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Employing electromagnetic fields (EMFs) in new wireless communication and sensing technologies has substantially increased the level of human exposure to EMF waves. This paper presents a useful insight into the interaction of electromagnetic fields with biological media that is defined by the heat generation due to induced currents and dielectric loss. The specific absorption rate (SAR) defines the heating amount in a biological medium that is irradiated by an electromagnetic field value. The paper reviews the radio frequency hazards due to the SAR based on various safety standards and organisations, including a detailed investigation of previously published work in terms of modelling and measurements. It also summarises the most common techniques utilised between 1978 and 2021, in terms of the operational frequency spectrum, bandwidth, and SAR values. The European Union’s Horizon 2020 innovation programme under grant agreement H2020- MSCA-ITN-2016 SECRET-722424 and the U.K. Engineering and Physical Sciences Research Council (EPSRC) under grant EP/E022936/1 supported this research. This study is also sponsored by the FCT/MEC through national funds and, where applicable, co-financed by the ERDF under the PT2020 Partnership Agreement under the UID/EEA/50008/2020 project. This study is part of the POSITIONII project, which is funded by the European Commission’s Joint Undertaking under the grant number Ecsel-7831132-Postitio-II-2017-IA. Moreover, this paper is also partially funded by British Council “2019 UK-China-BRI Countries Partnership Initiative” programme, with project titled “Adapting to Industry 4.0 oriented International Education and Research Collaboration”. In addition, this project has received funding from Universidad Carlos III de Madrid and the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant 801538.

This study presents an underground subsurface wireless sensor for drainage infrastructure water level monitoring. It operates from 800 to 2170 MHz to cover the required GSM850/900, GSM1800/1900 and Universal Mobile Telecommunications System (UMTS) bands. The system consists of a wideband antenna, transceiver, data acquisition unit and an ultrasonic sensor. The proposed antenna is a three-dimensional inverted double F antenna and has an envelope size of 90 × 63.5 × 32 mm 3 , which is acceptably small for a cramped subsurface passageway environment. The antenna design was developed using software simulation to optimise its key parameters of return loss and radiation pattern, these were evaluated both in free space and in the partially underground environment. The design developed was then realised in hardware and tested in a representative subsurface location: a utility manhole chamber. It was found that the location of the antenna in the chamber had a significant effect on its performance, but a location that was acceptable for operational purposes was found by experiment. The overall system, including a transceiver, was demonstrated to operate satisfactorily for utility monitoring purposes, including acceptable levels of path loss for communication with mobile communication base stations.

A closely packed wideband multiple-input multiple-output (MIMO)/diversity antenna (of two ports) with a small size of less than 18.5 mm by 18.5 mm is proposed for mobile communication applications. The antenna can be orthogonally configured for corner installation or by placing it on a back-to-back structure for compact modules. To enhance the isolation and widen the bandwidth, the antenna is structured with multiple layers having differing dielectric constants. The feeding through a via significantly reduces the ground waves. A multi-fidelity surrogate model-assisted design exploration method is employed to obtain the optimized antenna geometric parameters efficiently. The antenna design was investigated using electromagnetic simulation and a physical realization of the optimal design was then created and subjected to a range of tests. The specific parameters investigated included reflection coefficients, mutual coupling between the input ports, radiation patterns, efficiency and parameters specific to MIMO behavior: envelope correlation coefficient and pattern diversity multiplexing coefficient. It was found that the antenna has an impedance bandwidth of approximately 4 GHz, mutual coupling between input ports of better than &minus, 18 dB and an envelope correlation coefficient of less than 0.002 across the operating band. This makes it a good candidate design for many mobile MIMO applications.

A proposed method for achieving active load-modulation technique without using a quarter-wavelength transmission line is discussed and evaluated. The theoretical analysis shows that the active load-modulation can be achieved without using a quarter-wavelength line, where the main amplifier sees a low impedance when the input signal level is low, and this impedance increases in proportion to the amount of current contributed from the peaking amplifier. The peaking amplifier sees an impedance decreasing from infinity to the normalized impedance. To validate the method, a circuit was designed, simulated and fabricated using two symmetrical gallium nitride (GaN) transistors (6 W) to achieve a peak power of 12 W and 6 dB output backoff efficiency. The design operates with 400 MHz bandwidth at 3.6 GHz and showed an average efficiency of 50% at 6 dB backoff and an efficiency of 75% at peak power. The designed circuit was tested with CW and modulated signals, the amplifier showed an Adjacent Channel Power Ratio (ACPR) of 31–35.5 dB when tested with a wideband code division multiple access signal of 6 dB peak-average-power ratio (PAPR) at 35.5 dBm average power. Additional 20 dB of linearity improvement was achieved after adding a lineariser.

The design and implementation of an asymmetrical Doherty power amplifier are discussed, where two Cree GaN High Electron Mobility Transistors (HEMTs) devices are used for designing an asymmetrical Doherty power amplifier to achieve saturated power of 48 dBm and optimal back-off efficiency of 8 dB in the frequency band of 3.3&ndash, 3.5 GHz. Rogers RO4350B material is used as a substrate material, a back-off of 8 dB was achieved with an average gain of 10 dB. Load-pull data are an important tool for determining the optimum load impedance that the transistor needs to see. Additionally, the measured efficiency was 50% when the designed amplifier was tested by a modulated signal of 8 dB peak-to-average-power ratio when the average output power was 40 dBm. At the same time, the linearity of the designed amplifier was measured and found 31.8 dB which can be improved using a digital pre-distorter. The gain phase measurement can be used as an indicator for compensating the phase difference between the two cells.

Power amplifiers in modern and future communications should be able to handle di erent modulation standards at di erent frequency bands, and in addition, to be compatible with the previous generations. This paper reviews the recent design techniques that have been used to operate dual-band amplifiers and in particular the Doherty amplifiers. Special attention is focused on the design methodologies used for power splitters, phase compensation networks, impedance inverter networks and impedance transformer networks of such power amplifier. The most important materials of the dual-band Doherty amplifier are highlighted and surveyed. The main problems and challenges covering dual-band design concepts are presented and discussed. In addition, improvement techniques to enhance such operations are also exploited. The study shows that the transistor parasitic has a great impact in the design of a dual-band amplifier, and reduction of the transforming ratio of the inverter simplifies the dual-band design. The o set line can be functionally replaced by a P-network in dual-band design rather than T-network.

Potential demodulating ability in biological tissue exposed to radio-frequency (RF) signals intrinsically requires an unsymmetrical diode-like non-linear response in tissue samples. This may be investigated by observing possible generation of the second harmonic in a cavity resonator designed to have fundamental and second harmonic resonant frequencies with collocated antinodes. Such a response would be of interest as being a mechanism that could enable demodulation of information-carrying waveforms having modulating frequencies in ranges that could interfere with cellular processes. Previous studies have developed an experimental system to test for such responses: the present study reports a circuit model to facilitate calibration of any non-linear RF energy conversion occurring within a test-piece inside the cavity. The method is validated computationally and experimentally using a well-characterised non-linear device. The proposed model demonstrates that the sensitivity of the measurement equipment plays a vital role in deciding the required input power to detect any second harmonic signal, which is expected to be very weak. The model developed here provides a lookup table giving the level of the second harmonic signal in the detector as a function of the input power applied in a measurement. Experimental results are in good agreement with the simulated results.

This paper examines the benefits and drawbacks of these services, reviewing differences in infrastructure, power requirements, sensing devices, and other factors. Technologies covered include Radio Frequency Identification, GSM, GPS, A-GPS, Smart Antennas, Distributed Antenna Systems, Localization by Cell-ID, Localization by Prediction (Dead Reckoning method), Angle of Arrival (AOA), Localization by Finger Printing, Localization by Time of Arrival (TOA), Localization by Observed Time Difference of Arrival (TDOA), and Hybrid Localization-based AOA-TOA.

In this work, miniaturised tunable two-antenna multiple-input–multiple-output (MIMO) systems composed of printed F-slot shaped are developed to operate in the global positioning system (GPS), personal communication system (PCS), distributed control system and Universal Mobile Telecommunications System bands. The two-element MIMO antenna occupies a volume of 50 × 37.5 × 1.6 mm3, and is printed on an FR4 substrate. Initially, the frequency tunability of the MIMO antennas was verified by lumped capacitors with values between 0.75 to 2.75 pF to achieve a tuning range from 1.55 to 2.07 GHz while the low mutual coupling between the radiators was accomplished by adding an I-shaped branch to a cut-away ground plane. The two antennas are then loaded with varactors to simultaneously achieve miniaturisation and tunability. Simulation and measurement results demonstrate the successful implementation of a tunable MIMO with coupling reduction mechanism for a portable handheld wireless transceiver. The channel capacity of the proposed antenna is investigated and found to be close to that of a un-correlated system with efficient diversity in which the mutual coupling across the full bandwidth was better than −13 dB. Owing to the compact size and ease of manufacture, the proposed antennas can be a promising solution for adaptive MIMO systems in handheld devices.

This article presents a printed crescent-shaped monopole MIMO diversity antenna for wireless communications. The port-to-port isolation is increased by introducing an I-shaped conductor symmetrically between the two antenna elements and shaping the ground plane. Both the computed and experimental results confirm that the antenna possesses a wide impedance bandwidth of 54.5% across 1.6-2.8 GHz, with a reflection coefficient and mutual coupling better than -10 and -14 dB, respectively. By further validating the simulated and the measured radiation and MIMO characteristics including far-field, gain, envelope correlation and channel capacity loss, the results show that the antenna can offer effective MIMO/diversity operation to alleviate multipath environments. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:348-359, 2014.

This study presents a miniaturised multiple input and multiple output /diversity antenna which is suitable for high data-rate communication systems such as mobile ultra-wideband (UWB). This antenna assembly comprises two identical planar inverted-F antennas, a T-shaped structure connecting them and a finite ground plane. The T-shaped structure improves the impedance matching and suppresses the mutual coupling between the antenna elements over a wider bandwidth than previously reported. The compact envelope dimension of this antenna is 50 × 90 × 7.5 mm3. Theoretical and experimental S-parameters are illustrated for this antenna that fully cover the UWB operating frequency band of 3.1–10.6 GHz, with a reflection coefficient and mutual coupling better than −10 and −20 dB, respectively. Acceptable agreement is obtained between computed and measured radiation patterns, gains, envelope correlation coefficient and channel capacity loss. The proposed antenna is an attractive candidate to provide pattern diversity and enhance channel capacity in a rich scattering environment.

A miniaturized modified planar inverted-F antenna (PIFA) is presented and experimentally studied. This antenna consists of a planar rectangular monopole top-loaded with a rectangular patch attached to two rectangular plates, one shorted to the ground and the other suspended, both placed at the optimum distance on each side of the planar monopole. The fabricated antenna prototype had a measured impedance bandwidth of 125%, covering 3 to 13 GHz for reflection coefficient better than -10 dB. The radiator size was 20×10×7.5 mm3, making it electrically small over most of the band and suitable for incorporation in mobile devices. The radiation patterns and gains of this antenna have been cross-validated numerically and experimentally and confirm that this antenna has adequate characteristics for short range ultra-wideband wireless applications.

This study presents two multiband planar printed monopole antennas with an optimised size of 57×37.5×0.8 mm3 for mobile/wireless and low-band ultra-wide band (UWB) applications. Both these antennas are constructed as a chorded crescent-shaped radiator, a feed line and a defected ground plane. The experimental and computed results confirm that one design offers a wide usable impedance bandwidth of 114.3%, with a reflection coefficient |S11

In order to compute the scattering properties of individual sand/dust particles dispersed in air, as in a true sand/dust storm, we need to derive a permittivity value which is representative of sand/dust particle with its layer of water. As a first step in this, we applied Looyenga’s formula to extract the permittivity of the dry sand/dust component from the mixture which is measured. Measured results could be used to estimate electromagnetic wave propagation impairments in communication systems.

In electromagnetic scattering computation the scattering properties of a single particle can be evaluated if the solid permittivity of its constituent material is known. If the material is not homogeneous (e.g. water layer on the particle surface) both the structure and the permittivity of the different parts must be known. However, it is possible (at least at microwave frequencies) to define an equivalent homogeneous permittivity $$\left( {\bar \varepsilon } \right)$$ which would produce the same scattering properties for a homogeneous particle of the same outer shape as the real one. In this work a novel measurement method of a single particle scattering for evaluating the dielectric properties of dust is derived. Results are compared with another work on bulk/scaled measurement (Part II) of dust permittivity at the same frequency band (X-band) gave matched results.

A case study of an active transmitting patch antenna revealed a characteristic loop locus of DC power versus RF output power as drive frequency was varied, with an operational bandwidth substantially smaller than the impedance bandwidth of the radiator. An approximate simulation technique, based on separation of the output capacitance of the power transistor, yielded easily visualized plots of power dependence on internal load impedance, and a simple interpretation of the experimental results in terms of a near-resonance condition between the output capacitance and output packaging inductance.

The scattering parameter formulation for the envelope correlation in an(N,N)MIMO antenna array has been modified to take the intrinsic antenna power losses into account. This method of calculation provides a major simplification over the use of antenna radiation field patterns. Its accuracy is illustrated in three examples, which also show that the locations of the correlation minima are sensitive to the intrinsic losses.

A four-sided folded, slotted and shorted rectangular patch antenna for 3G mobile handsets is described. The antenna provides a17.65% bandwidth with S11

A novel quadrifilar spiral antenna is proposed for use in personal communications mobile terminals exploiting the “big low earth orbit” (big-LEO) satellite system (uplink 1.61–1.63 GHz; downlink 2.48–2.5 GHz). Feeding of the antenna from the outer periphery is proposed, to avoid the complexity and space requirements of a centre feed; an externally fed antenna can also be fed either by coaxial cables or stripline. Moreover, the hybrid phasing network and the matching network (if required) can be located outside and possibly printed in the same plane as the antenna structure. Versions of the design were investigated using the method of moments and in hardware realizations. These had a finite ground plane with air dielectric between the spirals and the ground plane. Performance suitable for satellite mobile system applications was demonstrated, including dual-band matching. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:987–990, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25090

A circularly polarized quadrifilar helical antenna, operated at 2.4 GHz and intended for applications in satellite mobile communications, is designed using genetic algorithms (GA). The antenna was firstly considered as a test on an infinite ground plane and then optimized further on small size handsets. The performance of the optimized antenna design was analyzed using commercial simulators in terms of impedance match, axial ratio, and gain. Results for the optimal antenna met the design objectives subject to certain parametric constraints. The design shows the capabilities of GA as an efficient optimization tool for selecting globally optimal parameters to be used in simulations with an electromagnetic antenna design code, seeking convergence to specific design specifications. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2668–2671, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24694

A circularly-polarized conical-beam microstrip patch antenna, with a coplanar conductor pattern, is realized as an array of a number of stripline-fed slotted patches whose diagonal corners are cut off and whose feedlines meet at a central point. Prototypes of such an antenna array with four patches are studied and tested. The results show reasonable agreement between measured and theoretical results. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 78–81, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23956

A novel internal triple-band folded planar antenna for mobile handsets is introduced, formed by modifying the geometry of a rectangular patch antenna to include a shorting pin, folded sides, a shorted microstrip stub and a notch. The size of the antenna is successfully reduced to a volume of 34 times 34 times 7 times mm 3 . The antenna is mounted on a finite ground plane of 50times100 times mm 2 . The impedance bandwidth achieved was 29.7% (equivalent to return loss%%10%dB); this covers the DCS1800, PCS1900 and UMTS 2000 bands. The characteristics of the proposed antenna, including impedance bandwidth and far field radiation patterns are discussed theoretically and experimentally; the simulated and measured results show good agreement. The tuning effects of the geometry parameters on impedance matching of the proposed antenna are also investigated.

Radio Frequency Identification (RFID) systems has gained increasing popularity with multiple deployments to existing wireless sensors in a view to achieve energy and overall operational efficiency at a much lower cost. This paper presents the design and analysis of a UHF (860–868MHZ) passive tag using HFSS (High Frequency Structural Simulator) platform. It explores specific tag geometry characteristics that affect overall tag antenna performance and presents the optimised result. The simulation results and parametric analysis are compared. Further simulations on HFSS platform is carried out to theoretically demonstrate the reflections of the tag when deployed as sensors to multiple levels of a cistern.

The bandwidth of a microstrip patch antenna is expressed in terms of minimum achievable reflection coefficient using an equivalent circuit and the Bode-Fano theory. The bandwidth-reflection coefficient product is found to be proportional to antenna height and largely independent of feed probe position, for small bandwidths. The product can be computed directly from a numerical evaluation of the first-order Bode-Fano integral. Curves are presented showing how the product becomes limited by the feed probe inductance at very large bandwidths. It is concluded that this effect is unlikely to be a limit on the potential bandwidth of a practical patch antenna. If as a minimal correction the feed inductance is tuned out, the realized bandwidth with low order matching or optimal over-coupling shows the expected relationship to the theoretical limit.

A new analytical model is proposed for the conical beam radiation pattern of the TM 02 -mode circular patch antenna. It is based on a correct physical description of the radiation mechanism at low-elevation angles, and yields better agreement with measurement than available models.

It is shown that a conical beam 5.2-GHz antenna suitable for HIPERLAN application, but working in horizontal polarization, can be realized as a group of microstrip patch radiators in a ring formation. Layouts with three and four patches are described, and radiation patterns are found to agree well with predictions from a simple array model. The three-patch form is smaller and gives a closer approximation to an azimuth-independent pattern. Patterns are very similar to those achieved in vertical polarization with previously reported disk antenna realizations, giving peak radiation at about 50/spl deg/ elevation. Two methods of impedance matching are found to give satisfactory results. A dual-polarized conical-beam microstrip antenna, with a strictly uniplanar conductor pattern, is also presented and realized as an array of three square patches whose corners meet a central feed point. For the second polarization, the antenna functions as a series fed array. Fairly good conical beam patterns have been obtained, though only moderate polarization purity appears to be obtainable from three-element arrays.

The phase asymmetry of a 20-GHz slant-path transmission matrix during ice events is modeled in terms of two layers consisting of plate- and needle-type crystals. The microwave propagation data are consistent with the meteorological reports that the plate forms are usually predominant at higher altitudes than the needle forms.

A measurement of the complete transmission matrix of a slant path using a switched polarization satellite beacon needs to be properly calibrated in both amplitude and phase to yield the best characterization of the hydrometeors which affect propagation. For accuracy, error contributions from the satellite and earth station should be separated and corrected individually. It is shown how this can be achieved by indirect methods which rely on minimal theoretical expectations of the form of the transmission matrix. Methods to calibrate the amplitude and phase offsets of the satellite and receiver, and to assess the satellite polarization purity, are demonstrated using the Olympus satellite at 20 GHz. Small amplitude imbalances in the beacon are found and corrected, but a large correction for an unexpected phase offset is required. Good polarization purity of the beacon is inferred.

A miniaturised modified planar inverted-F UWB antenna with a broadband rectangular feeding structure, for a Wireless Body Area Network (WBAN), is proposed. The antenna is designed to cover a wide frequency range from 3 GHz to 13 GHz. Details of the proposed antenna design and measured results are presented and discussed. To validate the results, a prototype was fabricated and tested. The measured and computed results are in good agreement. The overall size of the antenna including the ground plane is 50 mm × 50 mm × 8 mm, making it suitable to fit into a standard enclosure of a UWB wireless transceiver.

In this work, an efficient autonomous solar energy system has been implemented for domestic use. It concentrates the solar heat to turn water into steam which in turn is channelled to drive an electricity turbine-generator. The prototype utilizes two heliostat reflector units, each of which has been designed to accurately and independently track the sun throughout the day and reflect its light constantly on a water tank through a Fresnel lens. The algorithm is flexible enough to accept up to 100 units and can accommodate the four seasons of the year. The performance of the system including the instantaneous generated power can be monitored wirelessly using a specially designed Computer User Interface. The final system produced a concentration efficiency of 86%–92%.

A novel conical beam patch antenna design, suitable for local area network applications, uses a central cylindrical connection from the disc to ground to achieve a large reduction in resonant size. Results on efficiency, bandwidth, and pattern are reported for several prototypes.

Size constraints and mutual coupling on the performance of a two-element PIFA assembly are investigated for a design frequency of 2.4 GHz. A benchmark antenna assembly, employing a normal metallic ground plane is compared with an EBG modified ground plane. The height of the antenna elements over the EBG is optimised, and an isolation factor of 9.12 dB is achieved for a gap of 2.5 mm. Prototype structures have been constructed and measured for both cases.

In this paper, the power added efficiency and/or the power efficiency of an active antenna was measured using a specially devised setup. This was based on previous work by the authors whereby the power input to a radiating element of an active patch antenna was measured using a small sensing patch. Combined with an estimate of patch radiation efficiency, the method also yields a measurement of total radiated power. Because of weakly frequency-dependant calibration factor, the method has advantages of convenience and can be integrated with modern computer based measurement systems.

In this paper, the power input to the radiating element of an active microstrip patch antenna is measured using a small sensing patch, which is weakly coupled to a radiating edge. Combined with an estimate of patch radiation efficiency, this also yields a measurement of total radiated power. The method has advantages of convenience, insensitivity to room reflections, and a weakly frequency-dependent calibration factor.

A new Uniplanar Compact Electromagnetic Bandgap (UC-EBG) structure is proposed in order to achieve a smaller unit cell operating at lower frequency stop band. Its design is detailed and an experimental result is presented. The property of the proposed UC-EBG is compared with the distorted uniplanar compact EBG (DUC-EBG). The application of the proposed UC-EBG is examined. A parametric study on various thicknesses of dielectric substrate is also discussed.

A shaping technique for the radiator of an active patch antenna is described, and shown experimentally to reduce harmonic radiation and therefore enhance the conversion efficiency. The perturbation of resonant mode frequencies in a deformed rectangular patch radiator is discussed as a possible basis for systematic design. Field simulation software (SONNET) was also used to further study and confirm the effects of shaping.

Fano integrals are assessed as objective measures of antenna bandwidth and bandwidth improvement schemes. Comparing a U-slot dual resonant antenna with a singly resonant form suggests the integral is invariantly related to overall volume. (3 pages)

The Bode-Fano integral is proposed as an objective tool for assessing the bandwidth of antennas and special schemes for antenna bandwidth improvement. The limiting gain-bandwidth product is a measure of optimum power transfer from source to loads. For loads represented in Darlington canonical form, optimum tolerance can be calculated using the Bode-Fano theory. In this paper for the first time the Fano limit has been calculated for loads with up to four reactive elements. It was note that even though increasing the substrate height of coaxially feed microstrip patch antennas will increase the antenna bandwidth; however this is only true up to some specific heights and afterwards the antenna potential bandwidth decreases for future increase in substrate height. Also for double resonance circuits it was concluded that the maximum potential bandwidth is obtained if the two resonant frequencies are the same.

A shaping technique for the radiator of an active patch antenna is described and shown experimentally to reduce harmonic radiation and therefore enhances the conversion efficiency. The perturbation of resonant mode frequencies in a deformed rectangular patch radiator is discussed as a possible basis for systematic design. Various techniques of plotting results were employed for maximum understanding.

This chapter looks at various aspects of signal transmission, network methods and impedance matching