Dielectric resonator antennas have attracted antenna designers for decades because they combine several properties that conventional metal patches struggle to match. Because the radiating element is a dielectric rather than a conductor, they suffer essentially no conductor loss at high frequencies, which translates into high radiation efficiency and useful gain. They are also mechanically and thermally stable, compact, and easy to integrate with printed circuits. Cylindrical resonators in particular carry a long pedigree: they were used in filters and oscillators well before researchers exploited them as radiators, and the first experimental study of a cylindrical DRA was carried out by S.A. Long in 1983. Compared with a hemispherical resonator, the cylinder gives the designer one extra degree of freedom, since the aspect ratio between radius and height can be tuned to hit a desired frequency and bandwidth.

The cylindrical geometry offers a further advantage that is central to the new design. A CDRA supports transverse electric (TE), transverse magnetic (TM) and hybrid electromagnetic (HEM) resonant modes, giving access to a wide variety of radiation patterns. HEM modes, the workhorses of cylindrical DRAs, are neither purely electric nor purely magnetic: their fields are transverse to the direction of wave propagation, and unlike TE or TM modes they do not require perfect conducting boundaries to exist, which allows efficient radiation into free space. The resonance frequency of an HEM mode depends on the height and radius of the cylinder, so geometry directly sets the spectrum. Coupling a centre-fed microstrip line to the cylinder excites the HEM11δ, HEM12d and HEM13d modes, where the indices describe azimuthal, radial and axial field variations. The fundamental HEM11δ behaves like a horizontal magnetic dipole, producing broadside radiation with a wide beam, while the two higher-order modes contribute higher directive gain.

Achieving multiband behaviour, however, has historically forced designers into compromises. Stacked resonators and parasitic-loaded structures increase the overall antenna volume; elaborate feed geometries and slotted ground planes raise fabrication complexity; designs using two or three resonator elements consume more area or height. A hybrid CDRA with a dual ‘C’-shaped patch, for example, lacks compactness because of its large volume, while two-segment cylindrical DRAs achieve triple-band operation only by stacking one resonator on top of another. The new study sidesteps these problems with two deceptively simple moves: a carefully placed notch in the resonator and a modified feed.

The resonator itself is a cylinder of Rogers RT Duroid 6010, a ceramic-loaded laminate with a moderate dielectric constant of 10.2, having a radius of 10 millimetres and a height of 2.5 millimetres. It is bonded to an FR4 substrate 1.6 millimetres thick with a dielectric constant of 4.4, over a ground plane measuring 35 by 24 millimetres. The decisive modification is the removal of a cylindrical volume of 18 by 6.75 by 2.5 millimetres from the centre of the disc. This central notch lowers the effective dielectric constant of the whole volume and reduces the Q-factor, which widens the bandwidth. It also transforms the outline of the resonator into a shape the author describes as resembling a pair of headphones. Without the notch, the same antenna on the same feed resonates only once, at 6.15 GHz with a narrow 6.5 percent bandwidth. With the notch in place, the structure resonates at three frequencies: 5.915 GHz, 10.76 GHz and 12.45 GHz, with fractional bandwidths of 18.57 percent, 9.9 percent and 15.8 percent respectively, and a return loss as deep as −32.7 dB, indicating strong impedance matching.

The dimensions of the notch proved critical. In a parametric study conducted with the HFSS 14 simulation software, the separation between the two arms of the headphone shape was varied between 5, 6.5 and 8 millimetres; the desired bands appeared only at 6.5 millimetres, with weaker bandwidth on either side. Similarly, notch lengths of 15, 18 and 19 millimetres were compared, and only the 18-millimetre version delivered both wide bandwidth and a good impedance match. The feed underwent its own evolution. A centre-fed microstrip line alone produced no resonance in the target bands. Adding a single stub yielded two bands, at 6.1 GHz and 13.1 GHz with bandwidths of 6 percent and 5.7 percent. Only when a second stub was added did the structure deliver three well-matched bands with the full bandwidths reported. The resulting flipped ‘F’-type feed, placed on the substrate beneath the resonator, acts as a frequency tuner: adjusting the lengths and widths of its two stubs optimises the coupling between feed and resonator and broadens each band. Microstrip feeding was chosen over alternatives because it is simple to fabricate, offers wider bandwidth than aperture coupling, and, unlike probe feeding, requires no drilling.

To confirm the simulations, the headphone-shaped antenna was fabricated from a slab of the Duroid material, machined into shape and bonded to the substrate with Araldite adhesive chosen for its very low dielectric constant, so that it would not perturb the results. Measurements were made with a PNA E8362B network analyser, and the simulated and measured return loss curves were found to be in very close agreement. The fabricated antenna resonated at 5.915 GHz, 10.76 GHz and 12.45 GHz, matching the predicted bands and covering WLAN frequencies along with the X and Ku bands. HEM modes were observed to be excited at all three resonances, confirming the mode analysis behind the design.

Radiation performance held up as well. Patterns measured in an anechoic chamber showed linear polarisation and stable broadside radiation across the entire operating range, with co-polarisation levels consistently above cross-polarisation in both the E-plane and the H-plane. Measured gain ranged from 4.5 dBi to a peak of 7.05 dBi at 10.88 GHz, with a horn antenna serving as the reference in the gain measurement. Radiation efficiency was evaluated using the Wheeler method, which compares the antenna’s reflection coefficient measured in free space with that measured in the chamber and requires no reference antenna. The theory predicted better than 87.5 percent efficiency, and the measurements confirmed more than 85 percent across all operating frequencies.

Compared with recently published multiband DRAs, the new design stands out for its profile. Earlier triple-band cylindrical designs reached 8, 9.6 or even 10 millimetres in height, and one rectangular DRA required 19.6 millimetres, whereas the proposed antenna achieves comparable or wider bandwidths in its lower and upper bands at only 2.5 millimetres thick, without stacking layers. Because it operates in both the C and X bands, the author notes it could serve as a satellite antenna and is well suited to air traffic control, weather monitoring, vehicle speed detection, defence tracking and radar applications. With a simple geometry, an easy feed mechanism, strong agreement between simulation and measurement, and good gain and efficiency throughout its bands, the headphone-shaped CDRA offers a compact route to the multiband operation that modern wireless systems increasingly demand.

Subject of Research: Multiband cylindrical dielectric resonator antenna design with modified microstrip feeding

Article Title: The multiband cylindrical dielectric resonator antenna with modified microstrip feeding

Article References: Vinodha, E. (2026). The multiband cylindrical dielectric resonator antenna with modified microstrip feeding. Frontiers in Antennas and Propagation, 4, Article 1850686. https://doi.org/10.3389/fanpr.2026.1850686

Image Credits: AI Generated

DOI: 10.3389/fanpr.2026.1850686

Keywords: dielectric resonator antenna, CDRA, multiband antenna, microstrip feed, HEM modes, WLAN, X band, Ku band, radiation efficiency, bandwidth enhancement, HFSS simulation, radar applications

The study, published in Frontiers in Antennas and Propagation by Hatice-Andreea Topal and Teodor Lucian Grigorie, is notable less for any single record-breaking number and more for its methodology. Rather than analyzing one antenna in one simulator, the team designed both structures analytically using classical horn theory implemented in Matlab, then validated each design in two independent electromagnetic environments, Ansys HFSS and CST Studio Suite. This cross-platform approach is a kind of computational sanity check that many antenna papers skip, and it paid off: the two simulators produced results in close agreement, with only minor differences attributable to their fundamentally different numerical engines.

The pyramidal horn, fed by a standard WR-90 rectangular waveguide with interior dimensions of 22.86 by 10.16 millimeters, was designed around a target gain of 22.70 decibels at a central frequency of 10 gigahertz. The aperture measures 178.5 by 145.8 millimeters, with an axial flare length of roughly 323 millimeters. In simulation, the antenna exceeded expectations, delivering a maximum gain of 23.63 decibels with an extraordinarily narrow half-power beamwidth of between 6 and 10 degrees. The reflection coefficient, a measure of how much signal bounces back toward the transmitter instead of radiating into space, dipped as low as minus 39.43 decibels at 8.22 gigahertz in HFSS and an impressive minus 56.18 decibels at 8.07 gigahertz in CST, both comfortably below the minus 10 decibel threshold that defines good impedance matching.

Those numbers matter because a well-matched antenna wastes almost none of the precious power an amplifier sends its way. The Smith chart analysis from the simulations showed the input impedance hugging the ideal 50-ohm target, landing near 50 plus or minus 3 ohms with negligible reactance, which corresponds to a voltage standing wave ratio of approximately 1.05. In practical terms, the pyramidal horn is a highly efficient, laser-focused energy projector, ideal for applications demanding pinpoint directivity and clean linear polarization.

The conical horn tells a different story. Fed by a BY89 circular waveguide 23.825 millimeters in diameter, with a dominant TE11 mode cutoff frequency of 8.67 gigahertz, the cone achieved a maximum gain of 16.43 decibels and a much wider beamwidth of about 30 degrees. Its reflection coefficient stayed below minus 10 decibels across the entire 9-to-12-gigahertz operating range, bottoming out at minus 43 decibels at 11.22 gigahertz in HFSS. That wider, symmetric beam is not a weakness; it is exactly what a reflector feed needs to illuminate a parabolic dish uniformly without spilling energy over the edges.

The real magic of the cone, however, is polarization. By exciting two orthogonal TE11 modes with equal amplitudes and a 90-degree phase difference through a bimodal wave port, the researchers generated circular polarization with an axial ratio below 3 decibels across the main lobe. This is a decisive advantage in satellite communications, where circularly polarized signals tolerate the orientation mismatch that inevitably arises between a moving spacecraft and a ground antenna. The conical design achieved circular polarization discrimination of 57 to 62 decibels on the principal axis, approaching the performance of the corrugated horns flown on operational satellites, yet without any corrugations or added polarizer hardware.

The comparative metrics sharpen the choice further. The conical horn posted an aperture efficiency of roughly 74 percent against about 64 percent for the pyramid, and its slightly higher radiation efficiency, about 96 percent versus 94 percent, follows from its smaller inner surface area and thus lower ohmic loss. Both exceed the 90 percent mark comfortably. In the satellite link budget, a 10 percent aperture efficiency improvement can translate into an additional half to one decibel of system gain, a meaningful margin when every watt counts. The pyramidal horn still dominates where linear polarization and maximum directivity are the goals, with a cross-polarization discrimination of about 62 decibels, far beyond the 30-decibel minimum recommended for satellite systems.

The dual-simulator exercise also yielded instructive engineering lessons about the tools themselves. Ansys HFSS, built on the frequency-domain finite element method with an adaptive tetrahedral mesh, consumed roughly 15 gigabytes of memory and delivered the highest accuracy on curved geometries. CST Studio Suite, using the time-domain finite integration technique with a hexahedral mesh, required only about 1.5 gigabytes and produced broadband results in a single run, though its Fourier-transform-based frequency extraction introduced a shift of roughly 1 gigahertz in the location of the reflection coefficient minimum. A gain discrepancy of 1.63 decibels between platforms for the conical horn fell squarely within the sub-2-decibel spread documented in prior comparative studies of the two solvers.

Beyond the immediate results, the study lays deliberate groundwork for what comes next. The smooth-wall conical horn is the direct geometric ancestor of the conical corrugated horn, the gold-standard feed for satellite reflector systems thanks to its symmetric radiation pattern and minimal cross-polarization. The researchers, working within a broader doctoral project, plan to add corrugations to the validated cone, optimize the corrugation depth and pitch to suppress surface currents, and fabricate a prototype for measurement. The present results serve as the reference baseline against which that upgraded design will be judged.

In an era when mega-constellations and flexible geostationary spacecraft are pushing antenna engineering to new heights, there is something almost reassuring about a study that goes back to fundamentals. The horn antenna, first sketched in the late nineteenth century and battle-tested through radar, radio astronomy, and every satellite generation since, still responds beautifully to careful analytical design and rigorous simulation. What this comparison shows is that the choice between pyramid and cone is not about which antenna is better, but about which task you need done, and for the circularly polarized world of satellite communications, the cone remains the shape of the future.

Subject of Research: Comparative electromagnetic design and simulation of pyramidal and conical horn antennas for X-band satellite communication systems

Article Title: Design and electromagnetic analysis of pyramidal and conical horn antenna for X-band satellite communication systems

Article References: Topal, H.-A., & Grigorie, T. L. (2026). Design and electromagnetic analysis of pyramidal and conical horn antenna for X-band satellite communication systems. Frontiers in Antennas and Propagation, 4, Article 1828476. https://doi.org/10.3389/fanpr.2026.1828476

Image Credits: AI Generated

DOI: 10.3389/fanpr.2026.1828476

Keywords: horn antenna, X-band, satellite communication, pyramidal horn, conical horn, circular polarization, waveguide, Ansys HFSS, CST Studio Suite, reflection coefficient, gain, reflector feed

Quantum illumination is one of the most counterintuitive ideas in quantum sensing. The transmitter prepares pairs of photons in a two-mode squeezed vacuum state, a fragile form of entanglement linking a signal mode and an idler mode. The signal is sent toward the region where a target may be lurking, while the idler is retained in the receiver. When the returned signal is recombined with the idler, the residual correlations between the two, even after the entanglement itself has been all but destroyed by loss and thermal noise, allow the receiver to decide between target present and target absent with a lower error probability than any classical strategy using the same transmitted energy. Crucially, the advantage is largest precisely in the regime that matters for plasma-shrouded targets: high loss, high thermal background, and vanishingly faint returned signals. That makes the scheme a natural candidate for sensing through a sheath, which recent modeling of terahertz and millimeter-wave propagation shows is simultaneously lossy and thermally bright, the worst possible combination for classical detection.

To quantify the advantage rigorously, the study, authored by Sai Krishna Thota, turns to the quantum Chernoff bound, the gold-standard figure of merit for discriminating two quantum states. For M independent copies of the transmitted probe, the probability of a detection error is bounded by one half times the exponential of minus M times the Chernoff exponent, so a larger exponent means errors are suppressed exponentially faster as more copies are sent. For the Gaussian states that describe microwave-frequency squeezed light, the exponent can be computed from the covariance matrices of the two hypotheses, target present and target absent, via a mathematical procedure called the Williamson symplectic decomposition. The plasma sheath enters this machinery through a single number: the round-trip transmissivity, computed from the classic Appleton-Hartree description of an unmagnetized, collisional plasma for a five-centimeter-thick sheath at an X-band carrier frequency of 10 gigahertz, the center of the band used by many operational radars.

The physical picture that emerges is one of two competing effects. As the electron density in the sheath rises from a tenuous 10^16 per cubic meter toward the blackout regime, the plasma frequency climbs toward the carrier frequency, and the round-trip transmissivity collapses from essentially unity to a floor of 10^-6. At the same time, the sheath radiates as a thermal body, raising the background photon number that the receiver must fight against. The target itself is deliberately pessimistic, reflecting only two percent of the light that reaches it, so even in a perfectly transparent sheath roughly ninety-eight percent of the transmitted photons are lost before they return. The study sweeps electron densities over four orders of magnitude and collision frequencies typical of re-entry conditions, mapping exactly where the quantum advantage lives and where it dies.

The headline result is strikingly clean. In the transparent regime, where the sheath barely attenuates the signal, the exact quantum Chernoff exponent computed in truncated Fock space is 4.07 times 10^-4, against 1.98 times 10^-4 for the best classical illumination scheme using coherent states, a ratio of just over 3.1 decibels. That quantum edge holds nearly constant, drifting only from 3.13 to 3.20 decibels, as the electron density rises by two full orders of magnitude and the transmissivity drops to 0.737. The advantage is smaller than the idealized 6-decibel limit familiar from textbook treatments because the signal brightness, the background, and the reflectivity are all finite, but it is remarkably robust. The plasma sheath, in other words, does not selectively destroy the entanglement-based edge while the channel remains open; quantum and classical schemes degrade together, and the quantum one stays roughly twice as good in error exponent terms.

Then comes the cliff. Near an electron density of 1.2 times 10^18 per cubic meter, the plasma frequency reaches the 10-gigahertz carrier and the X-band channel effectively closes. Past this cutoff, both exponents plummet by five to six orders of magnitude, and deeper in the blackout regime the advantage does something remarkable: it inverts. At an electron density of 2.51 times 10^18 per cubic meter, with transmissivity pinned at the 10^-6 floor, the classical exponent of 4.83 times 10^-10 actually exceeds the quantum exponent of 3.77 times 10^-10, a deficit of 1.08 decibels. The practical message is unambiguous. Quantum illumination buys you a genuine, quantifiable edge through a transparent or moderately attenuating sheath, but once the sheath crosses the plasma cutoff for your carrier frequency, no amount of entanglement rescues the link, and the fallback must be operational, such as shifting to a higher carrier frequency whose cutoff lies at a higher electron density.

One of the most valuable contributions of the work is its honesty about numerical limits. Computing exact Chernoff exponents for entangled two-mode states against a thermal background requires truncating the photon-number Hilbert space, and the thermal photon-number tail decays only as a power set by the background brightness. At a background of one thermal photon per mode, a cutoff of fourteen photon numbers converges to better than one percent accuracy, but at a background of forty photons the same accuracy would demand photon numbers beyond 190 and matrix operations scaling with the cube of a dimension of order a million, which is prohibitive on standard hardware. The author therefore validates the Gaussian approximation against exact Fock-space results at the single background where exact computation converges, and finds that the Gaussian bound systematically underestimates the true entangled-state exponent by a factor of 3.7 to 5.0, with the discrepancy shrinking as the background grows. The bias keeps the correct sign and preserves the physical scaling, so the Gaussian value serves as a correctly ordered proxy rather than an exact prediction, a caveat the paper states plainly rather than burying.

The study also builds a machine-learning surrogate designed to amortize the repeated Chernoff evaluations that a full mission analysis or a waveform optimizer would demand. The architecture is a four-layer residual multilayer perceptron with SiLU activations, trained on one hundred thousand samples across five random seeds, that predicts a correction to an analytic scaling prior rather than the exponent from scratch, with a soft gradient penalty enforcing the known dependence of the exponent on signal brightness, reflectivity, transmissivity, and background. The physics regularization pays off modestly in-distribution, delivering a median relative error of 16.7 percent against 21.0 percent for a plain network of identical capacity, and dramatically against a simpler physics-prior-plus-ridge baseline that errs by nearly two hundred percent. More importantly, the author holds out the top decile of electron densities as an out-of-distribution blackout test and reports the failure openly: error balloons to 57.8 percent median in that regime, so the surrogate is explicitly not trusted past the blackout gate, where the analytic expression is evaluated directly instead.

There are, of course, caveats between this analysis and a flying quantum radar. The per-copy exponents involved are tiny, on the order of 4 times 10^-4 even through a transparent sheath, meaning that a large number of transmitted copies is still required to push the error probability down to useful levels, and the two-percent target reflectivity already imposes a brutal loss budget before the sheath contributes anything. The model treats the sheath as a static, unmagnetized, uniform slab, whereas real re-entry plasmas are turbulent, magnetized, and rapidly varying. No receiver architecture, microwave quantum memory, or practical entanglement source is specified. Yet the paper’s central finding stands on exact computation rather than approximation: the quantum-illumination advantage is not an exotic curiosity that evaporates at the first sign of a hostile channel. It survives a lossy, thermally bright plasma sheath essentially intact, at roughly 3 decibels of error-exponent advantage, right up to the plasma cutoff where the physics of the sheath itself, not the fragility of entanglement, closes the door. For engineers contemplating sensing through re-entry blackout, that is a genuinely useful map of where the quantum edge begins, and where it ends.

Subject of Research: Quantum illumination sensing through a hypersonic plasma sheath analyzed via quantum Chernoff bounds

Article Title: Physics-regularized Chernoff-bound analysis of the quantum-illumination advantage through a plasma sheath

Article References: Thota, S. K. (2026). Physics-regularized Chernoff-bound analysis of the quantum-illumination advantage through a plasma sheath. Results in Optics, Article 101184. https://doi.org/10.1016/j.rio.2026.101184

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101184

Keywords: quantum illumination, plasma sheath, quantum Chernoff bound, radio-frequency blackout, hypersonic vehicles, quantum radar, two-mode squeezed vacuum, X-band, Appleton-Hartree, physics-informed neural network, Gaussian states, entanglement

The research team, led by Lan-Ping Qian and Ti-Lin Fang with colleagues including Xiuli Du, focused on geopolymer, a low-carbon cementitious material formed by activating aluminosilicate-rich industrial residues with an alkaline solution. Geopolymers already attract attention because they can slash the carbon emissions associated with conventional cement, and their amorphous three-dimensional aluminosilicate network provides a dielectric environment in which electromagnetic energy can be dissipated. But the field faces a raw-material bottleneck. Most geopolymer shielding studies rely on fly ash from coal-fired power plants and ground granulated blast-furnace slag, and as China’s energy structure shifts away from coal, high-quality fly ash is becoming scarcer. The researchers turned instead to two of the world’s most problematic waste streams: red mud and ore tailings.

The scale of these waste streams is enormous. Red mud, the highly alkaline residue left after alumina is extracted from bauxite, accumulates globally at a rate of roughly 60 to 120 million tons per year, with stockpiles exceeding 4.1 billion tons. Ore tailings, the ground rock left after metals such as copper are extracted, are generated at approximately 1.4 billion tons annually. Both materials are rich in iron-bearing and other metal-containing phases that can promote dielectric loss and interfacial polarization, giving them a latent electromagnetic function, and both contain active aluminosilicate components suitable for geopolymer synthesis. In the new work, red mud and two types of ore tailings, one from a Qinghai mining operation and one from Xinjiang, replaced 20, 40, and 60 percent of the fly ash in a fly ash and slag geopolymer matrix, with carbon fibers held constant at 0.5 percent by mass to provide a baseline conductive framework.

The mechanical results revealed sharply divergent roles for the two wastes. Red mud consistently undermined strength: at 60 percent fly ash replacement, the 28-day compressive strength dropped 49 percent relative to the control, from a robust baseline down to just 23.3 megapascals. X-ray diffraction explained why. Red mud is dominated by crystalline hematite and cancrinite, and these inert crystalline phases appear to hinder the dissolution and polycondensation reactions of the active aluminosilicates that build the binding gel. The ore tailings behaved in exactly the opposite fashion. Both tailings types raised compressive strength above the control at every replacement level, with the 60 percent replacement of the first tailings reaching 68.0 megapascals, the highest value among the fiber-free mixes. The tailings had been ball-milled before use, and their fine particles acted as micro-aggregates, filling pores and densifying the matrix. Nitrogen adsorption analysis confirmed that tailings substitution increased the proportion of harmless pores below 20 nanometers and reduced harmful pores above 50 nanometers, a pore refinement that translated directly into mechanical gains.

Electrical measurements underscored the complementary role of each component. The plain geopolymer control had a 28-day alternating-current resistivity of 426 ohm-centimeters, but substituting red mud or tailings for fly ash lowered that value moderately, because the alkaline red mud supplies abundant hydroxyl ions and iron oxide while the milled tailings release soluble calcium, sodium, and hydroxide ions and refine the pore network into connected capillary pathways. Adding 0.5 percent carbon fibers then produced a dramatic step change, cutting resistivity by roughly an order of magnitude across all systems. The lowest values reached 12.7 ohm-centimeters for the first tailings and 10.6 ohm-centimeters for the second at 60 percent replacement, 40.9 and 50.7 percent below the fiber-only control, evidence that a more effective conductive network had formed when the waste-modified matrix and the fibers worked together.

Electromagnetic shielding tests in the X-band, the 8.2 to 12.4 gigahertz range used by aviation, military, and weather radar, showed how these electrical changes translated into functional performance. Without carbon fibers, the plain geopolymer achieved a total shielding effectiveness of about 15 decibels, and replacing fly ash with the wastes raised the peak value by as much as 6 decibels, up to 22.7 decibels for the first tailings at 12.4 gigahertz. The mechanism was subtle and counterintuitive: the moderate conductivity increase improved impedance matching, allowing more electromagnetic waves to penetrate the material’s surface rather than bounce off it, so the waves could be absorbed and dissipated inside. Transmittance in the first-tailings matrix fell below 2.3 percent, and absorption rather than reflection became the dominant shielding pathway.

With carbon fibers present, the physics shifted again. The fibers established continuous conductive pathways, triggering a strong skin effect in which incident waves concentrate near the highly conductive surface and are largely reflected, dropping transmittance below 0.1 percent, while penetrating waves were dissipated through conduction loss, boosting the absorption component by roughly 14 decibels. Against this fixed fiber framework, the waste modifiers fine-tuned the balance between reflection and absorption. Red mud acted as an absorption-favorable modifier: the 60 percent red mud composite with fibers maintained a total shielding effectiveness above 25 decibels, peaked at 30.5 decibels at 8.2 gigahertz, and showed roughly 10 percent higher absorptance than the fiber-only control, though at the cost of the lowest compressive strength, 26.6 megapascals, among the fiber-reinforced mixes. The two tailings delivered the best all-around results. The first tailings at 60 percent replacement reached a measured peak shielding effectiveness of 32.2 decibels at 11.92 gigahertz with transmittance below 0.09 percent, while the second tailings achieved the study’s maximum of 33.5 decibels at 8.2 gigahertz and held transmittance below 0.06 percent.

Those numbers place the material in impressive company. At only 5 millimeters thick and containing just 0.5 percent carbon fiber, the composite achieves a thickness-normalized shielding effectiveness of 6.7 decibels per millimeter, higher than geopolymer systems loaded with 3.75 percent cork, 60 percent slag, or silica-coated carbon nanotubes, and it approaches the performance of composites that require five to ten times more carbon fiber. Mechanistic analysis using Cole-Cole plots revealed Debye-type relaxation behavior arising from interfacial polarization at the many heterogeneous boundaries between unreacted particles, gel matrix, and carbon fibers, while the frequency dependence of the imaginary permittivity confirmed that conduction loss, driven by the fiber network, was a major dissipation channel. Magnetic loss, by contrast, proved negligible across all formulations, since permeability values remained close to unity.

Microstructural imaging tied the story together. Scanning electron microscopy showed carbon fibers embedded snugly in the matrix without large interfacial gaps, bridging microcracks and suppressing their propagation, which explains the modest strength enhancement the fibers provided. Backscattered electron mapping revealed interwoven calcium-rich gels around slag particles and aluminum-silicon-rich gels around fly ash, with iron concentrated in discrete spots inherited from the waste precursors, confirming that the wastes were not inert passengers but active participants reshaping the gel chemistry and pore architecture. The authors are appropriately cautious, noting that because only one independently prepared specimen was evaluated for each mixture in the electromagnetic tests, differences of one to two decibels should be read as comparative trends rather than statistically validated improvements, and that small-specimen waveguide measurements are not fully equivalent to the shielding of full-scale wall panels.

Even with those caveats, the study’s implications are considerable. It demonstrates that red mud and ore tailings are not merely substitutes for dwindling fly ash but genuine matrix modifiers that can be chosen according to need: tailings when strength and balanced shielding are priorities, red mud when absorption-dominant shielding matters more than mechanical performance. As electromagnetic pollution intensifies and the construction industry confronts its carbon problem, the idea that the walls of future buildings could simultaneously lock away billions of tons of hazardous waste, bear structural loads, and quietly absorb radar and wireless signals transforms two environmental liabilities into a single functional material. The next steps, the researchers suggest, involve exploring different fiber contents, specimen thicknesses, frequency bands, and long-term durability, moving this waste-to-shield concept from the waveguide bench toward the built environment.

Subject of Research: Electromagnetic shielding and compressive strength of carbon-fiber-reinforced geopolymer composites modified with red mud and ore tailings

Article Title: Tuning electromagnetic shielding and compressive strength of carbon-fiber-reinforced geopolymer using red mud and ore tailings

Article References: Qian, L.-P., Fang, T.-L., Xu, L.-Y., Shi, D.-D., Peng, K.-D., Li, Y., & Du, X. (2026). Tuning electromagnetic shielding and compressive strength of carbon-fiber-reinforced geopolymer using red mud and ore tailings. Case Studies in Construction Materials, 25, Article e06599. https://doi.org/10.1016/j.cscm.2026.e06599

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06599

Keywords: geopolymer, electromagnetic shielding, red mud, ore tailings, carbon fiber, fly ash, compressive strength, dielectric loss, impedance matching, industrial solid waste, X-band, sustainable construction

The material, developed by Zhi Lei, Daohai Zhang and colleagues, is built from two inexpensive and environmentally friendly ingredients: polyvinyl alcohol, a widely used water-soluble polymer, and gelatin, a natural biomass-derived protein. The researchers combined them into a semi-interpenetrating network in which the two polymer chains entangle during gelation and are further crosslinked by hydrogen bonds between abundant amino and hydroxyl groups. That molecular architecture produces a three-dimensional porous scaffold with high water content, and the same hydrophilic functional groups allow the gel to adhere to skin, paper, glass and metal through hydrogen bonding and intermolecular forces. Into this matrix the team dispersed composite nanoparticles made of carbon nanotubes decorated with ZIF-67, a cobalt-based metal-organic framework in which cobalt ions are coordinated by 2-methylimidazole ligands into a highly ordered porous crystal.

Each component of the hybrid filler plays a distinct role. Carbon nanotubes are lightweight and highly electrically conductive, so they raise the conductivity of the hydrogel dramatically, from 0.34 siemens per meter for the pristine polymer blend to 0.62 siemens per meter, an increase of 82.35 percent. ZIF-67, meanwhile, contributes cobalt centers that absorb electromagnetic waves, giving the composite dual loss mechanisms: dielectric loss from the conductive carbon network and magnetic loss from the cobalt ions. Together, the two phases dissipate incoming electromagnetic energy as heat rather than letting it pass through or bounce off. The porous hydrogel scaffold adds a third layer of defense, because its three-dimensional architecture provides anchoring sites for the nanoparticles and forces penetrating waves to undergo multiple internal reflections, each of which bleeds away more energy.

The synthesis relied on three complementary techniques. First, an in situ growth method deposited rhombic dodecahedral ZIF-67 crystals directly onto the surfaces of carboxylated multi-walled carbon nanotubes, with cobalt nitrate and nanotubes dispersed in methanol at controlled mass ratios and then added to a 2-methylimidazole solution under stirring for 24 hours. Second, a one-pot method dispersed the resulting CNTs@ZIF-67 particles uniformly into the aqueous PVA-gelatin precursor under ultrasonication. Third, cyclic freeze-thaw treatment physically crosslinked the polymers into a stable gel without any toxic chemical crosslinkers. Scanning electron microscopy confirmed that ZIF-67 had grown on the nanotubes and revealed how the fillers reshaped the gel’s internal pores.

Perhaps the most intriguing structural finding concerns electrostatics. When ZIF-67 alone was added, cobalt ions attracted electron-rich groups on the polymer chains, causing chain segments to bunch together and collapsing the gel’s three-dimensional network into a smoother, denser morphology. Carbon nanotubes alone infiltrated the pores and shrank them. But the combined CNTs@ZIF-67 particles produced hydrogels with more numerous and larger pores than either single filler. The researchers attribute this to an electrostatic confinement effect: oxygen anions on the nanotube surfaces neutralize the positive charge of the cobalt ions, weakening the electrostatic interactions between the composite and the polymer matrix and preserving the porous architecture that is so important for wave attenuation.

Spectroscopic analysis backed up the picture. X-ray diffraction peaks of the synthesized ZIF-67 matched the standard reference pattern at all expected crystal planes, indicating a complete lattice, while the hydrogels showed narrowed diffraction peaks consistent with grain-size changes induced by the cobalt-containing filler. Fourier transform infrared spectra showed no new functional groups after doping, meaning the nanoparticles disperse physically within the polymer framework rather than forming new chemical bonds. X-ray photoelectron spectroscopy, however, revealed subtle electronic interactions: the cobalt 2p binding energies shifted upward in the doped gels, suggesting that hydroxyl and carboxyl groups coordinate with cobalt ions and redistribute electron density at the interface.

Mechanically, the composite gel proved remarkably robust. The pristine PVA-gelatin hydrogel withstood compressive stresses of 18.75 megapascals at 71.07 percent strain; the shielding version reached 19.67 megapascals at 66.33 percent strain, with its compressive modulus rising from 38.1 to 41.8 kilopascals as the fillers constrained chain mobility and increased crosslinking density. A cylinder of the composite hydrogel just half a centimeter in diameter could lift a 200-gram weight without fracturing, and the material stayed firmly attached to paper, iron rods and glass rods for extended periods. Differential scanning calorimetry showed the additives did not disturb the gel’s crystallization behavior, and water content remained high at 88.99 percent, with water absorption of 265.92 percent, so the shielding function came without sacrificing the hydration and flexibility that make hydrogels attractive for wearables.

The electromagnetic results were the study’s centerpiece. Measured with a vector network analyzer across the X-band from 8.2 to 12.4 gigahertz, the plain hydrogel achieved an average shielding effectiveness of 14.70 decibels. The optimized composite, designated PGZC1, reached 33.32 plus or minus 1.67 decibels, a 126.7 percent improvement that comfortably exceeds the 20-decibel threshold generally considered effective shielding. Analysis of the reflection, absorption and transmission coefficients showed near-zero transmission and absorption dominating over reflection, confirming that the material neutralizes waves by soaking up their energy rather than bouncing them back toward other devices, a crucial distinction for crowded electronic environments. Conductivity of the optimized gel measured 0.512 plus or minus 0.026 siemens per meter.

Intriguingly, more nanotubes were not always better. When the researchers increased the carbon nanotube proportion in the CNTs@ZIF-67 filler, shielding effectiveness progressively declined. The explanation lies in agglomeration: nanotubes have enormous surface area and strong van der Waals attraction, so at higher loadings they clump together, disrupting the synergistic interplay between the carbon and the ZIF-67 phase and damaging the gel’s porous structure, which reduces the surface area and active sites available for wave scattering. The finding underscores a recurring lesson in composite design, that uniform dispersion at modest loadings can outperform brute-force addition of conductive filler.

The researchers propose a clear shielding mechanism that ties the observations together. The PVA-gelatin network forms the structural scaffold, while well-dispersed CNTs@ZIF-67 particles build a conductive network and help generate relatively independent, complete spherical pores. When a wave strikes the surface, part is reflected and part penetrates, losing energy as it crosses the conductive walls; reflected components bounce between wall layers, attenuating further with each pass. Because absorption dominates, the energy ultimately dissipates as heat within the material. The authors suggest the gel’s combination of high shielding, toughness, adhesion, water retention and processability positions it for flexible electronics, wearable devices and stretchable sensors, offering a green, low-cost route to protecting both devices and people from the electromagnetic clutter of modern technology.

Subject of Research: CNTs@ZIF-67 composite nanoparticles enhancing electromagnetic interference shielding in tough PVA-gelatin organic hydrogels

Article Title: CNTs@ZIF-67 conductive composite nanoparticles enhance the electromagnetic shielding performance of tough organic hydrogels

Article References: Lei, Z., Zhang, D., Hu, Y., Zhou, T., Diao, K., Liu, D., Song, A., Zou, K., & Qin, S. (2026). CNTs@ZIF-67 conductive composite nanoparticles enhance the electromagnetic shielding performance of tough organic hydrogels. iScience, 29(11), Article 114635. https://doi.org/10.1016/j.isci.2026.114635

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.114635

Keywords: electromagnetic interference shielding, hydrogel, carbon nanotubes, ZIF-67, metal-organic framework, polyvinyl alcohol, gelatin, X-band, wearable electronics, dielectric loss, magnetic loss, conductive composites

The work, carried out by Mert Karahan, Nazmiye Selvi, and Onur Battal of the Electronics and Communication Engineering Department at the Turkish Military Academy of the National Defence University in Ankara, introduces what the authors call the Optimized Reflector-Integrated CPW-Fed Antenna, abbreviated ORICPW-FA. The device is contained entirely within a standard 1U CubeSat structural frame, the basic 10-centimeter cube unit of the CubeSat standard, and is intended to fly as a communication payload on larger multi-unit spacecraft such as 3U and 6U platforms. Critically, it respects the 6.5-millimeter rail-protrusion limit that governs how far any component may extend beyond the satellite’s structural rails before launch.

The trade-off the researchers set out to resolve is a familiar one in small-satellite engineering. Fixed-profile antennas, which keep a constant shape throughout the mission, are mechanically robust but typically offer limited bandwidth, meaning they can only operate efficiently over a narrow slice of the radio spectrum. Deployable antennas, by contrast, can unfold into large reflecting structures that deliver high gain and wide bandwidth, but they introduce hinges, springs, and release mechanisms that add mass, cost, and above all risk. A deployment that fails leaves the mission mute, and every additional mechanism is another point of failure that must be qualified for the vibration, vacuum, and thermal extremes of launch and spaceflight.

ORICPW-FA sidesteps that dichotomy with a hybrid architecture. At its core is a coplanar waveguide-fed radiating element, a feeding technique in which the signal-carrying conductors lie in a single plane on the substrate, which simplifies fabrication and keeps the feed compact. That element sits inside a cavity-backed structure, a metallic enclosure that shapes the radiation pattern, suppresses unwanted backward radiation, and stabilizes performance across frequency. The novel ingredient is what happens behind the radiator: a motorized reflector whose height above the radiating element can be adjusted in flight. By moving this reflector, the antenna effectively reconfigures its own electromagnetic environment, shifting the frequency range over which its beam remains stable and well-formed.

The physics behind this tuning is rooted in how the reflector modifies the boundary conditions seen by the radiating element. The distance between a radiator and its backing reflector determines how the direct and reflected waves combine, which in turn shapes the antenna’s impedance, its gain, and the directionality of its beam. In a conventional design that distance is frozen at manufacture. In ORICPW-FA, a small motor adjusts it on command, allowing the spacecraft to sweep the reflector position until the antenna presents the best possible pattern at whatever frequency the mission requires at that moment. The result, according to the authors’ full-wave simulations, is a dramatic widening of the usable band: pattern-stable operation extends from 6.0 to 8.2 gigahertz in a fixed-reflector reference configuration to a continuous 6.0 to 9.4 gigahertz once the reflector height is tuned.

Those simulation results come with performance guarantees that matter to mission planners. Across the tuned range, the antenna maintains a maximum main-lobe deviation, the amount by which the beam’s pointing direction drifts, of no more than plus or minus three degrees, and a sidelobe level of at most minus twelve decibels. Main-lobe deviation matters because a communication beam that wanders off target wastes link margin and can drop contacts with ground stations, while high sidelobes scatter transmitted power into unwanted directions and make the satellite more susceptible to interference. Holding both parameters within tight bounds across a 3.4-gigahertz span, covering the C-band and much of the X-band, is what makes the concept attractive as a genuine payload rather than a laboratory curiosity.

To confirm that the simulated behavior survives contact with reality, the team built a laboratory prototype and measured it at three representative tuning states: 7.0, 7.8, and 8.6 gigahertz. The measurements relied on S11 reflection-coefficient tests, which characterize how efficiently the antenna accepts power from its feed, and on time-domain-gated radiation-pattern measurements, a technique that filters out reflections from the surrounding test environment so that the antenna’s true pattern can be isolated. The measured peak realized gains came in at 8.41, 7.78, and 7.58 decibels relative to an isotropic radiator at the three frequencies respectively, while the measured patterns showed main-lobe deviations between zero and three degrees and sidelobe levels of minus 17.6, minus 18.9, and minus 14.6 decibels.

Those numbers support the central claim of the paper: that a moving reflector inside a cavity can genuinely steer an antenna’s operating characteristics in flight, without any part of the structure needing to unfold beyond the satellite’s envelope. The authors are careful to delineate the boundaries of their validation. The laboratory campaign confirmed pattern behavior at three representative tuning states, but continuous full-band radiation-pattern validation and space-environment qualification, the punishing sequence of thermal-vacuum, vibration, and radiation tests that any flight hardware must endure, remain outside the scope of this study. That is a standard and honest caveat for an early-stage design, and it marks the path from a working prototype toward a qualified flight unit.

The broader context explains why the result resonates beyond a single antenna design. CubeSats have evolved from educational novelties into serious platforms for Earth observation, technology demonstration, and even deep-space missions, and their communication demands have grown accordingly. A 3U or 6U spacecraft carrying ORICPW-FA could, in principle, retune its downlink or uplink frequency in orbit to match different ground stations, adapt to changing mission phases, or dodge interference, all with a single antenna that occupies one cube unit and never deploys. The design also speaks to a growing trend in antenna engineering toward reconfigurability as an alternative to sheer aperture: rather than making the antenna bigger, make it smarter about where and how it radiates.

There are, of course, engineering questions that follow hardware into orbit. The motor and its control electronics introduce their own reliability considerations, and the long-term behavior of a moving mechanical assembly in vacuum will need to be demonstrated before the concept earns a place on a flight manifest. But the study’s core demonstration stands: a deployment-free, motorized, cavity-backed antenna that fits the strictest CubeSat form factor while spanning C- and X-band with stable, well-controlled beams. For mission designers weighing the risk of deployables against the limits of fixed antennas, ORICPW-FA offers a third option, one in which the antenna tunes itself instead of unfolding, and the satellite keeps talking no matter what frequency the mission demands.

Subject of Research: A deployment-free, motorized frequency-reconfigurable cavity-backed CPW-fed antenna for C- and X-band CubeSat communications

Article Title: A Deployment-Free, Motorized Frequency-Reconfigurable Cavity-Backed Antenna for C- and X-Band CubeSats

Article References: Karahan, M., Selvi, N., & Battal, O. (2026). A Deployment-Free, Motorized Frequency-Reconfigurable Cavity-Backed Antenna for C- and X-Band CubeSats. International Journal of Aeronautical and Space Sciences. https://doi.org/10.1007/s42405-026-01281-w

Image Credits: AI Generated

DOI: 10.1007/s42405-026-01281-w

Keywords: CubeSat, antenna design, frequency reconfigurable antenna, cavity-backed antenna, CPW-fed antenna, motorized reflector, X-band, C-band, small satellites, satellite communications, deployment-free mechanism, aerospace engineering

The core of the innovation lies in the filler itself, a three-component architecture the researchers call poly(Ani-co-Cz)@GO-Fe₃O₄. Graphene oxide, a two-dimensional carbon sheet decorated with oxygen-containing groups, serves as the structural backbone. Iron oxide nanoparticles, Fe₃O₄, contribute magnetism, while a copolymer of aniline and carbazole is grown onto the surface, providing the conjugated, electron-delocalized pathways that make the material electrically conductive. Aniline-based polyaniline is one of the most studied conducting polymers, prized for its stability and tunable conductivity, but it can be brittle and difficult to process. Carbazole, a fused-ring aromatic unit, brings additional rigidity, thermal robustness, and photoelectronic functionality. Co-polymerizing the two monomers onto graphene oxide yields a hybrid in which each component compensates for the weaknesses of the others, and the magnetic particles anchor a functionality that pure carbon fillers cannot supply.

To confirm that the hybrid really formed as designed, the team subjected the material to an extensive battery of characterization techniques. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy tracked the chemical bonds and surface chemistry, verifying that the copolymer had grafted onto the graphene oxide and that the iron oxide phase was present. Ultraviolet-visible spectroscopy and X-ray diffraction probed the electronic structure and crystallinity, while scanning electron microscopy, energy-dispersive X-ray analysis, and atomic force microscopy mapped the morphology and confirmed that the nanocomposite dispersed uniformly through the silicone rather than clumping into conductive islands. Vibrating sample magnetometry delivered perhaps the most striking result: the filled films exhibited superparamagnetic behavior, meaning they magnetize strongly in an external field but lose their magnetization when the field is removed, exactly the property needed for devices that must not retain magnetic memory or stick to one another.

Uniform dispersion is the make-or-break variable in polymer nanocomposites, and here the chemistry worked in the researchers’ favor. Hydrogen bonding between the oxygen groups on graphene oxide, the amine and imine sites along the copolymer backbone, and the siloxane network of PDMS creates strong interfacial interactions that restrain the mobility of neighboring polymer chains. Those interactions show up clearly in the mechanical tests. As nanocomposite loading increased, both tensile strength and tensile modulus rose significantly, transforming a soft, nearly featureless elastomer into a film that resists deformation and carries load. The trade-off is a reduction in flexibility, a familiar dilemma in composite design, though the authors report that the films retain useful elastomeric character, particularly at moderate filler contents, making them compatible with bendable and wearable form factors.

Dynamic mechanical thermal analysis added another layer of insight, revealing that the viscoelastic behavior of the films can be tuned by adjusting the filler loading. The glass transition temperature of the silicone shifted as the nanocomposite content changed, a direct consequence of the hydrogen-bonding network tethering polymer segments to filler surfaces and altering how segments relax under thermal agitation. In practical terms, this means an engineer could dial in not only the electrical and magnetic properties of a film but also its damping and thermal-mechanical response, an attractive degree of freedom for protective coatings that must survive vibration, flexing, and temperature swings in communication hardware.

The headline application, electromagnetic interference shielding, was evaluated across the X-band from 8 to 12.4 gigahertz, the frequency range used by radar, satellite links, and many wireless systems. The measurements showed that both direct-current and alternating-current conductivity of the films increased with nanocomposite loading, as expected when more conductive pathways thread through the insulating silicone. Shielding performance likewise climbed with loading, reaching a maximum shielding effectiveness of 1.2 decibels at 40 weight percent filler. That figure is modest compared with dense metal shields or high-loading carbon composites, but the significance lies in the mechanism and the multifunctionality: the films achieve shielding while remaining thin, elastomeric, and magnetically responsive, attributes that conventional metallic enclosures cannot match.

Dissecting the shielding mechanism revealed a synergistic interplay between reflection and absorption. Incoming electromagnetic waves are first partially reflected at impedance mismatches at the film surface, while the portion that penetrates is attenuated inside the material through dielectric losses, driven by interfacial polarization and conduction along the copolymer-graphene network, and through magnetic losses from the iron oxide phase. Notably, the analysis showed that dielectric loss dominates over magnetic loss in these films, indicating that the conductive copolymer and graphene oxide components do the heavy lifting in converting wave energy into heat, while the magnetic particles primarily add magnetic responsiveness and complementary attenuation pathways. This kind of mechanistic understanding is essential for rational design, because it tells future researchers which component to optimize when higher shielding is needed.

The broader context makes clear why multifunctionality matters. Most shielding research to date has focused on single-objective materials: carbon nanotube networks for conductivity, MXene films for ultrahigh absorption, or ferrite powders for magnetic loss. Each approach typically sacrifices something else, whether it is mechanical flexibility, processability, weight, or cost. By covalently and noncovalently integrating a conducting copolymer, graphene oxide, and superparamagnetic iron oxide into one filler, and then embedding that filler in a medically benign, optically transparent elastomer, the Mazandaran team has produced a platform in which shielding, mechanical reinforcement, electrical conduction, and magnetic function coexist in a single film. The authors highlight potential uses in flexible electronics, wearable electromagnetic shielding garments, and advanced protective coatings for communication devices.

The work also illustrates the pragmatic realities of translating laboratory nanocomposites into products. A shielding effectiveness of 1.2 decibels corresponds to attenuating roughly a quarter of the incident power, sufficient for reducing interference in low-to-moderate exposure scenarios but well below the 20 decibels or more demanded by military and high-power applications. Nonetheless, the loading-dependent trends in conductivity, modulus, and viscoelasticity provide a clear roadmap: optimizing percolation pathways, reducing the filler content needed for a given conductivity, or structuring the film with segregated or layered architectures could push performance substantially higher without sacrificing the flexibility that makes PDMS attractive in the first place. The fact that the researchers received no external funding for the study underscores the accessibility of the synthesis route, which relies on well-established polymerization and co-precipitation chemistry.

As the electromagnetic spectrum grows ever more crowded and flexible, skin-conformal electronics move from concept to clinic, materials that can do several jobs at once will increasingly define the state of the art. The PDMS films described here are not the final word on multifunctional shielding, but they demonstrate a compelling design principle: build the conductivity, magnetism, and mechanical reinforcement into a single nanoscale hybrid, and let strong interfacial chemistry knit it into an elastic matrix. If subsequent work can amplify the absorption component and trim the loading required, the same strategy could yield the thin, stretchable, magnetically addressable shields that the next generation of wearables, implants, and communication devices will need to operate cleanly in a noisy wireless world.

Subject of Research: Multifunctional PDMS nanocomposite films containing a poly(aniline-co-carbazole)@graphene oxide-Fe₃O₄ hybrid for electromagnetic interference shielding, mechanical reinforcement, and magnetic functionality.

Article Title: High performance PDMS nanocomposite films with poly(Ani-co-Cz)@GO–Fe₃O₄ toward enhanced EMI shielding, mechanical integrity, and magnetic functionality

Article References: Fallah, M., Lakouraj, M. M., & Norouzian, R.-S. (2026). High performance PDMS nanocomposite films with poly(Ani-co-Cz)@GO–Fe₃O₄ toward enhanced EMI shielding, mechanical integrity, and magnetic functionality. Polymer Bulletin, 83(11), Article 631. https://doi.org/10.1007/s00289-026-06684-z

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06684-z

Keywords: PDMS nanocomposite, EMI shielding, graphene oxide, Fe3O4 nanoparticles, polyaniline, polycarbazole, superparamagnetism, flexible electronics, X-band, dielectric loss, mechanical reinforcement, wearable devices