Research On Novel Multiband And Compact Antenna Based On Composite Right/Left-Handed Transmission Lines Structures

With the rapid development of the wireless communication systems, antennas aredeveloping towards the trend of multi-frequency and miniaturization as the front-enddevices for transmitting and receiving electromagnetic signals. The compositeright/left-handed transmission lines (CRLH-TL) structures provide a conceptual routefor implementing novel multi-frequency and compact antennas. CRLH-TL is anartificial transmission lines which both has the left-handed (LH) and right-handed(RH) characteristics at the same time. The LH characteristics dominate the lowerfrequency band, and at this LH band the phase velocity and the group velocity areanti-parallel. While the RH characteristics dominate the higher frequency band, atwhich band the phase velocity and the group velocity are parallel. In addition,CRLH-TL can realize a larger propagation constant at lower LH frequency region,and the phase relations of the CRLH-TL can be controlled by the element values oftheir constructing unit cell to obtain a desired result. Because of these characteristics,CRLH-TL can be used to realize multi-frequency antennas with arbitrary frequencyratios. They also can be used to obtain the compact antennas.The thesis studied the main theory of left-handed metamaterials (LHMs),including the characteristics and the determination methods. And then it also providedtwo methods to improve CRLH-TL unit cell structure. Based on this, the thesis alsodesigned novel CRLH-TL unit cell structures and applied them to realize multibandand compact antennas. The main work and contributions of this thesis are as follows:(1) Proposed a method to extract the circuit parameters and two approach toimprove the characteristics of the proposed CRLH-TL structure. Accordingto the characteristics of the equivalent circuit model, the thesis provides amethod to extract the circuit parameters from simulated S parameters.Electromagnetic simulation and circuit simulation show that the circuitmodel and the extraction method are reliable and correct. Based on the research of the circuit model and the dispersion relation, an approach toimprove the characteristics of the CRLH-TL structure is obtained: increasingseries capacitance CLor shunt inductance LLseparately or simultaneously,which both can enlarge the propagation constant and decrease the size of theunit cell structure.(2) Proposed a novel tri-band compact patch antenna based on the multi-layermushroom CRLH-TL structures. The multi-layer mushroom structureenhances the LH series capacitance CLby loading middle metal caps into theoriginal mushroom structure. The comparison research on the single layerand multi-layer structures demonstrate that, the multi-layer structure hasenlarged the propagation constant in the LH region. Anovel tri-band compactpatch antenna is designed based on the multi-layer mushroom CRLH-TLstructures. The novel antenna works at three operating frequencies. Thelowest resonant frequency belongs to mode n=1, which is in LH frequencyregion. The LH resonant frequency can be adjusted by tuning the size of thecentral CRLH-TL structure. The two higher resonant frequencies belong toRH frequency region, which can be adjusted by changing the patch size.Different scale of frequency ratio can be arbitrarily chosen by adjusting thedimensions of patch and CRLH-TL structures.(3) Proposed a novel multilayer and spiral-slot CRLH-TL structure andimplemented a multiband compact patch antenna based on it. The proposedCRLH-TL structure simultaneously enhances shunt inductance and seriescapacitance by loading spiral-slot and middle metal caps into originalmushroom structure. By this way, the multi-layer and spiral-slot structure hasfurther enlarged the propagation constant of the CRLH-TL in the LH region.In addition, a novel compact multi-frequency patch antenna with multilayerand spiral-slot CRLH-TL structures is designed and proposed. The operatingmodes of the multiband patch antenna is analysed and explained by transmission line theory and the simulated results prove the theory analysis.A novel antenna prototype with four resonances is fabricated, which has thedimensions of λ0/9.5×λ0/5.88×λ0/32.9at the first working frequency.Frequency ratio can be chosen by adjusting the dimensions of the outer patchand the central CRLH-TL structures.(4) Proposed a compact tri-band circularly polarized (CP) patch antenna withmultilayer and spiral-slot CRLH-TL structures. This novel CP antenna canrealize multiband by loading CRLH-TL structures and circular polarizationby Wilkinson divider. A CP antenna working at2.4GHz,3.1GHz and3.5GHz is fabricated. The lower two operating frequencies are controlled by thecentral CRLH-TL structure and the highest operating frequency is influencedby both the outer patch and the central CRLH-TL structures. Compared withthe previous multiband CP antennas, the frequency ratio between differentbands can be set arbitrarily in a wide range, by adjusting the size of differentparts of the proposed antenna. The length of the top patch is λ0/6.6at2.4GHzand it is much smaller than the conventional CP antenna. The measured andsimulated reflection coefficient curves are well matched.