The device at the center of the study is a Circular InterDigital Capacitor Sensor, or CIDCS, a one-port microwave component that doubles as both a resonator and a radiator. Its sensing element is a circular array of interlocking metal fingers, the interdigital capacitor, printed on one side of an FR4 substrate, with a ground conductor on the reverse. Interdigital capacitors are prized in microwave sensing because their tightly spaced fingers confine strong electric fields into a compact footprint, making them acutely responsive to whatever material is brought near them. In this design, a microstrip transmission line feeds the structure electromagnetically, and the whole sensor is tuned to resonate at a well-defined frequency.

The research team, Swati Todi and Poonam Agarwal of the university’s MicroSystems Lab, fabricated two prototypes that differed in exactly one respect. Prototype-I carried a complete metallic ground plane behind the interdigital structure, while Prototype-II had a partial ground plane, leaving the substrate exposed directly beneath the sensing fingers. Every other dimension was held constant, ensuring that any difference in performance could be attributed to the ground configuration alone. The researchers also built a rigorous electromagnetic simulation campaign in CST to quantify field distributions, radiation patterns, directivity, gain, and radiation efficiency for both versions before any liquid ever touched the devices.

The simulations told a story of trade-offs. Removing the ground beneath the IDC pushed the resonant frequency down dramatically, from 6.345 gigahertz to 2.149 gigahertz, while simultaneously raising the electric field strength around the fingers, boosting surface current, and increasing both electric and magnetic energy densities. The physical explanation is intuitive: with a full ground plane present, most of the electromagnetic field is trapped between the patch and the ground, locked inside the dielectric substrate, and only weak fringing fields extend upward toward the sample. Strip that ground away and the fringing fields surge, spilling out of the surface where they can interact with whatever liquid sits above the fingers. The catch was radiative: radiation efficiency collapsed from 26 percent in the fully grounded prototype to just 5 percent in the partially grounded one, and the front-to-back ratio fell from 15.91 to 2.08.

To contain liquid samples during testing, the team bonded small circular wells carved from polydimethylsiloxane, the flexible biocompatible polymer known as PDMS, onto the sensor surface. Cavity size mattered more than one might expect. Because most of the sensing action occurs in the fringing fields between adjacent fingers, the researchers systematically tested cavities with diameters of 4, 6, and 8 millimeters, filling each with carefully measured volumes of glucose dissolved in deionized water at concentrations spanning 0.0 to 0.5 grams per milliliter. The 6-millimeter cavity proved optimal, and it was there that the contrast between the two prototypes reached its peak.

The experimental results were striking. With the 6-millimeter cavity, the partially grounded Prototype-II outperformed its fully grounded sibling by 20.3 percent in sensitivity measured through resonant frequency shifts and by a remarkable 49.41 percent in sensitivity measured through return loss changes. The linearity of the sensor’s response also improved, with the linear regression coefficient rising by 2.26 percent for frequency-based readings and 0.324 percent for return-loss-based readings. As glucose concentration climbed across the tested range, the partially grounded sensor tracked the changes with a consistency that the fully grounded version could not match.

Reliability was put to the test as well. The team repeated their measurements three times for each configuration and computed standard deviations, mean absolute deviations, coefficients of variation, and limits of detection. The coefficients of variation came out at 1.68 percent and 0.16 percent for the fully grounded device and 5.64 percent and 0.26 percent for the partially grounded one, indicating high precision with negligible relative variability. The estimated limits of detection confirmed that both prototypes could resolve low glucose concentrations, with the partially grounded design again holding the edge in raw responsiveness.

The physics behind the improvement can be understood through perturbation theory, the framework that governs how resonant frequencies shift when a dielectric material perturbs an electromagnetic field. The stronger the electric field in the sensing region, the larger the perturbation caused by the sample, and the bigger the resulting frequency shift. Removing the ground conductor beneath the interdigital capacitor simultaneously strengthens the fringing fields, reduces parasitic capacitance between the fingers and the ground, and alters the effective capacitance and inductance of the structure, all of which conspire to amplify the sensor’s reaction to subtle permittivity changes. The fully grounded prototype, by contrast, buries its field energy in the substrate where the sample cannot reach it.

What distinguishes this work from earlier studies is the rigor of the comparison. Partial and defected ground planes have been used liberally across the microwave sensor literature to shrink devices, widen bandwidths, and improve impedance matching, but most reports optimize a single design without quantifying what the ground modification actually bought. Here, both prototypes share identical sensing geometry on identical FR4 substrates, so the measured sensitivity gains are a direct, controlled measurement of the ground plane’s role. The authors note that most prior glucose sensors also rely on FR4, reinforcing the practical relevance of their approach for low-cost implementations.

The broader implication is a design principle rather than a single device. When a one-port microwave structure must serve primarily as a sensor, sacrificing antenna metrics such as directivity, gain, and radiation efficiency is a worthwhile price for a device that responds more sharply to the dielectric fingerprint of the liquid it examines. The researchers point toward future work refining the sensing geometry, exploring alternative sample placement, and extracting additional parameters such as Q-factor, phase, and signal-to-noise ratio for multi-dimensional analysis. For applications like glucose monitoring, where detecting minute variations in permittivity is the whole point, the message is clear: sometimes the ground beneath a sensor is better left partly removed.

Subject of Research: Effect of complete versus partial backside ground conductors on the sensitivity of a microstrip-fed circular interdigital capacitor microwave glucose sensor

Article Title: Sensitivity analysis of the microstrip-fed circular InterDigital capacitor sensor with complete and partial backside ground conductor

Article References: Sensitivity analysis of the microstrip-fed circular InterDigital capacitor sensor with complete and partial backside ground conductor. (n.d.). https://doi.org/10.3389/fanpr.2026.1813732

Image Credits: AI Generated

DOI: 10.3389/fanpr.2026.1813732

Keywords: microwave sensor, interdigital capacitor, partial ground plane, glucose sensing, permittivity, resonant frequency, PDMS microfluidics, FR4 substrate, sensitivity analysis, electromagnetic simulation, biosensor, antenna trade-off

The device, which the authors call a Funnel Morph Antenna, gets its name from a distinctive hybrid geometry that combines a narrow tapered neck with a flared upper section, resembling the profile of an industrial funnel. That shape is not merely aesthetic. In millimeter-wave antenna design, one of the central challenges is impedance matching: the antenna must present a consistent 50-ohm load to the feeding circuitry across a wide range of frequencies, or else a large fraction of the captured signal simply reflects back instead of being delivered to the rectifier. The funnel profile solves this by providing a gradual electromagnetic transition from the narrow neck, which concentrates surface currents toward the radiating aperture, to the wide flared portion, which extends the effective radiating area. Because the current path changes smoothly rather than abruptly, discontinuities that would normally cause reflections are minimized, and the antenna maintains a reflection coefficient below minus 10 dB across the full operating band.

The physics behind the wideband behavior is a story of overlapping resonances. Simulations of the surface current distribution at 28, 33, and 38 GHz reveal that different parts of the structure dominate at different frequencies. At 28 GHz, current concentrates around the narrow neck and lower radiating sections, corresponding to a low-frequency resonant mode. By 33 GHz, the current spreads into the flared upper portion, indicating the formation of additional resonant modes, and at 38 GHz the maximum current density shifts to the upper flare and the cavity regions, where shorter current paths support the highest-frequency operation. The superposition of these multiple modes is what stretches the usable bandwidth to a full 14 GHz, an unusually wide span for a compact millimeter-wave element. A multilayered cavity embedded in the ground plane adds further control, confining the electromagnetic fields and stabilizing the directional radiation pattern.

Perhaps the most provocative engineering decision in the study is the choice of substrate. Millimeter-wave devices almost always use specialized low-loss laminates such as Rogers RT/duroid, because ordinary FR4, the fiberglass material found in countless consumer circuit boards, suffers from significant dielectric losses and dispersion at frequencies above 26 GHz. The team deliberately chose 1.6-millimeter FR4, with a dielectric constant of 4.3 and a loss tangent of 0.02, to test whether a genuinely low-cost design could still perform. Despite the higher insertion losses, careful optimization of the flare and feed geometry allowed the antenna to achieve wideband matching, and the geometry proved robust against the etching tolerances of standard printed circuit board fabrication, holding its performance even with dimensional deviations of plus or minus 0.05 millimeters. The single element measures only about 17.5 by 20.3 millimeters and delivers a simulated peak gain of roughly 5 dBi with radiation efficiency above 85 percent, figures that measurements closely confirmed.

A single antenna, however, captures only a modest amount of power, so the researchers replicated the element into a four-element array. To feed all four elements uniformly, they designed a 1-to-4 Wilkinson power divider, a classic microwave component built from three cascaded two-way dividers, each using quarter-wavelength transmission lines with a characteristic impedance of twice the system impedance and a 100-ohm isolation resistor between output ports. The divider performed impressively across the band, with insertion loss between roughly minus 6 and minus 6.2 dB, meaning each output port receives almost exactly one quarter of the input power, and port-to-port isolation exceeding minus 45 dB around 30 GHz, indicating negligible signal leakage between channels. The array elements were spaced 5 millimeters apart, half a wavelength at the operating frequency, to maximize gain while suppressing unwanted side lobes.

The assembled array, measuring 78.25 by 70.57 millimeters, delivered a measured peak gain of 10.25 dBi and an efficiency of about 93 percent at 28 GHz. The theoretical maximum gain for four ideal elements with a single-element gain of 4.8 dBi would be 10.82 dBi, so the measured value falls only 0.57 dB short, a gap the authors attribute to feed network losses, conductor losses in the microstrip lines, and the dielectric losses inherent to FR4. The aperture efficiency, a measure of how effectively the physical area of the array is used, remained a steady 78.5 percent despite mutual coupling between elements and phase errors across the wide band. The beam pattern is notably narrow, with a half-power beamwidth of about 9 degrees at 28 GHz and 10 degrees at 38 GHz in one plane, and side lobe levels between minus 7 and minus 9 dB, giving the array a focused, directional sensitivity well suited to harvesting energy from distant base stations.

Converting captured radio waves into usable electricity falls to the rectifier, and here the team navigated some practical constraints. They selected the 1SS351 Schottky diode from ON Semiconductor, chosen because its electrical characteristics closely match the widely used HSMS-2852 diode, which has recently become obsolete. Using Keysight Advanced Design System software, the researchers modeled the diode with harmonic balance and S-parameter simulations to extract its input impedance, finding a value of 47.4 minus j20 ohms near 28 GHz. Matching that impedance to the antenna required careful design work. The team compared half-wave and full-wave rectifier topologies and found that the full-wave bridge significantly raises the imaginary component of the input impedance, which would degrade efficiency without elaborate matching. A matched half-wave rectifier using microstrip lines emerged as the practical choice, especially after lumped-element matching proved infeasible because it required component values, such as a 53-ohm resistor and a 2.7-nanohenry inductor, that are difficult to source off the shelf.

The full system was validated experimentally in an anechoic chamber, with a broadband vector signal generator driving a standard gain horn antenna as the transmitter and the fabricated rectenna prototype as the receiver, separated by one meter, comfortably within the far-field region for these frequencies. Both antennas were carefully polarization-aligned, and the incident power density was calibrated with a broadband power sensor before characterization. When the array harvested 68.7 microwatts of incident RF power, the system produced an output voltage of 250 millivolts across a 2-kilohm load, corresponding to an overall RF-to-DC conversion efficiency of 45.5 percent. That figure is competitive with, and in several respects superior to, prior millimeter-wave rectennas, many of which required expensive substrates, waveguide structures, or far higher input power levels to reach comparable efficiency.

The comparison with earlier work underscores why the low-cost approach matters. Previous millimeter-wave rectennas have relied on Duroid substrates, substrate-integrated waveguides, liquid crystal polymer packaging, on-chip CMOS integration, and even air-filled waveguide Fabry-Perot resonators. Some achieved higher peak gains or higher conversion efficiencies, such as a waveguide-fed 35 GHz design reaching 68.5 percent, but typically with narrower bandwidth, more complex manufacturing, or substantially greater incident power. Others, like CMOS folded dipoles, achieved extreme miniaturization at the cost of negative gain. The new design occupies a distinctive middle ground: a full 26 to 40 GHz operating range, a respectable 10.25 dBi gain, a narrow 9-degree beamwidth, and 45.5 percent conversion efficiency, all on ordinary FR4 fabricated with standard PCB technology.

The implications reach beyond the laboratory. As 5G and future 6G networks densify urban radio environments, the ambient millimeter-wave energy available for harvesting will only grow, and devices that can scavenge it without batteries could power IoT sensors, wearables, and backscatter tags indefinitely. The authors suggest that future work will explore miniaturization techniques and adaptive beamforming to improve integration into compact real-world devices. If battery production costs, which are high relative to the energy those batteries actually store, continue to drive the push toward green electronics, a funnel-shaped patch of fiberglass that drinks from the 5G spectrum may prove to be one of the more elegant shortcuts to energy-autonomous wireless devices.

Subject of Research: A wideband funnel-geometry rectenna array for radio-frequency energy harvesting in the 26–40 GHz 5G millimeter-wave band

Article Title: Wideband funnel geometry rectenna array for RF energy harvesting in 26–40 GHz 5G networks

Article References: Megahed, A. A., Mousa, M. E., Al-Gburi, A., & Elabd, R. H. (2026). Wideband funnel geometry rectenna array for RF energy harvesting in 26–40 GHz 5G networks. Results in Optics, 25, Article 101178. https://doi.org/10.1016/j.rio.2026.101178

Image Credits: AI Generated

DOI: Not provided

Keywords: rectenna, RF energy harvesting, 5G, millimeter-wave, antenna array, Wilkinson power divider, Schottky diode, FR4 substrate, wireless power transfer, impedance matching, IoT, Results in Optics