Abstract
Electric dipoles and magnetic dipoles are the most fundamental particles in electromagnetic (EM) theory. Based on specific combinations of the electric and magnetic dipoles, three fundamental directional dipoles – the circular, Huygens, and Janus sources – exhibiting interesting near-field directionalities have been proposed, offering a greater level of electromagnetic wave manipulation. While a Huygens source is well known for having directive radiation, a Janus source – a pair of co-located, orthogonally directed electric and magnetic current sources with a 90° phase shift – is known to have a quasi-isotropic radiation pattern and a near-field directionality in terms of coupling to a waveguide. Although the Huygens source has been widely used in antennas and metasurfaces, the applications of the Janus source are heretofore limited. The existing Janus sources are limited to either ideal current sources or passive particles, hindering their efficient use in directional devices.The thesis is focused on creating an active Janus source to demonstrate the near-field directionality and developing the application prospect. The comparison for studying the existing Janus sources is first summarized, and the advantages and necessity of proposing an active Janus source are pointed out. Subsequently, a theory is constructed from the field of electromagnetism to reveal the reasons why directionality is exhibited by the Janus source in the near-field.
Firstly, we report, to the best of our knowledge, the first realistic active Janus source, which we achieve by properly tuning the current running through two filaments placed parallel plate waveguide (PPW) environment. It is shown that the Janus and Huygens sources achieve similar far-field performance as their equivalents in 3D space. Investigations in the near-field indicate that the active sources possess directive coupling properties consistent with earlier works featuring ideal Janus and Huygens sources. Furthermore, the quadruple current filament active source is introduced – a reconfigurable active directional dipole that facilitates the implementation of all three fundamental directional dipoles at a single frequency. The near-field directionality of the directional dipoles is also demonstrated through wave coupling to nearby waveguides.
Secondly, near-field mutual coupling suppression is demonstrated using the active Janus sources constructed in a PPW environment. The weakening of the coupling between two close neighboring Janus sources is achieved by modulating the near-field directionality between them. It is shown that when two such sources are placed within close proximity (from 0.08 to 0.24 wavelengths), the mutual coupling between them is reduced by around 1000-fold compared to two dipoles of similar proximity. Particularly, the simultaneous achievement of strong mutual coupling suppression and quasi-isotropic radiation makes the Janus source an ideal candidate for consideration in future compact multi-input multi-output (MIMO)communication systems.
Thirdly, a three-dimensional (3D) active Janus source is proposed to achieve near-field directionality over a wide bandwidth and a power efficiency much improved over the existing passive Janus source. It is shown that a class of quasi-isotropic antennas are actually active Janus sources (which is referred to as the Janus antenna). A well-designed Janus antenna is demonstrated, which (a) exhibits near-field directionality over a 3-dB broad bandwidth of 28.9%, and (b) in a practical usage environment, achieves a power efficiency ≈60 times higher than a passive Janus source. This work elucidates the connection between the “Janus dipole” concept in physics and the “quasi-isotropic antenna” concept in antenna design and paves the way for the design of future directional devices with much improved bandwidth and efficiency.
Finally, a compact MIMO system with high isolation is demonstrated. Using the constructed 3D active Janus source (Janus antenna), firstly, the weakening of the near-field coupling using near-field directionality modulation is demonstrated, and the isolation degree reaches above 28 dB when the two antennas are separated by 0.12 wavelengths. Furthermore, a four-cell array of 2×2 is constructed, and the results show that the isolation degree can reach 18.5 dB. Finally, a 4×4 with the whole size of the array in the sub-wavelength range is designed with the coupling between the antenna units up to -19.5 dB.
| Date of Award | 25 Aug 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Man Hon Alex WONG (Supervisor) |