Antoniades and Eleftheriades (IEEE Antennas and Wireless Propagation Letters, vol. 8, 2009) demonstrated a compact (22 x 30 mm) coplanar-waveguide-fed printed monopole loaded with a single negative-refractive-index transmission-line (NRI-TL) metamaterial unit cell, which achieves broadband dual-mode operation: at 5.5 GHz the loaded monopole radiates as a short folded monopole, while near 3.55 GHz it acts as a balun for the ground-plane currents so that the top edge of the antenna's own ground plane becomes the dominant radiator, producing an orthogonally polarized dipolar mode; the two orthogonal modes give a dual-resonance return loss with a measured 4.06 GHz impedance bandwidth and about 90% efficiency at both frequencies. The demonstrated behavior, however, depends on the antenna's own finite ground plane, its measured patterns are not fully symmetric and show measurable cross-polarization, and its sensitivity to substrate loss, MTM-geometry manufacturing tolerances, and platform interactions was not characterized. The open problem is to determine whether this dual-mode broadband, high-efficiency operation can be preserved when the antenna is integrated with larger or nonstandard platform ground planes, to reduce the pattern asymmetries and cross-polarization while retaining the dual-resonance broadband match, and to quantify the robustness of the dual-mode behavior to fabrication and platform-level variations. While alternative MTM-loaded monopoles and balanced NRI-TL-loaded printed dipoles sidestep ground-edge radiation, and ground-plane booster technologies exist in other antenna families, no prior work is known to demonstrate this antenna class operating on larger or nonstandard platform ground planes with characterized pattern symmetry, cross-polarization, and tolerance performance.

Background

A printed monopole antenna is a flat metal strip fed against a ground plane on a dielectric substrate, widely used in compact wireless terminals; feeding it with a coplanar waveguide (CPW, a planar transmission line with a signal strip between two ground strips on the same surface) keeps the whole radiator single-layer and low-cost. A negative-refractive-index transmission-line (NRI-TL) metamaterial is an artificial transmission line in which series capacitors and shunt inductors are embedded so that the line supports a left-handed (backward) propagation regime; a composite right-/left-handed (CRLH) line combines this with ordinary right-handed propagation. When such a unit cell is inserted into a monopole, the cell's near-zero phase shift can force in-phase currents on separate radiating arms, so the antenna behaves as a folded monopole (a shorter resonant radiator with enhanced radiation resistance), as formalized in the authors' earlier folded-monopole model for electrically small NRI-TL antennas (IEEE AWPL, vol. 7, pp. 425-428, 2008, DOI 10.1109/LAWP.2008.2008773). Antoniades and Eleftheriades (2009) integrated a single NRI-TL pi unit cell (a series gap capacitance between the top monopole and a bottom patch, plus shunt inductances realized by vias and a thin strip) into a CPW-fed monopole on FR4, a standard fiberglass-epoxy printed-circuit-board substrate. The measured consequences were: (i) at 5.5 GHz, in-phase currents on the monopole and the bottom strip produce a folded-monopole mode with x-directed polarization; (ii) near 3.55 GHz the unit cell behaves as a series resonator approaching a short circuit, so the loaded monopole acts as a balun (a balanced-to-unbalanced transformer) that reverses the current phase on one CPW ground section, making both ground top edges carry in-phase currents — the ground-plane top edge, about half a guided wavelength long, becomes the dominant radiator and radiates a y-polarized dipolar mode orthogonal to the 5.5 GHz mode. Because the two modes are orthogonal in polarization and pattern, their resonances combine into a dual-resonance return loss covering 3.14-7.20 GHz (measured -10 dB bandwidth 4.06 GHz, spanning the 3.3-3.8 GHz WiMAX and 5.15-5.85 GHz WiFi bands), with measured efficiencies near 90% at 3.55 and 5.5 GHz (verified independently by the gain-comparison and Wheeler-cap methods, the latter measuring efficiency from the difference between resonant and detuned input resistance). This orthogonal dual-mode behavior makes the antenna attractive for pattern-diversity multiple-input multiple-output (MIMO) terminals. The paper also identified tuning levers (ground-plane width and length, bottom-patch length, thin-strip width). The unresolved issues follow directly from the mechanism: the 3.55 GHz radiator is the ground edge of the antenna's own finite ground plane, so the mode's existence depends on ground-plane geometry; the measured patterns at 5.5 GHz are asymmetric due to ground-plane blockage, and at 3.55 GHz the pattern nulls are partly filled and cross-polarized (radiation in the orthogonal polarization component) because residual monopole currents interfere constructively; and the sensitivity of the dual-mode response to substrate loss, manufacturing tolerances of the embedded MTM geometry (via diameter, thin-strip width), and platform-level interactions was not experimentally characterized.

Problem Statement

Starting from the demonstrated design of Antoniades and Eleftheriades (2009) — a CPW-fed printed monopole on FR4 (22 x 30 mm) loaded with one NRI-TL metamaterial pi unit cell, whose 5.5 GHz folded-monopole mode and 3.55 GHz ground-edge dipolar mode (excited because the loaded monopole acts as a balun for the CPW ground currents) jointly produce a measured 4.06 GHz dual-resonance impedance bandwidth with about 90% efficiency and orthogonal polarizations at 3.55 and 5.5 GHz — determine whether this dual-mode broadband, high-efficiency operation can be made platform-integrable and manufacturable. Specifically: (1) establish, by analysis and fabricated prototypes, whether the 3.55 GHz ground-edge radiating mode and the 5.5 GHz folded-monopole mode, and hence the dual-resonance broadband match, are preserved when the antenna is integrated with the larger or nonstandard ground planes of real host platforms (host ground planes substantially larger than the antenna's own 22 x 30 mm stand-alone plane, or of nonstandard host shape such as a non-rectangular platform), instead of only the antenna's own stand-alone ground plane, and if the original geometry does not survive, provide design modifications that restore the two orthogonal modes on such platform ground planes; (2) reduce the measured radiation-pattern asymmetry at 5.5 GHz (attributed to ground-plane blockage) and the null filling and cross-polarization at 3.55 GHz (attributed to constructive interference of residual monopole currents) while preserving the broadband dual-resonance impedance match and the orthogonal-polarization property; and (3) characterize, experimentally or by validated systematic simulation, the sensitivity of the dual-mode response to substrate losses and to manufacturing tolerances of the embedded metamaterial geometry (via diameter, thin-strip width) and to platform-level interactions. These three items share one objective — turning the demonstrated high-efficiency dual-mode broadband MTM-loaded monopole into a robust, product-integrable dual-mode (pattern-diversity capable) antenna that preserves the dual-resonance broadband match and the orthogonal polarizations, reduces the pattern asymmetry and cross-polarization, and has its sensitivity to substrate loss, metamaterial-geometry tolerances, and platform interactions characterized on larger and nonstandard platform ground planes. The pattern asymmetry and co/cross-polarization ratio at 3.55 and 5.5 GHz characterize item (2)'s improvement, and 'preserving the broadband dual-resonance impedance match' means maintaining a -10 dB dual-resonance match across the 3.3-3.8 GHz WiMAX and 5.15-5.85 GHz WiFi bands.

References

  1. M. A. Antoniades and G. V. Eleftheriades, "A Broadband Dual-Mode Monopole Antenna Using NRI-TL Metamaterial Loading," IEEE Antennas and Wireless Propagation Letters, vol. 8, pp. 258-261, 2009. DOI: 10.1109/LAWP.2009.2014402, https://doi.org/10.1109/LAWP.2009.2014402
  2. M. A. Antoniades and G. V. Eleftheriades, "A Folded-Monopole Model for Electrically Small NRI-TL Metamaterial Antennas," IEEE Antennas and Wireless Propagation Letters, vol. 7, pp. 425-428, 2008. DOI: 10.1109/LAWP.2008.2008773, https://doi.org/10.1109/LAWP.2008.2008773
  3. M. A. Antoniades and G. V. Eleftheriades, "A compact and broadband NRI-TL metamaterial monopole antenna," in Proc. 13th International Symposium on Antenna Technology and Applied Electromagnetics and the Canadian Radio Science Meeting (ANTEM/URSI), 2009, pp. 1-4. DOI: 10.1109/ANTEMURSI.2009.4805042, https://doi.org/10.1109/ANTEMURSI.2009.4805042
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