Novel metamaterial frequency discriminating devices for next generation wireless communication systems

Ahmed, Kafil Uddin (2013) Novel metamaterial frequency discriminating devices for next generation wireless communication systems. Doctoral thesis, London Metropolitan University.


In this research, novel metamaterial structures based on microstrip integrated circuit technology have been investigated for filter application in the next generation of microwave wireless communication systems. A number of novel microstrip planar filter structures have been developed that are able to meet the stringent requirements for high-performance systems and subsystems, such as sharp-cutoff frequency response, low passband insertion-loss and high return-Joss, high out-af-band rejection, compact size, low cost and ease of integration. The filters are approximately 70% smaller than their conventional counterparts. Analysis and mathematical modeling of these microstrip devices involved the use of transmission-line theory and EM simulation tools which were based on the method of moments and finite element analyses. Measured results were used to validate the predicted behavior and performance of these devices. In fact there was good agreement with the theory and simulation modeling. A Composite right/left-handed (CRLH) metamaterial unit-cell whose ground-plane is defected with a rectangular dielectric slot was used to develop a single and multi-pole bandpass filters. The unit-cell comprises of serial inter-digital capacitors whose junction is connected to a short-circuited inductive stub. By defecting the ground-plane of the unit-cell 's structure with a dielectric slot, which is located immediately below the unit-cell, enables substantial tuning of the filter's centre frequency in the order of 26.5%. This was achieved with minimal effect on the unitcell's insertion- and return-loss performance as well as its selectivity. The filters were fabricated on conventional dielectric substrate and their performance measured to verify the design methodology. The proposed technique eases the trade-off constraints that plague conventional filter designs and makes possible the realization of challenging filter specifications constituted from CRLH unit-cells using distributed transmission-lines. A novel multilayer technique is presented that enables (i) the control of the filter's bandwidth, (ii) significantly improves its passband selectivity, and (iii) enhances its out-of-band rejection without affecting the filter's overall dimensions. The technique involves implementation of identical filter structures on both sides of the dielectric substrate that are interconnected through vias. The filter circuit is laid on top of another identical substrate with a ground-plane. This structure results in a device that exhibits a sharp selectivity that is substantially smaller than traditional filter constructions. The sharpness of the filter is due to the upper and lower transmission zeros present on either side of the filter's passband being shifted closer together which causes reduction in its bandwidth. This effect is opposite to that encountered in conventional multilayer filter structures. It is shown the filter's bandwidth can be controlled while maintaining a good passband insertion- and return-loss performance, i.e. <1.2 dB and >10 dB, respectively. Furthermore, the proposed filter is relatively easy to fabricate using conventional technology. A diplexer was developed based on the proposed multilayer technique and was shown to yield a high out-af-band rejection and high isolation between the two very closely spaced channels. Normally isolators are used to provide isolation between channels in order to prevent inter-channel interference. The technique is relatively simple to realize and cost effective to manufacture. Design methodology and experimental results are presented that show good correlation between the measured and simulation results. This diplexer should find application in multi band wireless communication systems. A triplexer deSign is also presented whose multiband performance was created using CRLH unit-cell structures. The three passband channels of the triplexer have a common input splitter and transmission-line matching network is used to ensure high isolation between the channels is maintained in order to prevent undesired interaction between the channels which would otherwise adversely affect the triplexer's passband response. This was achieved using stepped impedance lines (SIL). No isolators were necessary using the proposed technique. It is also shown that by curving the SIL can provide enhancement in the triplexer's loss performance, bandwidth, and suppression of high order spurii. The design analysis and performance of the triplexer was verified via fabrication and measurement. A novel compact microstrip wideband bandpass filter was presented for wideband application that possesses good insertion- and return-loss, sharp frequency selectivity and high out-of-band rejection. The proposed filter comprises of two inter-digital capacitors with four inductive stubs. It employs a T-shaped open stub that is inserted between the symmetrical unitcell structures.

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