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The rapid development of wireless communication, space-based radar systems and active military radars in the past decades gives rise to the necessary of information security and electromagnetic waves pollution, which arouse the requirement of perfect electromagnetic absorbers in both military and civil areas. Since the isolation of graphene and the first experimental demonstration of so-called perfect metamaterial absorber carried out in the early 21st century, a large amount of research has been directed at graphene based metamaterial absorbers. This thesis delivers pioneering developments on the design of printed graphene metamaterial absorbers on electromagnetic ranges including radio frequency, microwave frequency, and THz bands. This thesis focuses on the design and optimization of broadband metamaterial absorbers for civil and military applications and aims to provide novel ideas and combed design approaches for further development of the metamaterial absorbers with outstanding properties. This thesis presents several broadband printed graphene metamaterial absorbers with competitive properties. For example, the two multilayer structure metamaterial absorbers in chapter 3 provide ultra-wide operation bandwidth covering from C band to Ka band, and from S band to Ku band, respectively; the ultra-thin metamaterial absorber in chapter 4 achieves a relative absorption bandwidth of 178% with a thickness of merely 0.3 mm; the novel printed graphene AMC based radar absorber has pioneering advances compared with conventional metallic AMC based absorbers; the genetic algorithm optimized absorber indicates potentials of coding metamaterial absorbers. For the first time, the printed graphene AMC based radar absorber is experimentally demonstrated. Furthermore, the printed graphene metamaterial absorbers presented in this thesis are angle-insensitive and polarization-independent, and possess good flexibility thanks to the printed graphene patterns and flexible dielectric substrates. This work significantly provides novel approaches and more design flexibility to metamaterial absorbers, and expands the applications of printed graphene in the design and fabrication of metamaterial absorbers.

Novel feedback signal interference concept for bandpass filter (BPF) design is proposed in this thesis. This new concept was utilized to design wide stopband BPF with superior performance for WLAN applications. The proposed filtering structure consists of two simple coupled-line couplers. The first coupler was employed within the main signal path and the second coupler which is open circuited at the opposite ends was used for the feedback circuit. This new filtering structure was fabricated using low temperature co-fired ceramic technology. The fabricated BPF exhibits an insertion loss (IL) of -1.3 dB with a 3dB fractional bandwidth of 13% at a centre frequency of 2.4 GHz. Furthermore, an attenuation level of -15 dB is achieved up to 4.7fo. Using stepped impedance coupled-lines in the feedback; the stopband performance of the proposed structure can be significantly improved while keeping passband performance intact. Furthermore, the feedback signal interference concept proved to be versatile and can be used to design high selectivity microstrip BPF using composite right/left handed transmission line unit cell in the feedback circuit. The measured results show that roll-up/down rate of more than 300 dB/GHz can be achieved with low IL.The spectrum based quality factor for CRLH TL based BPF is explored and thoroughly studied in this thesis to design a low phase noise oscillator. The proposed metamaterial BPF has higher spectrum based quality factor within the left-handed region due to the slow-wave propagation. This intriguing feature enables the design of a free-running oscillator with excellent phase noise performance operating at a frequency of 2.05 GHz. The fabricated oscillator demonstrates a phase noise of -126.7 dBc/Hz at 100 kHz frequency offset and a FOM of -207.2 dBc/Hz at a 1 MHz frequency offset, being one of the very best reported so far. Many microstrip multi-band BPF design techniques are presented in the literature that offers superior performance in terms of IL, 3dB FBW, and high selectivity. These methods mainly lack the ability to obtain high performance with compact size. Coupled slotted open stubs are used to design a miniaturized dual-wideband BPF. Interdigital capacitor and inner open stubs are used to improve rejection level within the stopbands and increase selectivity. The measurement results reveal that the fabricated dual-wideband BPF has two passbands with a 3dB FBW of 117% and 36%, at respective centre frequencies of 1 GHz and 6.65 GHz. The filter has a super compact size (0.09 Gammag × 0.05 Gammag) where Gammag denotes the guided wavelength at the centre frequency of the first passband and exhibits an attenuation level greater than 20 dB up to 12 GHz.

This thesis introduces, for the first time, a novel 4-bit, metal-oxide-metal (MOM) digital capacitor switching array (MOMDCSA) which has been implemented into a wideband CMOS voltage controlled oscillator (VCO) for 5 GHz WiMAX/WLAN applications. The proposed MOMDCSA is added both in series and parallel to nMOS varactors. For further gain linearity, a wider tuning range and minor phase noise variations, this varactor bank is connected in parallel to four nMOS varactor pairs, each of which is biased at a different voltage. Thus, VCO tuning gain reduces and optimal phase noise variation is obtained across a wide range of frequencies. Based on this premise, a wideband VCO is achieved with low phase noise variation of less than 4.7 dBc/Hz. The proposed VCO has been designed using UMC 130 nm CMOS technology. It operates from 3.45 GHz to 6.23 GHz, with a phase noise of -133.80 dBc/Hz at a 1 MHz offset, a figure of merit (FoM) of -203.5 dBc/Hz. A novel microstrip low-phase noise oscillator is based on a left-handed (LH) metamaterial bandpass filter which is embedded in the feedback loop of the oscillator. The oscillator is designed at a complex quality factor Qsc peak frequency, to achieve excellent phase noise performance. At a centre frequency of 2.05 GHz, the reported oscillator demonstrates, experimentally, a phase noise of -126.7 dBc/Hz at a 100 kHz frequency offset and a FoM of -207.2 dBc/Hz at a 1 MHz frequency offset. The increasing demands have been placed on the electromagnetic compatibility performance of VCO devices is crucial. Therefore, this thesis extends the potential of highly flexible and conductive graphene laminate to the application of electromagnetic interference (EMI) shielding. Graphene nanoflake-based conductive ink is printed on paper, and then it is compressed to form graphene laminate with a conductivity of 0.43×105 S/m. Shielding effectiveness is experimentally measured at above 32 dB as being between 12GHz and 18GHz, even though the thickness of the graphene laminate is only 7.7µm. This result demonstrates that graphene has great potential for offering lightweight, low-cost, flexible and environmentally friendly shielding materials which can be extended to offering required shielding from electromagnetic interference (EMI), not only for VCO phase noise optimisation, but also for sensitive electronic devices.

In the thesis presented, three novel uni-planar left handed transmission lines based on Complementary Split Ring Resonators (CSRR) and Complementary Spiral Resonator (CSR) metamaterial particles are proposed for the first time and successfully applied for the performance enhancement of conventional coupled line filters and the design of a full scan leaky wave antenna. Based on the implementation of Complementary Split Ring Resonators (CSRR) a fully planar Composite Right left handed transmission line (CRLH-TL) plus a Dual Composite Right Left Handed transmission line (D-CRLH-TL) are proposed, designed and studied. The CRLH transmission line is realized loading a microstrip host line with CSRR metamaterial particles and capacitive gaps on the conductor strip of the line, while the D-CRLH transmission line is built bridging the capacitive gaps between the CSRR's with inductive connections to induce dual propagation properties in the structure. A further performance enhancement is achieved by a second proposal based on the implementation of Complementary Spiral Resonators (CSRs). The studies applied to the structure reveal that conditions for D-CRLH propagation are created in a transmission line composed only by CSR metamaterial resonant particles as the result of the unique electromagnetic properties acquired by the CSRs etched on the conductor line of a microstrip waveguide. The CSRR/CSR based D-CRLH transmission lines proposed are applied in the design of two enhanced coupled line filters. The implementation is possible for the first time in this work as result of the highly versatile layout features of the left handed transmission lines proposed which among other important geometrical features, allows couplings in both sides of the structure. Enhanced filtering features are reached by the metamaterial based coupled line filters proposed in terms of selectivity and size reduction in comparison with conventional coupled line filters. In the case of the CSRR loaded metamaterial coupled line filter a size reduction of 49% is achieved compared with a filter of similar performance. A further enhancement is reached by the second coupled filter proposed based on CSR metamaterial resonant particles. The CSR coupled filter proposed reach a roll off rate improvement of 8 dB/GHz and size reduction of 43% in comparison with a conventional coupled line filter of the same order. When this is compared against a conventional coupled line filter of similar performance but higher order, a remarkable size reduction of 77% is achieved by the proposed CSR coupled line filter. Finally, the CSR loaded D-CRLH transmission line is applied in the design of a fully planar leaky wave antenna. Using the CSR structures etched on the conductor line of a microstrip as part of the radiation mechanism altogether with the D-CRLH propagation features of the structure. A full scan radiation pattern is created with backward, forward and broadside radiation as result of the left handed propagation band, right handed propagation band and the balanced transition between them. Two leaky wave antennas are designed to operate at the centre frequencies of 12.5GHz and 6.0GHz. In order to corroborate the performance the CSR D-CRLH leaky wave antenna operating at 6.0GHz is fabricated and measured showing a scanning range of 30 deg with a maximum gain of 13.2 dBi.

In this thesis metamaterial radar absorbers and plasmonic structures have been investigated. Following a brief overview covering metamaterial structures, and their applications in various areas of Microwave Engineering, a novel thin metamaterial wideband radar absorber, formed by two layers of resistive Hilbert curve arrays, is proposed and analysed numerically in HFSS, revealing a reduction in Monostatic Radar Cross Section (RCS) of more than 10 dB from 9.1 to 18.8 GHz (70% fractional bandwidth) for both polarizations. The structure has thickness of only 0.11λ to 0.24λ at lowest and highest frequencies respectively. The lateral dimensions are only 0.13λ to 0.3λ per unit cell at lowest and highest frequencies respectively which is several times smaller than that of recently reported circuit analogue absorbers operating in the similar frequency band. Furthermore, a wideband terahertz Hilbert curve array is proposed and analyzed both theoretically and numerically, showing an absorption bandwidth of more than one octave. This was followed by study of plasmonic cloak for subwavelength conducting objects. It was demonstrated that a plasmonic cloak designed for a conducting sphere will work for non spherical conducting objects of similar dimensions as well. Finally spoof plasmonic structures were investigated. A novel plasmonic structure based on a modified Apollonian fractal array of cylindrical coaxial apertures in an aluminium sheet was proposed and analyzed. The structure exhibits negative group velocity with less than 3.5 dB attenuation. Plasmonic structure based on Sierpinski array of apertures was also investigated and found to give quite good extraordinary transmission bandwidth.