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This letter presents an ultra-wideband crossover based on metallic-nanowire-filled membrane (MnM) from dc to 110 GHz. Two designs are proposed with reduced insertion loss and high isolation. Design Type 1 presents a 1.2 dB insertion loss and 19 dB isolation up to 80 GHz, with a phase imbalance of 14° at 80 GHz. This important phase imbalance is due to CPW that passes under the top microstrip (MS) line. To improve the device, a CPW was used in both paths. The improved design Type 2 shows a 1.5 dB insertion loss, 0.2 dB insertion loss imbalance, and 3.3° phase imbalance at 110 GHz. The latter presents a measured isolation of 38 dB up to 70 GHz and a simulated isolation better than 30 dB up to 110 GHz.

A finite element method (FEM)-based simulation approach to predict the tunability in composite materials was developed and tested with analytical data. These tests showed good prediction capabilities of the simulation for the test data. The simulation model was then used to predict the tunability of a network-structured composite, where the dielectric phase formed clusters in a paraelectric network. This was achieved by simulating a reciprocal core-shell unit cell of said network. The simulation showed a high tunability for this network model, exceeding the tunability of the analytically evaluated layered, columnar, and particulate model. The simulation results were experimentally verified with a Ba0.6Sr0.4TiO3/Mg3B2O6 (BST/MBO) composite, where core-shell granulates were made with a two-step granulation process. These structured samples showed higher tunability and dielectric loss than the unstructured samples made for comparison. Overall, the structured samples showed higher tunability to loss ratios, indicating their potential for use in tunable radio frequency applications, since they may combine high performance with little energy loss.

After being postponed for two years, the inaugural edition of the IEEE Microwave Theory and Technology Society (MTT-S) International Conference on Microwave Acoustics & Mechanics (IC-MAM) was held from 18 to 20 July 2022 in the Training Center of Rohde & Schwarz in Munich, Germany. It was financially sponsored by the IEEE MTT-S (Figure 1) and technically cosponsored by the European Microwave Association.

A complete tunability electromagnetic simulation model for the Ba$_{0.6}$Sr$_{0.4}$TiO$_3$ (BST), with 𝜀𝑟 ≈ 2000, and Mg$_2$B$_2$O$_6$ (MBO), with 𝜀𝑟 ≈ 7, composites is proposed here. The model is based on electrostatics, to simulate the effects of bias fields distribution in the composite varactor at the unbiased state to create the biased state for all volumetric mixture compositions. A bulk-ceramic varactor approach is chosen for the fabricated varactors. Varactors are fabricated with different volume compositions of BST and MBO, ranging from 10 to 100 vol-% of BST. Simulated results of the varactor model are then verified with the measured results of the varactor. The simulated and measured tunability shows considerable discrepancy at room temperature, which leads to Curie temperature 𝑇$_𝐶$ investigation of the fabricated varactors. It has been observed that a shift in 𝑇$_𝐶$ is directly proportional to the discrepancies in the simulated and measured tunability. After incorporating the 𝑇$_𝐶$ shifts in the model, the results show close proximity between measured and 𝑇$_𝐶$ -shifted simulated tunabilities with differences being reduced from around 32% to 2% for 80 vol-% BST varactor.

The composite system Ba0.6Sr0.4TiO3/Mg3B2O6 (BST/MBO) was investigated in regard to the influence of sintering temperature and composition on the dielectric properties. Dilatometry measurements were carried out at up to 1200 °C. At 1050 °C, all composites with an MBO content of 22.0 vol% or more showed densities over 90 % of the theoretical value. An in-situ XRD study was conducted at up to 1200 °C. The formation of Mg2TiO4 was detected starting at 1120 °C. These data sets were used to make a set of compositions ranging from 17.8–79.7 vol% MBO. At 1050 °C, the element distribution at the BST/MBO interfaces was investigated via EDX and ToF-SIMS. It was observed that titanium migrates from the BST into the MBO, mostly in the interface region. The microstructure of the different compositions was determined via SEM and image analysis. Traces of a third phase were found even for samples sintered at 1050 °C. The dielectric properties of the samples were measured at 13.56 MHz and compared to the expected behavior from the literature. It was found that compositions in the middle of the spectrum showed reduced permittivity, loss, and tunability, resulting in an overall increase in the material quality factor when compared to the chosen model. This was attributed to the passivation of the BST at the interfaces.

This letter presents a planar endfire Yagi-Uda antenna phased array fed by a corporate network. The phase of each radiating element is controlled by a loaded line phase shifter based on liquid crystal technology. The phase shifters are meandered to minimize the dimension of the array and are measured to provide maximum differential phase shift of 400 ${^\circ }$ with a figure-of-merit of ${54}{{^\circ }\rm {dB}^{-1}}$ at 28.4 GHz. Each phase shifter is tuned individually by applying bias voltage through high impedance chromium line together with dc blocks. The array is designed for full-range scanning in E -plane with $>\!{360}{^\circ }$ phase shifters. Measurements show that it can be steered from −40 ${^\circ }$ to 40 ${^\circ }$ by applying dc bias in the range of 0–5 V and from −50 ${^\circ }$ to 60 ${^\circ }$ with degraded patterns. The maximum gain of the phased array antennas is measured to be 4.5 dBi. The array beam is able to be fast steered within approximately 1 s.

Since the dielectric permittivity of ferroelectric materials depends on the electric field, they allow designing switchable and continuously tunable devices for adaptive microwave front ends. Part of the ongoing research is the field of all-oxide devices, where epitaxial oxide conductors are used instead of polycrystalline metal electrodes, leading to epitaxial ferroelectric layers and resulting in high device performance. In particular, they allow engineering the acoustic properties separated from the electric ones due to the structural similarity between the dielectric and conducting oxide films. Two major results are reported in this work. First, a highly accurate model for the microwave impedance of ferroelectric varactors is derived that tracks the superposition of induced piezoelectricity and field extrusion into the substrate caused by thin electrodes. In difference to previous works, this model covers both a wide frequency and biasing range up to 12 GHz and 100 V/ [Formula: see text]. Second, the high model accuracy enables the determination of all relevant electric and mechanic properties based on a mere microwave characterization. This approach will be especially valuable when independent measurements of mechanical properties of the thin-film materials are impeded by a high integration of the devices. Though derived for all-oxide varactors, the presented model can as well be adapted for thin-film bulk acoustic wave resonators (FBARs) and varactors with conventional metal electrodes when eventual dead layers at the interface are modeled correctly.

This letter investigates the feasibility of the combination of additive manufacturing and liquid crystal (LC) loaded dielectric waveguides (DWGs) at ${W}$ -band (75–110 GHz). The waveguide is extruded such that it has a cavity in its center, used for LC hosting. Reproducible cross sections are achieved along the DWG and high agreement between differential phase shift and figure-of-merit (FoM) is obtained when the DWG is biased electrically or magnetically. Deembedding the structure shows attenuation constants between 0.12 and 0.33 dB/cm which lead to a high FoM in the range of 100°/dB to 160°/dB. According to simulations, higher values are obtainable if larger LC cavities are realized.

In this paper, the gap waveguide technology is examined for packaging liquid crystal (LC) in tunable microwave devices. For this purpose, a line based passive phase shifter is designed and implemented in a ridge gap waveguide (RGW) topology and filled with LC serving as functional material. The inherent direct current (DC) decoupling property of gap waveguides is used to utilize the waveguide surroundings as biasing electrodes for tuning the LC. The bed of nails structure of the RGW exhibits an E-field suppression of 76 dB in simulation, forming a completely shielded device. The phase shifter shows a maximum figure of merit (FoM) of 70 °/dB from 20 GHz to 30 GHz with a differential phase shift of 387° at 25 GHz. The insertion loss ranges from 3.5 dB to 5.5 dB depending on the applied biasing voltage of 0 V to 60 V.

This letter presents a fully dielectric beam-steering lateral antenna operating at the V-band (50 GHZ). The antenna consists of a semicircular Rexolite body with an embedded liquid crystal (LC) cavity. An electrode network is used in order to align the LC molecules such that they form reconfigurable areas of high and low permittivity. The area of high permittivity is used to steer the direction of the beam. Due to the dielectric nature of the antenna, it shows enhanced fabrication simplicity and a weight reduction factor of 21 when compared to previous work. In addition, the bias electrodes may have arbitrary shape and are easily accessible, ensuring a high versatility for future designs. The steering mechanism is nearly frequency-independent, and the antenna is matched throughout the whole frequency range. Beam-steering angles of 17 $^\circ$ are reported with a gain of 16 dBi and a maximum sidelobe level of $-$ 7.6 dB.

This paper presents an overview of fully dielectric antenna arrays with integrated dielectric power dividers developed at Technische Universität Darmstadt as an extension of previous work. The power dividers are based on the principle of multimode interference and offer one- and two-dimensional power division in a single step, which allows the realization of small and lightweight devices. In order to prove the concept, 1 × 4 and 4 × 4 fully dielectric antenna arrays with integrated power dividers milled from Rexolite are designed, realized and characterized, operating between 90 GHz and 105 GHz. To assess miniaturization and beamsteering capabilities, three 1 × 4 arrays composed of different materials, Rexolite, Preperm L300 and Preperm L440, are compared regarding gain, size, weight and element spacing. Depending on array size and material, gain is between 12.5 dBi and 22 dBi, accompanied with sidelobe levels below - 7.5. All demonstrators are realized by milling, but the designs show the potential to be 3D printed or injection molded for large scale manufacturing.

For the first time, a liquid crystal (LC) filled lens antenna is investigated for beam steering in this paper. The LC’s anisotropy is applied inside a semicircular shaped parallel-plate waveguide to achieve a deflection of the propagating electromagnetic wave at the interface of lower and higher permittivities. Using an electrode network, the direction of the sector of higher permittivity can be adjusted and, therefore, so can the beam direction. The main advantage of this antenna concept is that neither phase shifters nor switching networks are needed, resulting in a very simple approach for beam steering. Operating at $V$ -band, from 50 to 75 GHz, the first demonstrator proves the antenna concept by reconfiguring the beam in predefined directions of −30°, 0°, and +30° to simplify the electrode biasing network. Furthermore, it is feasible and being demonstrated to adjust the beamwidth and to generate multiple beams. In the measurements, the antenna exhibits a measured input reflection below −10 dB over the complete frequency range. For the unsteered radiation pattern, the sidelobe level is between −8 and −12 dB, whereas these values degrade to −4.5 to −6 dB for the steered radiation pattern at ±30°. The measured gain is between 13 and 15 dBi.

Low loss, ferroelectric, fully-printed varactors for high-power matching applications are presented. Piezoelectric-induced acoustic resonances reduce the power handling capabilities of these varactors by lowering the Q-factor at the operational frequency of 13.56 MHz. Here, a quality factor of maximum 142 is achieved with an interference-based acoustic suppression approach utilizing double metal–insulator–metal structures. The varactors show a tunability of maximum 34% at 300 W of input power. At a power level of 1 kW, the acoustic suppression technique greatly reduces the dissipated power by 62% from 37 W of a previous design to 14.2 W. At this power level, the varactors remain tunable with maximum 18.2% and 200 V of biasing voltage.

This paper presents a planar quasi Yagi-Uda antenna based on liquid crystal (LC) technology for millimeter-wave applications. The antenna is built on the inner surfaces of two parallel glass substrates, with thin layer of LC filled in the feed part. It is fed by a two-step coaxial-to-microstrip transition. The antenna achieves a measured bandwidth of 31 % with a VSWR < 2, better than 15 dB front-to-back ratio, better than –13 dB cross polarization and 9.0 dBi gain at 28 GHz. Measured scattering parameters, as well as far field patterns agree very well with the simulation results. The planar quasi Yagi-Uda antenna is compatible with existing planar reconfigurable feed network based on LC technology, complementary to the broadside radiating array.

This paper presents a continuously tunable parallel-polarized dielectric image guide phase shifter, operating at W-Band (75 GHz to 110 GHz). The core material of the image guide is liquid crystal (LC), which is used to change the propagation constant of the guided wave. Four bias electrodes provide the field necessary to tune the LC. The parallel polarization reduces parasitic coupling to DC structures processed on the ground plane. Therefore, no stepped-impedance structure is necessary for the bias electrodes. Due to the image guide topology, the phase shifter can be used in substrate integrated environments. A maximum differential phase shift of 520° at 110 GHz is achieved with a bias voltage of ±150 V while the insertion loss is between 3.7 dB to 4.3dB. From 87GHz on, the resulting maximum Figure-of-Merit (FoM) is higher than 100°/dB, reaching a maximum of 125°/dB at 103 GHz.

Barium strontium titanate (BST) thin-film varactors promise very good performance in RF frontends in terms of low loss and high tunability. However, their application is commonly limited to lower GHz frequencies. For higher frequencies, acoustic resonances drastically reduce the device quality in metal-insulator-metal (MIM) structures under bias voltage. In this work, this limitation is overcome by replacing the metal electrodes with conducting oxides that structurally match to BST and, therefore, avoid acoustic impedance mismatch. A detailed analytic model is derived, incorporating electric and acoustic behavior. Four samples with an oxide bottom electrode and varying BST thickness are characterized and fitted by the derived model with very high accuracy. Each shows a significantly reduced degradation due to acoustic loss. A model-wise comparison of stacks with metal and oxide electrodes demonstrates the strong benefit of all-oxide varactors, the possibility of complete suppression of acoustic resonances.

In this work, we present a liquid crystal based fully dielectric phased array for beamsteering at W-Band. The array consists of 1 x 4 tapered dielectric rod antennas, including liquid crystal phase shifters and a single multimode interference power divider as feeding network. With this approach, a fully dielectric beamsteering antenna is realized in a very lightweight, compact and low-loss design. Each rectangular phase shifter has a tapered dielectric rod antenna at its end. For beamsteering, dielectric subwavelength phase shifters are filled with a novel liquid crystal mixture. The realized demonstrator achieved a maximum scanning range of ± 10°. Moreover, the measured antenna gain is ranging between 13dB to 14dB with a side lobe level below – dB. The input reflection coefficient is below –10 dB over the whole W-band.

In this work, multimode interference is investigated for the design of a two-dimensional fully dielectric power divider, well suited for the usage of dielectric waveguides. Most important, power division is achieved in a single device without the need of cascading multiple dividers. This allows to design a very compact and lightweight power divider, well applicable for dielectric rod antenna arrays. As a proof of concept for the used technique, a 16-way power divider with 4 × 4 output ports, made out of Rexolite, is realized, working in a frequency range between 90 and 105 GHz. For the S-parameter measurements, a special measurement setup, including a modular pin probe technique as well as radiation taper for waveguide termination, is proposed. The measurements are in good agreement with the simulations with a power split of − 15 dB for all output ports within the desired frequency range. This is equal to an additional insertion loss of 3 dB. To demonstrate the usability for antenna arrays, a fully dielectric rod antenna array is realized based on the proposed power divider. With this array, a gain of 22.5 dBi at 97.5 GHz was achieved.

This work addresses the piezoelectric induced reduction of quality factor in fully-printed metal-insulator-metal (MIM) barium strontium titanate (BST) thick film varactors designed for high power operation. An acoustically optimized varactor design is presented and compared to a non-optimized high-power varactor. The design is utilized to present a narrowband acoustic suppression technique based on defined weights. The acoustically optimized varactor consists of 162 varactor cells in a capacitive matrix. The cells in the matrix are interconnectable allowing for a variable unbiased capacitance and power rating. Due to this setup, surface acoustic waves are interrupted, and the reduced size of the cells allows for a reduced BST layer thickness, shifting the acoustic resonance away from the operational frequency. Therefore, an inverted behavior in comparison to the high-power varactor is observed with an increasing quality factor with biasing voltage. Compared to the high-power varactor, the acoustically optimized varactor design shows a 40% increased quality factor in biased state. By applying the narrowband acoustic suppression technique, an increase in quality factor of 145% is achieved compared to the unsuppressed design. In comparison to the high-power varactor, the acoustical suppressed design shows an increase in quality factor of 480% at the first acoustic resonance frequency.

This study presents the first continuously tuneable, interference-based W-band single-pole double-throw (SPDT), fully implemented in subwavelength (sWL) dielectric fibre topology. Key components are the phase shifters in the two branches of the SPDT, which are realised as line sections filled with liquid crystal. They allow not only to adjust the signal ratios at the two output ports but also to tune the operating frequency between 93 and 110 GHz with isolation >25 dB. The first proof-of-concept SPDT demonstrator is realised and characterised at W-band. However, concept and technology are scalable to higher frequencies. The complete SPDT with the WR10-to-sWL fibre transitions exhibits an insertion loss (IL) at the thru-port between 5 and 7 dB in the frequency range from 85 to 110 GHz, where back-to-back measurements of the transitions itself already indicate an IL of up to 1.6 dB. The measurements at the isolated port of the SPDT exhibit transmission coefficients of less than -25 dB between 93 and 110 GHz with a minimum of -40 dB.

A fast dielectric characterization method is on-wafer probing of circular, capacitive test structures. However, this was found to be accurate only for limited frequencies in the megahertz range. At higher frequencies, the measured impedance data deviate from the expected characteristic. The effective capacitance shows a negative slope, although it should increase up to infinity at electrical resonance. This prevents from extracting accurate material data at RF frequencies. The modeling of this phenomenon as an appealing negative series inductance is presented, as well as the robustness of its compensation. The resulting wide-frequency fitting additionally enables the characterization of the bottom electrode material. This compensation is applied to measurements of the recently presented all-oxide varactor based on the conducting oxide SrMoO3 (SMO). Previously, only independent conductivity measurements for nonintegrated SMO thin films were available. This compensation now allows for a valid confirmation of the low expected resistivity $45~\mu \Omega \text {cm}$ of integrated thin films.

This paper presents a comparison and performance evaluation of three different technologies for the implementation of tunable filters utilizing semiconductor varactors, commercially available barium strontium titanate (BST) thin film and screen-printed BST thick-film varactors. To identify the most suitable technology for applications in low power receiving up to high-power transmitting stages, the performance evaluation is carried out with the same tunable hairpin filter design. While the center frequency of the proposed tunable filter structure is tuned by varactors loading the filter resonators, the bandwidth is controlled by coupling varactors between adjacent resonators. The filters are designed for a center frequency range from 700 MHz to 1 GHz and for bandwidth tuning from 60 to 150 MHz. The tunable filters are evaluated and compared with regard to their tunability, quality factor, linearity (intermodulation products of third order), and power handling capability. In conclusion, the tunable hairpin filter based on semiconductor varactor diodes offers the largest center frequency tuning range of 390 MHz with a transmission between −1.5 and −7 dB, whereas the BST thin- and thick-film-varactors-based filters exhibit high linearity with an IIP3 between 39 and 60 dBm. Moreover, large signal characterization reveals that the BST thick-film-varactors-based filter structure has the best power handling capability of up to 41dBm.