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We present a new reactive power control method for hybrid high-voltage direct-current (HVDC) systems that features a line-commutated converter-based (LCC) rectifier and a voltage-sourced converter-based (VSC) inverter. Our principal objective is to regulate the reactive power of the LCC rectifier side via co-operative control of the DC voltage and switched shunt devices (SSDs) such as capacitors and reactors. To achieve these objectives, we used three operative modes: reactive power, voltage, and power factor control. For each mode, we show how to determine the DC reference voltage. Also, to maintain the voltage within operational bounds and enhance the range over which reactive power is controllable, our method features the cooperation of SSDs. Furthermore, we show how to determine the required SSD capacities. MATLAB and PSCAD simulations confirmed that our method improves reactive power controllability at the LCC rectifier side of a hybrid HVDC system.

openaire: EC/H2020/736876/EU//VISORSURF The realization of a programmable metasurface, enabled by a custom application-specific integrated circuit (ASIC), is presented in this paper. The ASIC is used to provide an adaptive complex impedance load to each of the metasurface unit cells. Various technology nodes are analyzed for the implementation of tunable complex impedance loading elements before one is selected for the final implementation, in which four complex loads are placed within each integrated circuit, and each load is controlled by two digital-to-analog converters. Furthermore, the ASICs populate the back of the metasurface to form a mesh network to enable programmability. The paper includes practical limitations that affect the realization, as well as an example adaptive metasurface absorber that builds upon the practical tuning range of the ASIC. Perfect absorption for both transverse electric and transverse magnetic polarization is demonstrated.

This paper proposes multilevel linear polarization shift keying (MLPolSK) modulation, based on a high polarization-dependent gain optical amplifier (PDG-OA), for spectral efficient free-space optical (FSO) communication. At the transmitter end, multiple linear states of polarization (SOPs) are adopted, and different SOPs are mapped for MLPolSK modulation according to the PDG-OA. First, in the receiver, scintillation is effectively mitigated by a polarization-independent semiconductor optical amplifier (SOA) with gain saturation. Next, various SOPs are transformed into different signal intensity levels by an OA with high PDG characteristics. Finally, transformed signal is directly detected by a single photodiode (PD) with multilevel-thresholds decision. The feasibility of the proposed technique is evaluated experimentally using a reflective semiconductor optical amplifier (RSOA), with a high PDG-up to 20 dB. A Mach–Zehnder modulator (MZM)-based fading simulator is used to accommodate lognormal distribution simulated turbulence effects into experiments. Experimental results illustrate that multilevel linear SOPs can be distinguished effectively by the proposed technique. Consequently, the MLPolSK detection is simplified, and the spectral efficiency (SE) is improved by up to 2 bit/s/Hz with effective scintillation mitigation in FSO.

This paper proposes a semiconductor optical amplifier (SOA)-based multilevel polarization shift on–off keying (MPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, the MPS-OOK signal is modulated with a constant linear state of polarization (SOP) at the high-intensity level and various SOPs at the low-intensity level in order to improve the spectral efficiency (SE) with the transmitted power efficiency. At the receiver end, first, a polarization-independent SOA was introduced to optically suppress the turbulence-induced scintillation effect and equalize the intensities of the various SOPs in the deep gain saturation state without polarization distortion. Then, a linear polarizer (LP) with a high extinction ratio (ER) was deployed to convert the equalized SOPs into a known intensity. Finally, the converted MPS-OOK signal was detected using a single photodiode (PD) and distinguished using a multilevel fixed-threshold decision (M-FTD). The proposed technique was evaluated using experiments. A Mach–Zehnder modulator (MZM)-based fading simulator was introduced to emulate the turbulence-induced scintillation effect. The experimental results demonstrated that the scintillation effect was effectively mitigated and the SE was improved by up to 2 bit/s/Hz using the proposed four-level polarization shift on–off keying (4PS-OOK) transmission.

In this paper we present the theoretical considerations and the design evolution of a proof-of-concept reconfigurable metasurface, primarily used as a tunable microwave absorber, but also as a wavefront manipulation and polarization conversion device in reflection. We outline the design evolution and all considerations taken into account, from the selection of patch shape, unit cell size, and substrate, to the topology of the structure that realizes the desired tunability. The presented design conforms to fabrication restrictions and is co-designed to work with an integrated circuit chip for providing tunable complex loads to the metasurface, using a commercially available semiconductor process. The proposed structure can perform multiple tunable functionalities by appropriately biasing the integrated circuit: Perfect absorption for a wide range of incidence angles of both linear polarization states, accommodating a spectral range in the vicinity of 5 GHz, with potential also for wavefront control, exemplified via anomalous reflection and polarization conversion. The end vision is for such a design to be scalable and deployable as a practical HyperSurface, i.e., an intelligent multi-functional metasurface capable of concurrent reconfigurable functionalities: absorption, beam steering, polarization conversion, wavefront shaping, holography, and sensing., 15 pages (incl. author bios), 16 figures. Published online at 19 August 2020 in the IEEE Transactions on Antennas and Propagation, Vol. 68 (tentative)

An analytical mode-matching method based on the common area concept and mixed coordinate formulations is proposed for a multiple-row periodic array of magnetodielectric circular cylinders (PAMCs). Complex dispersion relations for single- and double-row PAMCs are computed with Davidenko’s method and tested using commercial electromagnetic (EM) analysis packages, thus verifying that our solutions are very accurate and numerically efficient. The EM bandgap formations of a single-row PAMC are presented with respect to different cylinder spacing. We also observe that a resonance gap, which results in a sharp band-stop phenomenon, can appear in a double-row PAMC with high-permittivity cylinders.

For elements of an infinite planar phased array in free space, fundamental bandwidth bounds are derived from the optical theorem for periodic scatterers. The bounds are based on Gustafsson’s theory of fundamental bandwidth limits for electrically small antennas of arbitrary shape, but extended to periodic configurations. The element bandwidth bound is found to be a function of the strengths of the induced static dipole moments in constant background fields as well as the unit-cell dimensions and the scan angle. Explicit bandwidth bound expressions are found for narrowband and ultrawideband array cases. They are numerically tested using example free-space array designs.

For passive, lossless impenetrable metasurfaces, a design technique for arbitrary beam control of receiving, guiding, and launching is presented. Arbitrary control is enabled by a custom surface wave in an orthogonal polarization such that its addition to the incident (input) and the desired scattered (output) fields is supported by a reactive surface impedance everywhere on the reflecting surface. Such a custom surface wave (SW) takes the form of an evanescent wave propagating along the surface with a spatially varying envelope. A growing SW appears when an illuminating beam is received. The SW amplitude stays constant when power is guided along the surface. The amplitude diminishes as a propagating wave (PW) is launched from the surface as a leaky wave. The resulting reactive tensor impedance profile may be realized as an array of anisotropic metallic resonators printed on a grounded dielectric substrate. Illustrative design examples of a Gaussian beam translator-reflector, a probe-fed beam launcher, and a near-field focusing lens are provided.

High-voltage dc (HVDC) systems are being adopted for grid connections by grid operators and transmission owners. Among the converter topologies, the line-commutated converter (LCC) is more economical than the voltage-sourced converter (VSC); however, its reactive power cannot be controlled. Hybrid HVDC, a mixed topology containing both LCC and VSC, demonstrates the benefits of both topologies. In this paper, a hybrid HVDC control scheme with an LCC sending end and a VSC receiving end is proposed. It is used to control sending-end reactive power. Using the proposed secondary controller, hybrid HVDC — using only shunt devices, filters, or capacitor banks — can have the same reactive abilities as VSC HVDCs. This reduces the cost of connecting the voltage-weak grids and ensures that the voltage-weak grids are not afflicted by voltage stresses. The proposed secondary controller was verified to be effective through PSCAD/EMTDC simulations.

Fundamental impedance matching bounds for electrically small loop antennas are derived based on spherical wave scattering in the low-frequency limit. A small lossless loop antenna is represented as a two-port network between the excitation port and the fundamental TE spherical wave port. The low-frequency asymptotic expression for the wave reflection coefficient is found and the classical Fano approach is applied to derive integral equalities for the magnitude of the reflection coefficient. A basic loop, one with a series-C element, and one with a parallel-C element are analyzed. The bandwidth upper bounds as functions of the antenna dimension, the port impedance, and the circuit parameters are presented.

An indirect impedance measurement approach that does not require direct cable attachment or large space using a two-port network is presented. Using a straight wire monopole as an interrogating antenna and measured impedances of three calibration standards, the input impedance of a small spherical helix dipole over a ground plane is retrieved. It is found that accurate result is obtained around the dipole resonance frequency. The accuracy and sources of error are discussed.

The radiation quality factors of vertically and horizontally polarized single-mode dipole antennas over a ground plane are investigated and compared with their free-space counterparts. The theoretical Q results are validated using simulation and measurement results of small spherical helix dipole antennas. For vertically polarized antennas, the bandwidth can be enhanced approximately by a factor of two.

Based on a common-area concept and insertion of an infinitesimal perfect magnetic conductor (PMC) wire, a new mode-matching method for mixed coordinate systems is proposed for the analysis of periodic magnetodielectric, perfect electric conductor, and PMC circular cylinder arrays. Our scattering solutions were computed and they showed favorable agreements with other known results in terms of power scattering coefficients, resonant frequency of nanostructures, and the sum rule of extinction width. The low-frequency solutions of periodic arrays were also formulated and compared with the full-wave counterparts.

For lossless, electrically small dipole antennas, the impedance matching bandwidth upper bounds are derived based on spherical wave scattering by the antenna in the low-frequency limit. The antenna is represented as a lossless matching network between the antenna port and the dominant TM 01 spherical wave port. The classical Fano approach is taken to derive integral identities for the magnitude of the antenna port reflection coefficient and bandwidth upper bounds. Unlike previously known Fano-type bounds, the new bandwidth bounds incorporate the effect of antenna details via the low-frequency electric polarizability.

Recent theoretical developments suggest that the low-frequency electric polarizability is a key factor in determining bandwidth bounds for electrically small dipole antennas. Using example designs of cylindrical shape, bandwidth properties in terms of the radiation Q and the associated electric polarizabilities are numerically investigated. Strong correlation is observed between the polarizability and the achieved bandwidth values. It suggests that a structure with the bounding surface that is close to that of the given design space is a good candidate for maximum bandwidth performance.

Based on quasi-conformal coordinate transformation, lens designs for conformal array antenna applications are presented. Potentially over a broad bandwidth for the two-dimensional TE polarization, a physically linear array can be transformed into a virtual curved array, or vice versa, using a lens having only nonmagnetic medium parameters. Lens designs for transforming between linear and cylindrical arrays in conformal conducting ground planes as well as their predicted scanning performances are shown. Such lenses can be designed and applied to existing array antennas to enable new scanning capabilities.

The recently introduced transformation-electromagnetics techniques provide a new methodology for designing devices that possess novel wave-material interaction properties. They are based on the form invariance of Maxwell's equations under coordinate transformations. These methods provide an extremely versatile set of design tools that employ spatial-coordinate transformations, where the compression and dilation of space in different coordinate directions are interpreted as appropriate scalings of the material parameters. The most famous transformation-optics device is the cloak of invisibility. However, a wide variety of other devices are also possible, such as field concentrators, polarization rotators, beam splitters, beam collimators, and flat lenses. In this paper, an overview of transformation-electromagnetics device design techniques is presented. The paper begins by introducing the underlying design principle behind transformation electromagnetics. Several novel transformation-based device designs are then summarized, starting with electromagnetic cloaks that have spherical shell or cylindrical annular shapes, More general cloaking designs of noncircular annular geometries are treated, and the application of cloaking to RF/microwave antenna shielding is also discussed. Following this, device designs that employ transformations that have discontinuities .on the domain boundary are presented. Unlike those used for cloaks, this type of transformation is capable of modifying the fields outside of the device. Examples of this type of transformation-electromagnetics device are presented, which include flat near-field and far-field focusing lenses, wave collimators for embedded sources (e.g., antennas), polarization splitters and rotators, and right-angle beam benders.

A radiation configuration is designed for a monopole antenna embedded within transformation media and buried inside a recess in a conducting ground plane. Both the ground plane near the base of the antenna and the antenna itself, which are physically located inside a recess, are mapped to a virtual location above the ground plane by a negative-index material block. The directivity pattern is identical to that of a monopole on a flat ground plane, radiating the maximum vertically polarized field in the horizon. Radiation characteristics are tolerant to small variations in material parameters from ideal values. Estimates for pattern and impedance bandwidths are provided based on numerical simulations.

Several fundamental questions about the operation of receiving antennas are addressed, such as ldquoWhy does a receiving antenna scatter an incident field?rdquo and ldquoUnder what conditions does a receive antenna capture all of the available incident power?rdquo A new method is described by which the received power can be maximized for an arbitrary receiving antenna. The technique is first illustrated for two-dimensional infinite receiving arrays of electric and/or magnetic dipole elements, which result in simple plane waves for the scattered (re-radiated) fields. Optimal results (for maximum received power) are derived for several cases, and it is established that half the available incident power may be received by an array of electric (or magnetic) elements in free space, and that all available incident power may be received by an array that combines electric and magnetic elements, or one that incorporates a ground plane. Next, an arbitrary finite three-dimensional antenna enclosed by a mathematical spherical surface is treated using spherical vector wave functions. It is shown that half the available incident power can be received by such an antenna consisting of either TM or TE only elements, while all available incident power can be received when both TM and TE elements are used. It is also shown that the absorption efficiency for any optimal arbitrary antenna is 50%.

A beam scanning/switching method utilizing a transformation electromagnetics device is presented. A flat focusing lens based on the coordinate transformation approach is designed such that the field radiated by an antenna at the lens focus is transformed into a directive beam emitted normal to the lens surface. The main beam can be scanned by moving the antenna in the focal plane. Physical movement of the source or the lens can be avoided by placing an array of radiators in the focal plane. Selective excitations of individual antennas in a linear array configuration result in switched beams. Full-wave numerical simulations are performed, and the results for 2D lens configurations with line sources are provided.

The radiation quality factor and gain of antennas are compared for two source configurations between TM/TE-only spherical mode radiation and combined TM + TE mode radiation. It is shown that the change to the combined TM + TE mode radiation can reduce the quality factor by up to 50% and increase the gain by maximum 3 dB simultaneously. The condition for maximum gain increase provides a guideline for antenna designs for successfully achieving wide bandwidths and small sizes at the same time.

A small printed antenna structure is presented featuring directive radiation patterns over ultrawide frequency band without relying on large antenna dimensions. The antenna comprises virtually two antennas-one electric-type antenna and one magnetic-type antenna-excited simultaneously in close proximity via one feed point. This combination of radiated fields from different types of sources result in directive radiation patterns without increasing antenna sizes as typically observed in antennas using only one type of sources. Two printed prototypes of the combined antenna are fabricated and their measured VSWR and radiation gain pattern characteristics are discussed.

The exact radiation quality factor (Q) of a horizontal planar dipole antenna over a conducting ground plane is presented. The stored energies as well as the radiated power are evaluated in the 2-D spectral domain. In the half space above the antenna, the energies are found from surface integrals at the antenna height, following existing techniques. Stored energies between the antenna and the ground plane are evaluated using volume integrals. The effect of the ground plane on Q is discussed.

Utilizing the modulation capability of LEDs, there have been many studies about convergence technology to combine illumination and communication. The visible light communication (VLC) system has several advantages such as high security, immunity to RF interference and lower additional cost than comparing to LEDs just for illumination. However, modulation bandwidth of LEDs is not enough for various wireless communication systems. Since the commercial LEDs are designed only for lighting systems; we need an effort to enhance the modulation characteristics of LEDs. When the area of LED is increased, internal junction capacitance of LED is also increased depending on the area of LEDs and then the RC delay time of LED is increased. As a result, the modulation bandwidth of LEDs is limited by large RC delay time. In addition, frequency response of commercial white LED is degraded by the slow response time of the used yellow phosphor. Thus, modulation bandwidth of VLC system is limited to several MHz which is not enough to accommodate high data rate transmission. In this paper, we designed equalization circuit using RLC component for compensating the white LEDs frequency response. Also, we used blue filtering to improve frequency response of white LEDs, which is degraded by yellow phosphorescent component. Power loss by optical filtering and distance is compensated by convex lens. Consequently, we extend the modulation bandwidth of VLC system from 3 MHz to more than 180 MHz, and it allows NRZ-OOK data transmission up to 400 Mbps at 50 cm.

White light-emitting diodes (LEDs) are widely used for lighting due to their energy efficiency, eco-friendly, and small size than previously light sources such as incandescent, fluorescent bulbs and so on. Optical wireless visible light communication (OWVLC) based on LED merges lighting and communications in applications such as indoor lighting, traffic signals, vehicles, and underwater communications because LED can be easily modulated. However, physical bandwidth of LED is limited about several MHz by slow time constant of the phosphor and characteristics of device. Therefore, using the simplest modulation format which is non-return-zero on-off-keying (NRZ-OOK), the data rate reaches only to dozens Mbit/s. Thus, to improve the transmission capacity, optical filtering and pre-, post-equalizer are adapted. Also, high-speed wireless connectivity is implemented using spectrally efficient modulation methods: orthogonal frequency division multiplexing (OFDM) or discrete multi-tone (DMT). However, these modulation methods need additional digital signal processing such as FFT and IFFT, thus complexity of transmitter and receiver is increasing. To reduce the complexity of transmitter and receiver, we proposed a novel modulation scheme which is named differential pulse amplitude modulation. The proposed modulation scheme transmits different NRZ-OOK signals with same amplitude and unit time delay using each LED chip, respectively. The ‘N’ parallel signals from LEDs are overlapped and directly detected at optical receiver. Received signal is demodulated by power difference between unit time slots. The proposed scheme can overcome the bandwidth limitation of LEDs and data rate can be improved according to number of LEDs without complex digital signal processing.

A simple method is presented for suppressing cable leakage current for printed dipole ultrawideband (UWB) antennas using thin leakage-blocking slots. Their effects on VSWR and pattern characteristics are demonstrated by comparing the performances of two identical coplanar waveguide (CPW)-fed printed dipoles except for the presence of the slots. Two thin slots along the feeding CPW trace effectively block a strong leakage current component from flowing over the outer surface of the feeding coaxial cable. While not significantly affecting the antenna's VSWR performance over wide bandwidth, the slots have the effects of increasing the antenna gain in the azimuth plane and reducing the strengths of strong radiation lobes close to the direction of cable attachment especially in the low-frequency range.

Exact expressions for the radiation quality factor Q and the gain pattern function of the corresponding antenna are derived and their properties are analyzed for crossed electric and magnetic dipole moment configuration. Both omni-directional and directional antennas with linear, elliptical, or circular polarization can achieve the same minimum radiation Q. The maximum gain of the antenna ranges from 3/2 to 3. The maximum gain of three can be achieved only for linear polarization when the source vectors make a right angle and the amplitudes of the TM and the TE modes are equal. The increase in the maximum gain from 3/2 for omni-directional antennas to 3 for directional antennas does not require any increase in the radiation Q, which suggests that practical directional antennas with higher gain may be realized with smaller size compared with small omni-directional antennas assuming similar operation bandwidths.

We propose received-signal-strength based real-time indoor positioning system in carrier allocation - visible light communication. The experimental result shows that estimated position errors are within 7 centimeters in 100cm × 100cm × 275cm space.

A high-frequency asymptotic ray solution is investigated for predicting the electromagnetic field build up and steady-state parameters of shielded enclosures or cavities, which are large with respect to wavelength. It is found that the ray solution can deterministically predict the early-time field build up after the source is switched on, but cannot predict the steady-state fields of high-Q enclosures because of the intractably large number of ray reflections required for convergence. However, it is demonstrated that the steady-state Q factor may be predicted from the early-time energy density build up at a point by coherently summing the power in each ray. The Q factor is obtained via its relationship to the cavity time constant, which may be extracted from the early time energy density curve. A clear indication of polarization diversity throughout the enclosure may also be obtained by plotting the polarization components of the early-time fields and energy density build up at different points. The advantage of the ray method is that it can be used to treat large closed cavities of relatively arbitrary shape.

A wideband vertical transition between co-planar waveguide (CPW) and parallel strip transmission line is presented. It functions as an unbalanced-to-balanced (balun) transformer as well. The transition is realized in back-to-back configurations for 50 /spl Omega/ and 75 /spl Omega/ characteristic impedance cases to verify their transition performances. The CPW-parallel strip-CPW transitions are found to have distinct resonant frequencies where the length of the parallel strips is equal to multiples of half wavelength, which is not characteristic of a single transition, but a result of two CPW-parallel strip transitions interacting with each other. Away from the resonant frequencies, the back-to-back configurations have low insertion loss over wide frequency band.

The energy storage and radiation quality factor of infinite planar phased arrays are studied for several cases of practical interest, including dipole arrays in free space and over a ground plane, as well as more general infinite phased arrays modeled with planar currents that excite only a small number of Floquet modes. The radiation Q is computed in a manner analogous to the classic Chu result for finite sized antenna elements [1], by calculating the stored electric and magnetic energies contained in a unit cell volume of the array (minus the energy density associated with the radiation field), and by calculating the radiated power from each unit cell. This is done using a full-wave moment method approach, with the appropriate Green's function for the geometry under consideration [2] (i.e., free-space, or over a ground plane). For lossless arrays in free space, in the absence of grating lobes, it is known that radiated power is contained only in the (0,0) Floquet mode, and that stored energy is contained only in higher order Floquet modes. This is in contrast to the Chu results for finite antennas, where each spherical wave mode contains some radiated power and some stored energy. The result is slightly different for arrays over a ground plane, where the (0,0) Floquet mode may contribute to the stored energies in the volume between the ground plane and the current elements. Both the radiated power and stored energies are evaluated in terms of the two-dimensional spectral domain Floquet mode series representation and the Fourier transform of the element current. Mode-by-mode correspondence of each Floquet mode contribution to the element input impedance is demonstrated, as is satisfaction of the complex Poynting theorem. The exact quality factor results are compared with approximate values based on the input impedance vs frequency from an independent calculation.

Although the radiation quality factor of small antennas in free space has been investigated extensively in the past, the exact effect of a conducting ground plane on the antenna bandwidth is still not clearly understood. In this paper, radiation quality factors of vertically polarized antennas above a ground plane are computed and compared with their free-space counterparts. The theoretical Q result is validated with simulation of a small spherical helix antenna.

An indirect impedance measurement approach that does not require direct cable attachment using a two-port network is designed and numerically evaluated. Using a monopole as an interrogating antenna, the impedance of a small spherical helix dipole over a ground plane is retrieved. The accuracy and sources of error are discussed.

We propose an asymmetric 3×2 multi-input multi-output (MIMO) system for polarization division multiplexing (PDM) transmission in visible light communication (VLC). In PDM transmission, independent channels are constructed by polarization orthogonality, which is vulnerable to misalignment of polarization between the transmitter and the receiver. Although PDM-based VLC system may not maintain polarization orthogonality, the proposed asymmetric MIMO system can achieve the polarization diversity required for a multichannel system. We experimentally demonstrate the performance enhancement of PDM VLC transmission using the proposed asymmetric 3×2 MIMO technique.

A sum rule for horizontal thin-wire dipole antennas backed by a perfectly conducting ground plane is presented. It is an integral identity for the extinction cross section of a conducting dipole multiplied by a weight function under a linearly polarized plane wave illumination. The weighted integral is found to be proportional to the static polarizability of the original and image dipoles in free space.

The only aspect of an antenna's receiving properties not available through reciprocity is on scattering by the antenna. Antenna scattering has received classical treatments [1, 2], where the scattered field expression and the associated scattering cross section are of interest under plane wave illuminations. Recently, received (absorbed) power, scattered power, and their relationships were investigated [3]. In [4], both scattered and received powers were expressed in terms of a common set of parameters — the vector wave expansion coefficients for the scattered field.

Small printed ultra-wideband (UWB) antennas that combine electric-type and magnetic-type radiators are presented. Combining the two types of radiators can extend the passband in the lower frequency direction compared with antennas of either individual type. Moreover, combined antennas can be designed to feature increased gain by allowing the electric fields radiated by the electric and magnetic sources combine in-phase in the desired direction. Two designs of the size 20 mm × 30 mm are presented together with measured impedance and radiation characteristics.

The transformation optics technique is employed in this paper to design two optical devices - a two-dimensional polarization splitter and a three-dimensional polarization rotator for propagating beams. The polarization splitter translates the TM- and the TE-polarized components of an incident beam in opposite directions (i.e., shifted up or shifted down). The polarization rotator rotates the polarization state of an incoming beam by an arbitrary angle. Both optical devices are reflectionless at the entry and exit interfaces. Design details and full-wave simulation results are provided.

Using the transformation electromagnetics technique, a 2D flat far-zone focusing lens design has been presented. The design was obtained by transforming a circular arc with the center located at the line source position into a planar interface. A near-zone focusing lens was obtained by concatenating two far-zone focusing lenses in a back-to-back configuration. Finally, a non-magnetic version of the lens design was also presented.

We have proposed to make use of the low-loss characteristic of chalcogenide glass material for infrared filter applications. A stop-band chalcogenide filter design based on a grating structure was presented. The study shows that by varying the grating's geometric structure, the bandwidth can be adjusted and the resonant frequency can be fine-tuned. However, high-Q characteristics can only be realized with thicker grating structures. This initial investigation indicates that a chalcogenide glass stop-band filter can achieve reflectances higher than 90% at infrared wavelengths.

For lossless receive antennas, the power absorbed and delivered to a conjugate matched load can be obtained by computing a net power lost within a volume that includes the antenna. Maximum achievable received powers by different planar array configurations were analyzed. It was found that both a grounded dipole array, and an array of electric/magnetic dipole combinations, are capable of receiving all of the incident power.

Background Methamphetamine, a derivative of amphetamine, has been well known to cause mental problems in humans; however, its physical effects are little known. Despite relevant information on the effect of methamphetamine abuse on bone quality being available, data regarding the frequency of osteoporosis in methamphetamine abusers are limited. Methods We selected 46 hospitalized male methamphetamine abusers and 188 reference male controls in whom any conditions affecting bone metabolism were ruled out. Bone mineral density (BMD) in the lumbar spine was measured by dual energy X-ray absorptiometry (DXA). We compared the BMD between methamphetamine abusers and controls and evaluated the frequency of osteoporosis in both groups. Results The mean BMD value was lower in methamphetamine abusers (mean±SD, 0.71±0.07g/cm 2 ) than in the controls (mean±SD, 0.98±0.14g/cm 2 ). The frequency of osteoporosis was 22% according to WHO diagnostic guidelines, and osteopenia at the lumbar spine was 76%. The correlation between the extent of methamphetamine abuse and BMD was very clear. Conclusion There was considerable loss of bone mineral in a high percentage of methamphetamine abusers. Our study is the first clinical study to determine the frequency of osteoporosis in male methamphetamine abusers.

An application of electromagnetic cloaks to shielding the radiation and circuit parameter characteristics of an antenna from other objects and antennas in highly scattering environments has been presented. The blockage and loading effects of scatterers on an antenna can be eliminated by enclosing them in cloaks designed to operate at the transmitting frequency of the antenna. In multiple-antenna radiation environments, each antenna can be enclosed in an electromagnetic cloak, which is designed to operate at the frequencies of other antennas and become transparent at the operating frequency of the enclosed antenna.

A near-IR metamaterial having a tunable index of refraction from negative, through zero, to positive values has been proposed. Liquid crystal layers are incorporated both as a superstrate and a substrate onto a conventional NIM. Tuning the permittivity of the liquid crystal layers controls the value of the effective permittivity in an averaging manner. Combined with proper magnetic resonances provided by a pair of silver strips separated by a thin layer of alumina, the effective index of refraction can be reconfigured between negative, zero, and positive values. The ability to vary the permittivity of the liquid crystal layers also provides a method to control the bandwidth of the negative index behavior.

Near-IR metamaterials exhibiting dual-band negative-index behavior have been presented. Magnetic resonances in two bands are obtained by incorporating two resonators of different dimensions into one period of an infinitely periodic design. Negative permittivities are provided by thin silver films via the naturally available negative dielectric function at near-IR wavelengths. The desired bands of the NIM behavior can be designed by proper combinations of the magnetic resonator dimensions and the silver film thicknesses. A two-dimensional example with two negative-index bands in the near-IR spectrum was demonstrated. The two-dimensional design was then extended to a three-dimensional doubly-periodic dual-band metamaterial design. It has been pointed out that the use of silver strips or meshes in place of thin continuous films significantly improves the loss characteristics of the metamaterial.

We experimentally demonstrated a gigabit visible light communication system employing polarization division multiplexing and adaptive modulation optical-discrete multitones by using a phosphor-based white light-emitting diode. An optimized pre-equalization circuit was used to prevent clipping of the baseband signal, which exceeded the linear operating range of the LED, and to control the power distribution of each subcarrier in order to obtain an efficient bit-loading profile. Using this technique, we achieved 1.025 and 1.016 Gbps transmissions for each polarization channel and a total of 2.041 Gbps transmissions were experimentally verified. In each transmission, bit error rate performances were below the forward error correction limit.

A planar dipole UWB antenna with an integrated filter to notch out the GPS frequency band is presented. The notch filter comprises lumped and distributed capacitors and inductors, which are integrated on the substrate close to the feed connector. Measurement results show that the gain of the GPS notch antenna is practically the same in the UWB pass band, but it is lower by 20 dB or so in the GPS band compared with the generic printed dipole antenna. Although the amount of suppression for GPS frequency components by the antenna alone may not be enough to satisfy the UWB emission limit, it can certainly relieve the performance requirements on a separate GPS notch filter.

In this paper, a measure of image-rejecting capability of an antenna for wireless communication devices is introduced and a loaded dipole antenna is designed to reject the image component for Korean personal communication system handset applications. The image-reject ratio for a receiving antenna is defined as the received power ratio of the image-frequency component to the desired frequency signal component for the same incident radio power density. An example image-reject dipole antenna shows that dramatic improvement in image-rejection capability can be realized without seriously compromising signal reception capability in the desired frequency band.

The development of ultra wide band (UWB) antennas for high resolution and high data transmission rates in modern communication systems is growing rapidly due to the difficult requirements of UWB antennas such as small size, non-dispersive and ultra wide-band characteristics. This paper describes a planar UWB slot antenna having frequency band notch characteristics. By inserting a V-shaped thin slot on the planar antenna, a narrow frequency band notch function has been created. Also, the notched frequency band can be varied by adjusting the length of the slot. Antenna performance including VSWR and azimuthal gain pattern of the new antenna are presented.

The transformation optics technique is applied to design three novel devices—a wave collimator, far-zone and near-zone focusing flat optical lenses and a right-angle bend for propagating beam fields. The structures presented in this paper are all two-dimensional (2D), however, the transformation optics design methodologies can be easily extended to develop 3D versions of these optical devices. The required values of the permittivity and the permeability tensors are derived for each of the three devices considered here. Furthermore, the functional performance of each device is verified using full-wave electromagnetic simulations. A wave collimator consists of a 2D rectangular cylinder where the fields (cylindrical waves) radiated by an embedded line source emerge normal to the top and bottom planar interfaces thereby producing highly directive collimated fields. Next, a far-zone focusing lens for a 2D line source is created by transforming the equi-amplitude equi- phase contour to a planar surface. It is also demonstrated that by aligning two far- zone focusing flat lenses in a back-to-back configuration, a near-zone focusing lens is obtained. Finally, a 2D square cylindrical volume is transformed into a cylinder with a fan-shaped cross section to design a right-angle bend device for propagating beam fields.