The classic Yagi-Uda antenna, invented in the 1920s, places a driven element at one end and a row of parasitic director elements in front of it, all along a single central axis, to beam energy in one direction. When engineers simply scale that geometry down to millimeter-wave frequencies, the gain enhancement tends to disappoint. The new design, described in Frontiers in Antennas and Propagation, takes a different route: it places paired directors symmetrically on both the top and bottom layers of the antenna structure, and deliberately shifts them away from the conventional central front region of the driven dipole.

That seemingly small change has large electromagnetic consequences. In a conventional planar Yagi, directors along a single central axis produce a relatively uniform current distribution with limited control over field confinement. The off-center, dual-layer arrangement modifies the current paths, strengthening the electromagnetic coupling between the driven element and the parasitic directors. Full-wave simulations show that the induced currents on the directors are largely in phase with those on the driven dipole, forming an effective guiding structure that reinforces energy propagation along the end-fire direction while suppressing backward and lateral radiation.

The researchers quantified the effect through a systematic series of comparisons. A basic printed dipole on its own achieves a peak gain of roughly 5 dBi. Adding a first set of directors raises that to more than 7 dBi. But with the modified director configuration, the realized gain climbs above 9 dBi at 28 GHz, all without enlarging the antenna footprint or profile. Control experiments with directors placed directly in front of the dipole, whether in vertical columns on the top or bottom layer or at the center, all plateaued near 7 dBi, confirming that the unconventional placement, not the mere number of directors, drives the improvement.

Parametric studies revealed that performance is highly sensitive to the spacing between the driven dipole and the directors, and to their relative positioning on the two layers. The team optimized these parameters for maximum gain and stable impedance matching at 28 GHz. The antenna is built on a Rogers RT/duroid 5880 substrate just 0.5 millimeters thick, with a low permittivity of 2.2 and a loss tangent of 0.0019, materials well suited to millimeter-wave operation. Simulations, run in CST Microwave Studio using a time-domain solver with adaptive hexahedral meshing, indicated a radiation efficiency of approximately 97 percent and a total efficiency of around 80 percent at the resonant frequency, under ideal conditions.

From the single element, the researchers scaled up to a four-element linear end-fire array. Evaluated as an active phased array, the configuration achieved a peak gain exceeding 13 dBi at 28 GHz with effective beam steering, while mutual coupling between adjacent elements stayed below -20 dB, indicating good isolation. A passive version of the array, driven by a corporate feeding network using quarter-wavelength impedance transformers to deliver equal-amplitude, equal-phase excitation to every element, reached a peak realized gain of about 12.8 dBi. The slight reduction relative to the phased array reflects losses in the feeding network, but the comparison confirmed that radiation performance is governed primarily by the antenna element itself.

The team then fabricated a prototype of the four-element array on a single-layer substrate and measured it experimentally. The measured reflection coefficients agreed well with simulations across the targeted 28 GHz band, with only minor discrepancies attributed to fabrication tolerances, substrate variations, connector and soldering effects, and measurement alignment errors. Measured E-plane and H-plane radiation patterns closely matched the simulated ones, showing stable end-fire radiation with high gain and suppressed sidelobes, and validating the design concept in hardware rather than simulation alone.

Because modern smartphones rely on multiple-input multiple-output (MIMO) techniques to multiply capacity, the researchers also evaluated a four-element MIMO configuration in which identical arrays are placed at four edges of a system circuit board. This placement enables pattern and spatial diversity while keeping mutual interaction low. All elements showed consistent impedance matching across the band, very low mutual coupling without any additional decoupling structures, and nearly identical end-fire radiation patterns regardless of position on the board, confirming uniformity and repeatability across ports.

Against a backdrop of competing millimeter-wave antenna designs, the numbers stand out. Most compact antennas reported in the recent literature achieve gains between 6 and 7 dBi, and designs reaching 8 dBi or more typically require larger footprints, dielectric resonator volumes, or auxiliary loading structures such as metamaterials, electromagnetic bandgap surfaces, or frequency-selective surfaces. The proposed element, by contrast, achieves 9 dBi in a 7 by 9.5 millimeter area with no external loading or backing at all, representing a gain improvement of roughly 15 to 40 percent over typical designs of comparable size.

The end-fire characteristic offers a practical bonus for handheld devices: when antennas are mounted along device edges, their directional beams can be steered away from the user’s hand, mitigating the body blockage that so severely degrades 28 GHz links. The authors note that the concept can be scaled to other millimeter-wave bands and adapted to multi-element and MIMO configurations, positioning the design as a candidate for next-generation 5G and beyond user equipment, where every decibel of gain and every square millimeter of board space counts.

Subject of Research: A compact dual-layer dipole-Yagi antenna with enhanced end-fire gain for 28 GHz millimeter-wave 5G user equipment

Article Title: A compact array antenna with enhanced-gain end-fire radiators for 28 GHz millimeter-wave user equipment

Article References: Jahanbakhsh Basherlou, H., Ojaroudi Parchin, N., & See, C. H. (2026). A compact array antenna with enhanced-gain end-fire radiators for 28 GHz millimeter-wave user equipment. Frontiers in Antennas and Propagation, 4, Article 1836522. https://doi.org/10.3389/fanpr.2026.1836522

Image Credits: AI Generated

DOI: 10.3389/fanpr.2026.1836522

Keywords: 28 GHz, 5G, millimeter-wave, Yagi-Uda antenna, end-fire radiation, gain enhancement, antenna array, MIMO, phased array, smartphone antennas, dipole antenna, user equipment

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