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This paper introduces a transmission line model analysis for dielectric-loaded self-resonant structures (SRSs). Equivalent circuit models, besides their fast computational time, usually offer some insights into the underlying physics of the electromagnetic structures, hence simplifying and improving the design and optimization criteria of these structures. A systematic approach for deriving impedance expressions of SRS on and within multi-layer dielectric media is presented. The process of extracting equivalent circuit parameters from impedance expressions is explained. The trends of equivalent circuit parameters are studied with different dielectric medium properties and various angles of incidence for an SRS with multiple resonant states highlighting the effect of changing the physical and electrical dimensions on the equivalent circuit parameters. The transmission line model responses are compared to full-wave simulation results, and model accuracy is assessed using the mean absolute error criteria. Furthermore, a new simplified method for calculating the forward transmission coefficient of an SRS within multi-layer media is proposed. This expression offers fundamental insights into the factors contributing to the $S_{21}$ response. Finally, experimental measurements for SRS embedded in a multi-layer dielectric media inside a waveguide is conducted and compared to the equivalent model with very good agreement.

5G technology provides users with a significantly better experience and is a key enabler of massive connectivity between people-to-people and people-to-machines as well as machines-to-machines. The low -latency transmission promised by 5G opens up the possibilities for implementing remote healthcare, self -driving cars, and virtual reality/augmented reality. 5G technology includes fully digital beamforming, multiple access technologies, and massive multiple-input-multiple-output (MIMO) to deliver higher data rates, higher bandwidth, and lower latency. Antenna along with other microwave systems plays an important role in achieving key features of 5G technology. Unlike a single base station antenna array technology, in 1990, the modern antenna system has evolved from a passive to an active antenna system with integrated radios and digital baseband processing units.

This study describes an experimental investigation into improving the detectability of self-resonant structures. Passive self-resonant structures have been used as an alternative to active devices in many applications including radio-frequency identification (RFID) due to their long lifetime and low manufacturing cost. They can be used to store information in terms of the presence or the absence of resonance peaks at a given frequency window. This work investigates practical validation of gain enhancement techniques, aiming to improve the resonance characteristics of passive RFID tags to increase their read range and allow them to operate in highly lossy mediums. Two passive resonator designs are studied: a uniform cross-shaped resonator and a folded (gammadion) cross having a physical aperture 45% less than that of the uniform cross. Arrays of elements have been used to improve the detectability of the structure. Measurement settings have also been investigated in order to achieve higher levels of detectability.

In this paper, a novel tunable tri-band antenna\ud is presented for concurrent, multi-band, single chain radio\ud receivers. The antenna is manufactured on a 50×100 mm FR4\ud printed circuit board (PCB), and is able to provide three concurrent,\ud independently tunable operating bands covering a frequency\ud range from 600 MHz to 2.7 GHz. The antenna performance\ud is investigated for both numerical and experimental methods\ud when using, first, varactor diodes and, second, digitally tunable\ud capacitors (DTCs) to tune frequencies, which shows the antenna\ud gain can be improved by up to 2.6 dBi by using DTCs. A\ud hardware-in-the-loop test-bed provides a system level evaluation\ud of the proposed antenna in a direct RF digitized, concurrent,\ud tri-band radio receiver. By measuring the receiver’s error vector\ud magnitude, we demonstrate sufficient isolation between concurrent\ud bands achieving 30 MHz of aggregated bandwidth as well\ud as strong resilience to adjacent blockers next to each band. The\ud data reported in this article are available from the ORDA digital\ud repository (https://doi.org/10.15131/shef.data.5346295).

Novel arrangements of active frequency selective surfaces (AFSSs) with integrated voltage control wiring are proposed for bistate (transparency/reflectance) operation at specified frequencies. The AFSSs are comprised of passive arrays of intertwined patterns of slots in a conductor screen and an active dipole array with p-i-n diodes placed on either the same or opposite sides of a thin dielectric substrate. Simulation and measurement results show that such AFSSs exhibit good isolation (~15 dB) between the translucency and reflection states at normal incidence that slightly decreases at oblique incidence. The proposed AFSSs maintain the high angular and polarization stability over broad fractional bandwidths inherent to the constituent periodic arrays of intertwined conductor patterns with substantially subwavelength unit cells. The merits of these AFSS arrangements also include resilience to parasitic effects of real switches, whose insertion loss in the ON state only enhances the AFSS ON/OFF isolation. Such AFSSs are essential elements for reconfiguring and controlling the electromagnetic architecture of buildings.

A low-profile quad-band loaded-slot antenna with independently controllable frequency bands is presented in this paper. Four slots are carefully designed to generate four separate frequency resonances, which were independently tuned by employing varactor diodes in each slot. The proposed concept was simulated using a full-wave solver and validated through measurements. The results show that each of the four bands can be independently controlled by altering the bias voltage of the respective varactor diode. Overall, the measured frequency tuning of the 1st, 2nd, 3rd, and 4th bands were 450–750 MHz, 600–1200 MHz, 1200–2100 MHz and 2100–2700 MHz, respectively. The proposed antenna has good potential for applications in the concurrently tunable multiband radio systems, including 5G.

A low-profile tri-band slot antenna with enhanced tuning range is presented in this paper. Tri-band frequency agility is obtained using three varactor-loaded slots on a planar structure. The tuning range of the proposed antenna is enhanced by suppressing the higher-order resonance modes, especially by controlling the surface currents and creating a fictitious ground on the antenna. Following full-wave simulations, a prototype was developed and measured to validate the concept. Results show a tuning range from 600MHz to 3300MHz, which is a 600 MHz increase in the overall tuning range than previously reported.

This invited paper considers a key next step in the design of radio architectures aimed at supporting low energy consumption in 5G heterogeneous radio access networks. State-of-the-art mobile radios usually require one RF transceiver per standard, each working separately at any given time. Software defined radios, while spanning a wide range of standards and frequency bands, also work separately at any specific time. In 5G radio access networks, where continuous, multiband connectivity is envisaged, this conventional radio architecture results in high network power consumption. In this paper, we propose the novel concept of a concurrent multiband frequency-agile radio (CM-FARAD) architecture, which simultaneously supports multiple standards and frequency bands using a single, tunable transceiver. We discuss the subsystem radio design approaches for enabling the CM-FARAD architecture, including antennas, power amplifiers, low noise amplifiers and analogue to digital converters. A working prototype of a dual-band CM-FARAD test-bed is also presented together with measured salient performance characteristics.

This work presents an analysis of a reconfigurable self-resonant structure using an equivalent circuit model. Reconfiguring the electrical length of the structure results in uniquely identifiable changes to its forward and reflected frequency response. Equivalent circuit models are known to reduce computational time drastically and to provide an insight into the physics of the electromagnetic problem. Equivalent circuit parameters have been calculated for different states of the self-resonant structure. The responses of these equivalent circuits are compared to a full-wave simulation with very good agreement.

In this paper, a switchable textile communication system working at 2.45 GHz ISM band is presented and studied for different locations within a realistic on-body environment. A 3D laser scanner is used to generate a numerical phantom of the measured subject to improve the accuracy of the simulations which are carried out for different body postures. For the off-body communications, the system is acting as an aperture coupled microstrip patch antenna with a boresight gain of 1.48 dBi. On-body communication is achieved by using a textile stripline, which gives approximately 5 dB transmission loss over 600 mm distance. The system is switched between on and off-body modes by PIN diodes. Common issues, such as shape distortion and body detuning effects which the textile antenna may experience in realistic use are fully discussed. Robust antenna performance is noted in the on-body tests, and an additional 3 dB transmission coefficient deduction was noticed in the most severe shape distortion case.

A new type of multiband shark-fin antenna printed on 1.6 mm thick FR4 for automotive applications is presented. The antenna is mounted over a 1 m × 1 m-metal ground plane which represents a car’s roof. The overall size of the antenna is 57 mm × 40 mm × 1.6 mm which is λ/6.5 × λ/9 × λ/231.5 at the lowest frequency of operation of 0.810 GHz. The antenna is in the form of a single-sided PCB and has a measured gain of 0–6.7 dBi over the entire range of 0.810–5.84 GHz covering the mobile telephone bands (GSM, DCS and PCS), both GPS bands and Bluetooth, WiFi and V2X bands on −6 dB measured reflection coefficient criteria. The measured co-polar radiation patterns in the azimuth (XZ-plane) and elevation (YZ-plane) planes depict omnidirectional radiation behaviour.

A miniature platform tolerant antenna is presented which is suitable for low frequency applications. A Split Ring Resonator (SRR) antenna loaded with lumped capacitances is proposed, and the antenna is compact, low proflle and easy to fabricate. It has a maximum dimension of ‚/9 and i10dB bandwidth of 1%. Miniaturized artiflcial magnetic conductor surfaces (AMCs) are designed using capacitance loaded metal patches with individual elements measuring just ‚=70. Placing the SRR above the AMC improves the bandwidth to between 1.5 and 3.5% dependent on the overall size of the AMC and produces a platform tolerant antenna measuring 0.11‚£0:17‚£0:019‚. The performance of the antenna over an AMC with and without vias is studied and discussed. The AMC mounted antenna performances in free space and over a ground plane are also compared.

The performance of a 2 × 2 microstrip patch array under crumpling conditions is presented. The antenna array was manufactured on textile material, and three cases of crumpling resembling real applications are discussed. The performance of the individual patch elements correlate with the overall performance of the array. Frequency shifts of up to 6% are measured for the array when crumpled, the boresight gain is reduced by up to 6 dB with significant changes in directivity and efficiency, up to 20%. The overall efficiency for the textile array increases for crumpling in one plane but falls in the other.

A compact multiband tunable PIFA system using a varactor as an active tuning component is presented covering 88-2175 MHz. The antenna was designed to cover the mobile telephone bands (GSM850, 900, 1800, 1900, UMTS) by controlling the value of the capacitance across the gap in the slotted PIFA and two broadcast bands, FM radio (76-108 MHz) & mobile television DVB-H (470-702 MHz), were added by designing an appropriate matching circuit into the system. The antenna measured λ/15 × λ/43 × λ/19 at 470 MHz and was easily integrated into a small mobile phone handset. Simulations are verified by measurements.

An electrically small tunable split ring resonator antenna is presented. The resonance of the antenna is dependent on the resonance of the split rings. By loading the split ring with varactor diodes, the antenna operating frequency can be reduced and tuned. By applying a DC voltage of 1 to 9 V, the antenna is capable of tuning within the frequency range of 365 MHz to 500 MHz with the measured reflection coefficient of ${-}10$ dB. The antenna is compact and low profile. At the operation frequency of 365 MHz, it measures $0.095 \lambda \times 0.046 \lambda \times 0.001\lambda$ , where $\lambda$ is the free space wavelength.

An automated, low-field disturbance probe positioning system for measuring 3-D electric field distributions inside vehicles or similar resonant environments is described. Correlations between measured and simulated electric field distributions for a simple rectangular cavity demonstrate that the probe positioner has little impact on the measured field levels. Comparisons between measurements and simulations for a real vehicle indicate that the predicted field population distributions are within the estimated uncertainties of the measurements.

In order to meet the high data rate, low latency and high energy efficiency requirements, the future radio units must utilise the frequency spectrum available at certain geographical location efficiently with a minimum amount of hardware. This requires frequency agility and concurrent multi-standard operation capabilities at the base and mobile terminals. To utilise the TV white spaces for mobile and wireless communication and enable their carrier aggregation with sub GHz LTE bands, this paper presents a single-chain, reconfigurable, RF digitising, dual- band receiver comprised of a tunable dual-band antenna, a reconfigurable digital down converter, a baseband processing unit and wideband LNA and ADC in the form of an oscilloscope. The presented dual-band receiver is tested through a hardware- in-the-loop test-bed which shows that up to 20 MHz aggregate bandwidths can be achieved. The receiver is able to provide equivalent error vector magnitude (EVM) performance across a wide range of frequencies avoiding any inter-band interference.

Frequency selective surfaces have been widely used to control radio propagation within buildings. In this paper we consider the application of a sparse FSS for improved coverage of radio signals between two isolated adjacent rooms. The scattering lobes which will occur from a sparse FSS design can therefore be exploited to alter the coverage pattern within the second room.

Understanding radio channel path characteristics is a very important factor for ensuring quality of service and high data rates for indoor wireless communication systems. Obstacles such as human occupants, doors, walls, windows, etc. all have a significant impact on received signals strengths and will contribute to fading, multipath and shadowing effects. This paper studies the effects of changes within a building, such as opening and closing doors, and the movement and number of human occupants, on indoor signal propagation at 5.8 GHz. The effects of different obstacles inside the rooms are compared and highlight which has the most significant effect on the received electric field strength.

Among the different assets tracking solutions Radio Frequency IDentification RFID is a promising technology for in-car communications due to its relatively low cost deployment and real time monitoring and processes. In this paper, a hidden, miniature, narrow band antenna of dimensions 0.18λ×0.20λ×0.005λ is presented which is easy to manufacture using conductive paint sprayed over a car body. The design relies on a resonator, a negative image of the radiator for improved efficiency when in close proximity to ground planes and is suited for use in radio frequency identification networks using the unlicensed RFID band (865.6- 867.6 MHz) is described. The term miniature comes from the fact that the antenna is physically small given the low proximity and insensitivity to the ground plane which facilitates a possible coating layer over the antenna for hidden applications.

An independently tunable low-profile dual-band high-impedance surface (HIS) antenna system is presented which employs a single layer cross bow-tie dual-band tunable HIS ground plane with a printed wideband monopole antenna designed to cover the UHF band. The unit cell of the proposed HIS ground plane is 19.71 mm by 19.71 mm with thickness of 1.74 mm. Simulations of the HIS ground plane have illustrated that either single or dual-band reconfigurable reflection phase resonances can be achieved between 0.95 GHz and 2.45 GHz by controlling C1 (4.55 pF~0.6 pF) and C2 (4.55 pF~0.6 pF) independently which represent the two groups of varactor diodes mounted on the surface elements of the HIS. Simulations with the wideband antenna 4 mm above the HIS ground plane illustrate an effective tunable operational bandwidth from 1.07 GHz to 2.75 GHz with a dual-band capability. Parametric studies indicate the radiation efficiency of the system increases as the antenna height is increased and the diode resistance is reduced. The radiation efficiency also increases with the frequency in general over the tunable frequency range and with the optimum parameters typical values from -4 dB to -2 dB can be achieved. The proposed HIS ground plane and the wideband antenna have been fabricated with Toshiba 1SV245 varactor diodes and it has shown an effective tunable bandwidth from 0.9 GHz to 2.8 GHz approximately with the dual band capability.

A response variation (RV) element is introduced to control the consistency of an adaptive wideband beamformer's response over the frequency range of interest. By incorporating the RV element into the linearly constrained minimum variance (LCMV) beamformer, we develop a novel linearly constrained beamformer with an improved output signal-to-interference-plus-noise ratio (SINR), compared to both the traditional formulation and the eigenvector based formulation, due to an increased number of degrees of freedom for interference suppression. In addition, two novel wideband beamformers robust against look direction estimation errors are also proposed as a further application of the RV element. One is designed by imposing a constraint on the RV element and simultaneously limiting the magnitude response of the beamformer within a pre-defined angle range at a reference frequency; the other one is obtained by combining the RV element and the worst-case performance optimization method. Both of them are reformulated in a convex form as the second-order cone (SOC) programming problem and solved efficiently using interior point method. Compared with the original robust methods, a more efficient and effective control over the beamformer's response at the look direction region is achieved with an improved overall performance.

A class of adaptive beamforming algorithms with real-valued coefficients is proposed based on the uniform linear array structure by introducing a preprocessing transformation matrix. It is derived from the beamformer with a minimum mean square error (MSE) or a maximum output signal-to-interference-plus-noise ratio (SINR), depending on the specific design criteria. The key parameter of the transformation matrix takes different values for different beamforming scenarios and three representative examples are studied: the linearly constrained minimum variance beamformer (and the generalized sidelobe canceller), the reference signal based beamformer, and the class of blind beamformers based on the constant modulus algorithm. Its advantage is twofold: 1) with real-valued coefficients, the computational complexity of the overall system is reduced significantly; 2) a faster convergence speed is achieved and given the same stepsize, the system arrives at a lower MSE (or a higher output SINR).