Cleverly Placed Directors Boost 28 GHz Antenna Gain for 5G Smartphones

Millimeter-wave 5G promises multi-gigabit data rates, but it comes with a punishing physics problem: signals at 28 GHz suffer severe free-space path loss and are easily blocked by the human body, walls, and even the user’s own hand. For a smartphone, where transmit power is limited and space is scarce, the antenna itself becomes the decisive factor in whether the link survives. A team at Edinburgh Napier University has now unveiled a compact dipole-Yagi antenna that squeezes unusually high gain out of a footprint of just 7 by 9.5 millimeters, using a director arrangement that breaks with nearly a century of Yagi-Uda convention.

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