data for the topic of PIM under modulated signal excitation has been carried out. The results obtained for a wide range of test scenarios exhibit a good agreement with theoretical predictions, thus providing a practical validation of the proposed technique.

This article presents the theoretical study, numerical simulation and fabrication of a phase shifter and a stub resonator for use in microstrip ridge gap waveguide (MRGW) technology, using a liquid crystal (LC) in the substrate as a reconfigurable material. The phase shifter and the stub resonator are filled with LC, and thanks to the LC’s dielectric anisotropy properties, the phase shift and the resonance response can be easily controlled using an external electric or magnetic bias field. The phase shifter was designed to operate in the range of 10 to 20 GHz, and the resonator was designed to operate in the range of 7.8 to 8.8 GHz. The phase shifter’s responses (including both phase shift and insertion losses), associated with both the parallel and perpendicular permittivity values of the LC, were computed and measured, and then the corresponding figure of merit (FoM) was extracted. The resonator’s frequency responses, associated with both the LC’s parallel and perpendicular permittivity, were computed. The resonator’s frequency responses, which provided different polarization voltages, were measured and compared to the simulation results. All technological issues related to both prototypes are also discussed here. The good agreement between the simulation and measurement results confirm this technology as a viable approach to the practical implementation of these microwave reconfigurable devices.

The effect of the phase of the input carriers has been traditionally neglected in the characterization of passive intermodulation (PIM) since standard two-tone PIM tests seem to be unaffected by phase variations of the excitation signals. However, the phase of the input carriers can be of relevance in many practical applications. This article is aimed at filling this gap in the technical literature. First, the existing theory explaining why the phases do not affect the measured PIM contribution for a two-carrier excitation but can be of relevance for generic multicarrier scenarios is summarized. PIM measurements for complex signals composed of several carriers with varying phases are then reported, enabling the practical characterization of this effect for the first time. Experimental results of the test campaigns are fully aligned with theoretical predictions, thus allowing us to identify those PIM contributions that can be affected by the carrier phases and assess the expected variation range in their amplitude level.

In this letter, a vertical transition from empty substrate integrated (ESI) coaxial line (ESICL) to ESI waveguide (ESIW) is presented. In contrast to in-line approaches, the proposed transition can connect both technologies with an independent substrate height, thus providing a compact structure. To transform the TEM mode into the TE10 mode, an aperture transversal to the propagation direction is used to couple the fields, and matching elements are added to improve the return loss level. A design guideline is presented and used to design two transitions. A back-to-back prototype is fabricated and measured to validate the proposed transition. The measured results show a return loss level better than 11.2 dB and an insertion loss lower than 1 dB, which makes the proposed transition promising for high-performance interconnection required in multilayer systems.