Abstract: We demonstrate a class of microstrip patch antennas that are stretchable, mechanically tunable, and reversibly deformable. The radiating element of the antenna consists of highly conductive and stretchable material with screen-printed silver nanowires embedded in the surface layer of an elastomeric substrate. A 3-GHz microstrip patch antenna and a 6-GHz 2-element patch array are fabricated. Radiating properties of the antennas are characterized under tensile strain and agree well with the simulation results. The antenna is reconfigurable because the resonant frequency is a function of the applied tensile strain. The antenna is thus well suited for applications like wireless strain sensing. The material and fabrication technique reported here could be extended to achieve other types of stretchable antennas with more complex patterns and multilayer structures.

Abstract: The concept of distributedly modulated capacitors (DMC) is proposed as a form of time-varying transmission-line structure offering nonreciprocal propagation and coupling characteristics. The nonreciprocity is achieved by taking advantage of the additional dimension of time-variance in the transmission-line property. The complete theory, based on: 1) the distributed parametric effect on a time-varying transmission line and 2) the distributed capacitive mixers, is presented with emphasis on the theoretical bounds of the isolation and insertion performances of DMC. Simulations are carried out and a prototype consisting of double-balanced varactor diodes on microstrip lines is implemented on a Rogers board. The measured results agree well with the theoretical derivations and the simulation results. It is thus confirmed that the DMC has great potential of being a circulator device with broadband, minimum insertion loss, and synthesizable isolation characteristics. It can be integrated into an RF system front-end that allows transmission and reception of signals at the same time and the same frequency.

Abstract: In this paper, a new type of microwave microfluidic sensor is proposed to detect and determine the dielectric properties of common liquids. The technique is based on perturbation theory, in which the resonant frequency and quality factor of the microwave resonator depend on the dielectric properties of the resonator. A microstrip split-ring resonator with two gaps is adopted for the design of the sensors (i.e., a double split-ring resonator). This resonator is both compact and planar, making it suitable for a lab-on-a-chip approach. Several types of solvents are tested with two types of capillaries to verify sensor performance. At 3 GHz, very good agreement is demonstrated between simulated and measured results.

Abstract: We propose an ultra-wideband plasmonic waveguide based on designer surface plasmon polaritons (DSPPs) with double gratings. In such plasmonic metamaterials, the DSPP waves in the region of lower frequencies of the dispersion curve can be tightly confined and hence effectively broaden the operating bandwidth. Based on such features, we design and fabricate a high performance DSPP filter, in which a transducer consisting of microstrip, slotline, and gradient corrugations is employed to feed electromagnetic energies into the plasmonic waveguide with high efficiency. The simulated and measured results on reflection and transmission coefficients in the microwave frequency demonstrate the excellent filtering characteristics such as low loss, wide band, and high square ratio. The high performance DSPP waveguide and filter pave a way to develop advanced plasmonic integrated functional devices and circuits in the microwave and terahertz frequencies.

Abstract: A low-cost single-layer wideband microstrip antenna array is presented for 60-GHz band applications. Each microstrip radiation element is fed by a modified L-shaped probe to enhance the impedance bandwidth. The proposed microstrip antenna array is excited by a novel coplanar waveguide (CPW) feed network, which not only has a simple structure by abandoning the traditional air bridges (wire bonds) above the CPW T-junctions but also provides pairs of broadband differential outputs. Experimentally, two 4 $\,\times\,$ 4 antenna arrays with different polarizations were designed and fabricated on the double-sided single-layer printed circuit board. The linearly polarized array exhibits an impedance bandwidth ( ${\hbox{SWR}}\leq 2$ ) of 25.5% and a gain of around 15.2 dBi. The circularly polarized array, employing the same CPW feed network to excite sequentially rotated circularly polarized elements, achieves an impedance bandwidth ( ${\hbox{SWR}}\leq 2$ ) of 17.8%, a 3-dB axial ratio bandwidth of 15.6%, and a gain of around 14.5 dBi.

Abstract: This communication presents a 3D frequency selective rasorber (FSR) with bandpass filtering response and wideband absorption characteristics. By loading an array of lumped resistors at one side of a microstrip-line based bandpass frequency selective structure (FSS), multiple resonators, including lossy resonators, are constructed. The bandpass performance with high selectivity is provided by resonators in the substrate region of the microstrip line. The absorption characteristic is obtained by the lossy resonators at the resistor-loaded side of the air region. All reflected waves at the resistor-loaded side can be effectively absorbed by appropriately choosing the resistance value. Physical mechanism of the FSR is analyzed with the aid of an equivalent circuit model and current distributions. As an example, a prototype of the designed FSR is fabricated and tested. Experimental results show that the insertion loss at the center frequency is 2.4 dB and a bandwidth of 114% for the absorption better than 10 dB in the upper rejection band is achieved under the normal incidence.

Abstract: This letter introduces a new technique of inkjet printing antennas on textiles. A screen-printed interface layer was used to reduce the surface roughness of the polyester/cotton material that facilitated the printing of a continuous conducting surface. Conducting ink was used to create three inkjet-printed microstrip patch antennas. An efficiency of 53% was achieved for a fully flexible antenna with two layers of ink. Measurements of the antennas bent around a polystyrene cylinder indicated that a second layer of ink improved the robustness to bending.

Abstract: A two-dimensional displacement and alignment sensor is proposed based on two open-ended transmission lines, each loaded with a split ring resonator (SRR). In this arrangement, the depth of resonance-induced notches in the reflection coefficients can be used to sense a displacement of the loading SRRs in two orthogonal directions. Since the operation principle of the sensor is based on the symmetry properties of SRR-loaded transmission lines, the proposed sensor benefits from immunity to variations in ambient conditions. More importantly, it is shown that in contrast to previously published metamaterial-inspired two-dimensional displacement and alignment sensors, the proposed sensor can be operated at a single fixed frequency. The concept and simulation results are validated through measurement.

Abstract: A novel compact microstrip diplexer for UMTS and WCDMA system is proposed in this letter. The diplexer is mainly composed of three dual-mode stub-loaded microstrip resonators. Two resonant modes of a stub-loaded resonator work together with one mode produced by the common stub-loaded resonator to form a passband or operating band of the proposed diplexer. Extensive study is then conducted on coupling scheme involved in various parts of the diplexer. In particular, the resonance of a resonator is employed to create a desired transmission zero in the stopband, thus improving the isolation between two passbands. The proposed diplexer is in final fabricated and measured. A good agreement between EM simulated and measured results evidently validates the proposed methodology.

Abstract: a b s t r a c t This paper proposes a two-dimensional alignment and displacement sensor based on movable broadside- coupled split ring resonators (BC-SRRs). As a basis for this sensor, a one-dimensional displacement sensor based on a microstrip line loaded with BC-SRRs is presented firstly. It is shown that compared to previously published displacement sensors, based on SRR-loaded coplanar waveguides, the proposed one-dimensional sensor benefits from a much wider dynamic range. Secondly, it is shown that with modifications in the geometry of the BC-SRRs, the proposed one-dimensional sensor can be modified and extended by adding a second element to create a high-dynamic range two-dimensional displace- ment sensor. Since the proposed sensors operate based on a split in the resonance frequency, rather than the resonance depth, they benefit from a high immunity to environmental noise. Furthermore, since the sensors' principle of operation is based on the deviation from symmetry, they are more robust to ambient conditions such as changes in the temperature, and thus they can be used as alignment sensors as well. A prototype of the proposed two-dimensional sensor is fabricated and the concept and simulation results are validated through experiment.

Abstract: Over the last years, a great number of substrate integrated circuits has been developed. These new circuits are a compromise between the advantages of classical waveguide technologies, such as high quality factor and low losses, and the advantages of planar circuits, such as low cost and easy compact integration. Although their quality factor and losses are better than for planar circuits, these characteristics are worse than in the case of waveguides, mainly due to the presence of the dielectric substrate. In order to improve the performance of the integrated circuits, a new methodology for manufacturing empty waveguides, without a dielectric substrate, but at the same time completely integrated in a planar substrate, is proposed in this work. A wideband transition with return losses greater than 20 dB in the whole bandwith of the waveguide allows the integration of the empty waveguide into the planar substrate so that the waveguide can be directly accessed with a microstrip line. Therefore, a microwave circuit integrated in a planar substrate, but at the same time with a very high quality factor (measured quality factor is 4.5 times higher than for the same filter in the substrate integrated waveguide), and very low losses is successfully achieved.

Abstract: Planar microwave angular displacement and angular velocity sensors implemented in microstrip technology are proposed. The transducer element is a circularly shaped divider/combiner, whereas the sensing element is an electric-LC resonator, attached to the rotating object and magnetically coupled to the circular (active) region of the transducer. The angular variables are measured by inspection of the transmission characteristics, which are modulated by the magnetic coupling between the resonator and the divider/combiner. The degree of coupling is hence sensitive to the angular position of the resonator. As compared with coplanar waveguide angular displacement and velocity sensors, the proposed microstrip sensors do not require air bridges, and the ground plane provides backside isolation.

Abstract: A dual-layer electromagnetic band-gap (EBG) mushroom structure is proposed in this paper. Based on the concept of slow-wave propagation, this structure can reduce the size of multi-element microstrip patch antennas (MPAs) by 61%. While the mushroom inner layer aids in the antenna miniaturization, the more compact upper layer acts as a band-stop filter further reducing the mutual coupling between the miniaturized patch antenna elements, which is otherwise not possible for a single-layer EBG structure. Various 2- and 4-element miniaturized MPAs are proposed for 2.5-GHz band applications, with antenna elements closely spaced at 0.5λ and low mutual coupling levels in the range of -28 dB to -50 dB. Furthermore, the achievable multiple-input-multiple-output (MIMO) channel capacities of these miniaturized multi-antennas are analyzed using the Kronecker channel model and tested in various propagation scenarios like the anechoic chamber, reverberation chamber and indoor. It is observed that the miniaturized MPAs can provide significant gains in MIMO capacity in all the signal scattering environments and are close to the theoretical limit of i.i.d. Rayleigh fading.

Abstract: In this paper, perfect magnetic conductor (PMC)-based packaging technique was used to improve the isolation performance among various microwave circuit components such as high-gain amplifier chains. In this approach, a periodic structure (such as metal pin rows) together with the ground plane of the substrate created a stopband for unwanted parallel plate or cavity modes as well as substrate modes, and thereby suppressed the problems of circuit resonances and related package phenomena. This paper describes two Ka-band amplifier chains that were tested with this new packaging technique. Firstly, a single amplifier chain was tested for maximum stable gain operation, and it was found that the stable gain of was achieved, whereas traditional metal wall package with RF absorber offered stable gain of 40 dB, thus showing significant isolation improvement. Secondly, two high-gain amplifier chains were placed side by side and their mutual isolation was tested. With the proposed gap waveguide packaging, a minimum isolation of 78 dB was achieved, whereas a complete metal shield provided a minimum isolation of only 64 dB over the band of interest.

Abstract: A wideband 2x2-slot element for a 60-GHz antenna array is designed by making use of two double-sided printed circuit boards (PCBs). The upper PCB contains the four radiating cavity-backed slots, where the cavity is formed in substrate-integrated waveguide (SIW) using metalized via holes. The SIW cavity is excited by a coupling slot. The excitation slot is fed by a microstrip-ridge gap waveguide formed in the air gap between the upper and lower PCBs. The lower PCB contains the microstrip line, being short-circuited to the ground plane of the lower PCB with via holes, and with additional metalized via holes alongside the microstrip line to form a stopband for parallel-platemodes in the air gap. The designed element can be used in large arrays with distribution networks realized in such microstrip-ridge gap waveguide technology. Therefore, the present paper describes a generic study in an infinite array environment, and performance is measured in terms of the active reflection coefficient S11 and the power lost in grating lobes. The study shows that the radiation characteristics of the array antenna is considerably improved by using a soft surface EBG-type SIW corrugation between each 2x2-slot element in E-plane to reduce the mutual coupling. The study is verified by measurements on a 4x4 element array surrounded by dummy elements and including a transition to rectangular waveguide WR15.

Abstract: We propose a broadband and high-efficiency transition from a microstrip line to a conformal surface plasmon (CSP) waveguide that is made of an ultrathin corrugated metallic strip, to transform the guide wave into a spoof surface plasmon polariton (SPP) in the microwave region. The transition consists of three parts: a convertor which converts the direction of the electric field from perpendicular to parallel to the strip, a matching area with gradient corrugations and a flaring metallic line to match both the momentum and impedance, and a CSP waveguide to support the SPP waves. A back-to-back transition sample is fabricated using the proposed method. Experimental results of S parameters and near-field distributions verify the excellent performance of the sample to transform guided waves to SPPs and transmit SPP waves in a wide band. The sample exhibits low energy loss when the CSP waveguide is bent or even twisted. The proposed transition may have potential applications in integrating conventional microwave devices with the SPP devices.

Abstract: We demonstrate direct quadrature modulator and demodulator monolithic microwave integrated circuits for future terahertz communications at 300 GHz based on the quadrature phase-shift keying (QPSK) modulation format. For the modulating and demodulating signal, we employed half-Gilbert cell mixers, which provide balanced signaling and moderate performance in conversion efficiency with a simple circuit configuration. In order to maintain the balance performance of the modulator and demodulator, passive baluns and couplers are implemented with thin-film microstrip lines, which exhibit less insertion loss than inverted microstrip lines (IMSLs), while the active mixers are based on IMSLs for short interconnections. The half-Gilbert-cell mixers have a wide enough operation bandwidth for high-throughput communications of more than 10% at 300 GHz. According to the static constellation of the modulator, imbalance is expected to be less than approximately ±0.6 dB ∠4°. A nonchip back-to-back experiment was conducted at up to 60 Gb/s, and 50-Gb/s operation was verified with a low bit error rate on the order of 10-8 or less. The results demonstrate that the QPSK modulation scheme can be applied to double the data rate at terahertz frequencies.

Abstract: This paper presents a novel method for designing Gysel power dividers with arbitrary power-division ratios as well as filtering responses. A coupling structure is utilized to replace the quarter-wavelength microstrip line in the power divider. By altering the coupling strength, the power ratio can be arbitrarily controlled with the highest ratio of 10:1. Furthermore, the coupling structures enable the filtering function of the power divider. Two transmission zeros are created near the passband edges, resulting in high-selectivity quasi-elliptic responses. Theoretical analysis is carried out, and closed-form equations are derived based on circuit theory and transmission line theory. For demonstration, three filtering Gysel power dividers are implemented with different power-division ratios (1:1, 3:1, and 10:1). In all of these circuits, power splitting and bandpass filtering functions are observed with good performance. Comparisons of the measured and simulated results are presented to verify the theoretical predications.

Abstract: A new wideband transition from microstrip line to substrate integrated waveguide (SIW) is introduced. Unlike most transitions that show reduced return loss over significant parts of a regular waveguide band, the presented configuration achieves return losses better than 30 dB in standard waveguide frequency bands from X to E. The new aspect of this transition is the addition of two vias to the widely used microstrip taper transition. Moreover, the influence of the substrate height is demonstrated. The results in each frequency band are compared with the data for the regular microstrip taper alone. A design formula for the placement of the vias and taper dimensions is presented and demonstrated to provide excellent results. The structures are simulated and optimized with CST Microwave Studio. Measurements performed on a Ku-band back-to-back prototype transition demonstrate a minimum return loss of 26.05 dB and maximum insertion loss of 0.821 dB over the entire Ku-band, thus validating the design approach.

Abstract: The gap waveguide technology was recently introduced as an alternative to hollow waveguides and substrate integrated waveguides for millimeter-wave applications. This paper presents the design of a 4 x 4 planar dual-mode horn array with low loss corporate feed network realized by using an inverted microstrip gap waveguide. The dual-mode horns are compact and designed to reduce the power losses in grating lobes. It is because the diameters of the horn apertures are larger than two wavelengths to allow more space for the feed network and thereby lower conductive losses. The measurements show very good agreement with simulations, with 10% bandwidth of the return loss, 25 dBi realized gain and about 60% aperture efficiency.

Abstract: Two new wideband four-way out-of-phase slotline power dividers are proposed in this paper. The half-wavelength slotlines are employed to construct the presented compact power dividers. Based on the proposed power-dividing circuit, a four-way power divider is implemented with compact size and simple structure. To obtain high isolation among the four output ports and good output impedance matching, another four-way out-of-phase slotline power divider with improved isolation performance is designed by introducing an air-bridge resistor and two slotlines with isolation resistors. The simulated and measured results of the proposed power dividers demonstrate reasonable performance of impedance matching, insertion loss, amplitude balancing, and isolation among the output ports.

Abstract: A broadband dual circularly polarized patch antenna with wide beamwidth is presented. The patch is excited by four cross slots via a microstrip line with multiple matching segments underneath the ground plane. The four cross slots and the multiple matching segments are optimized simultaneously to obtain the best performance. Measurements show that the antenna has 10-dB return-loss bandwidth of 24%, 10-dB isolation bandwidth of 19%, 3-dB axial-ratio bandwidth of 16%, and beamwidth of 110 ° . Moreover, the single feed makes it a good candidate for array design.

Abstract: In this paper, we propose an optimal design procedure for the focused antenna array based on the maximization of power transmission efficiency between two antennas. In the optimization process, the focused antenna array to be designed is used as the transmitting antenna and a testing antenna is used as the receiving antenna, positioned in a desired area to be focused. An optimal excitation distribution can thus be obtained for the antenna array to be focused in the desired area. Three 4 × 4 microstrip patch arrays with different focusing distances are designed to validate the optimization technique and demonstrate the focusing capabilities when the antenna array size is fixed. A 6 × 6 microstrip patch array is also built to reveal how the antenna size enhances the focusing effect.

Abstract: Compact fractal boundary microstrip antenna is proposed for circular polarization (CP). By replacing the sides of a square patch with asymmetrical prefractal curves, two orthogonal modes are excited for CP operation. The structure is asymmetric along the principal axes (x, y). The indentation parameter of the fractal boundary curve is optimized to design compact CP antennas. Experimental results show that 10-dB return loss and 3-dB axial-ratio bandwidths of the proposed fractal boundary Ant 2 are 162 and 50 MHz, respectively, at operating frequency of around 2540 MHz. Results show that an excellent CP is achieved with a single probe feed, besides reduction in the antenna size by applying fractal boundary concept.

Abstract: A microstrip-fed compact multi-band slot antenna using a single split-ring resonator (SRR) is proposed. The SRR acting as a loading element introduces multiple lower-order resonances in the antenna, which can be controlled by varying the SRR's dimensions as well as its position with respect to the arm of the slot, without altering the geometry of the radiating slot. The concept is validated by a full-wave simulation study and by measurement on a fabricated prototype. The proposed antenna has satisfactory gain and monopolar radiation patterns in all the operating bands.

Abstract: Defected ground structure (DGS)-integrated rectangular microstrip patch has been experimentally investigated with an aim to improve polarisation purity in radiated fields. Width to length ratio (aspect ratio) of a patch attributes different characteristic features. Therefore present experimental studies have been executed for four different aspect ratio values like 1.6, 1.3, 1.0 and 0.8. Folded defects have been employed in H-plane. Possibility of achieving high polarisation purity (over 25 dB isolation between co- to cross-polarised fields) with improved impedance bandwidth has been demonstrated. The variation in XP fields as a function of the patch aspect ratio has been investigated and a strong physical insight into the modal fields with and without DGS has been developed.

Abstract: Compact and high isolation microstrip filtering power dividers (PDs) have been presented in this letter. In order to effectively reduce circuit area, the compact folded net-type resonator is selected to build up the resonator-based PDs with a filtering response. The highly symmetric structure of the divider provides not only a good in-band isolation performance, but also a low magnitude imbalance and phase imbalance. For the demonstration, two design examples of in-phase and out-of-phase PDs with second-order Chebyshev filtering response have been designed and fabricated with microstrip technology. As a result, both dividers have an extremely small size of 0.19 λg by 0.19 λg. The measured results are in good agreement with the simulated predictions, showing that better than 30 dB isolation within the whole passband is achieved with a tradeoff of the bandwidth.

Abstract: A novel capacitive-coupled probe-fed circularly polarized (CP) microstrip antenna with coplanar parasitic ring slot patch has been proposed for the International Maritime Satellite (INMARSAT) and the Global Positioning System (GPS) applications. The key feature of the design is employing capacitive-coupled four-probe feeds to increase impedance bandwidth and adopting coplanar parasitic ring slot patch to enhance CP bandwidth. The feed network is compact and neat using only T-junction power dividers and phase shifters with shunted short-circuited microstrip stubs for lightning protection. The characteristics of the proposed antenna have been studied by simulation and experiment. The final antenna can produce a ${ S}_{11} 14 dB impedance bandwidth of nearly 27%, a 3-dB axial-ratio bandwidth of nearly 16%, and a gain of higher than 8 dB in the whole maritime satellite communication work band.

Abstract: The design and performance of a novel octave-bandwidth highly-directive half Maxwell fish-eye (HMFE) lens antenna are presented in superextended C band. The three-dimensional (3D) HMFE lens is implemented by gradient-refractive-index (GRIN) metamaterials and launched by an omnidirectional planar microstrip trapezoid printed monopole from the perspective of high integration, light weight and low profile. A new approach is proposed to design the GRIN metamaterial element in terms of a deep subwavelength feature by incorporating fractal geometry. Numerical and experimental results coincide well, showing that the lens enables a considerable gain enhancement of the monopole near 10 dB across a frequency range of 3 to 7.5 GHz while without significantly affecting the cross-polarization patterns and impedance matching. The near-field free-space measurement is also performed in an octave to afford a physical insight into the high gain, which is attributable to the accurate conversion of quasi-spherical waves to plane waves. Moreover, the truncation and homogenization effects of the lens on the antenna directivity are investigated to illustrate the fundamental mechanisms and afford the design guidelines.