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A detailed characterization of a high-power radio frequency (RF) broadband circularly polarized two-arm spiral antenna is designed to operate within the frequency range of 0.1–1.0 GHz. The impedance matching network technique is introduced to optimize its performance. The traditional spiral antenna is excited by a vertical or horizontal balun, whereas the proposed design is directly fed by a coaxial cable featuring a planar feeding section specially optimized to achieve broadband input impedance matching. The spiral antenna is designed as per the steady-state superconducting tokamak (SST-1) port space constraints. The simulated efficiency of the RF power coupling with the hydrogen plasma is ∼70 %. Through simulation, it was evident that the proposed antenna exhibited inherent resonance at 0.5 GHz with a reflection coefficient of −27.94 dB and an axial ratio is 3.39 dB respectively. The obtained outcomes unequivocally demonstrate the circular polarization of the designed antenna. Overall, the findings support the enhancement of plasma heating and current drive techniques in fusion research. [ABSTRACT FROM AUTHOR]

In this study, a plasma frequency selective surface (PFSS) was designed with an array of ordinary fluorescent lamps backed by a conducting plate. The proposed PFSS structure was used as a reflector with the spiral two arms Archimedean type Global Navigation Satellite System (GNSS) antenna to reduce the overall size of the antenna system. Characteristics and the performance of the proposed system were presented by using the results of simulations and experiments in the GNSS band. The optimum distance between antenna and PFSS was found to be around 35 mm to achieve a maximum gain in the simulation which is a lower profile than antenna with conventional conductive plate (66 mm at the GNSS center frequency). Experimental results show that the antenna system has mean values of around 6.7 dBiC gain, 1.27 dB axial ratio and less than -10 dB return loss in the range of 1.14 and 1.61 GHz. All these results show that the proposed novel antenna system is suitable for the reception of the GNSS signals with the advantage of its low profile design. [ABSTRACT FROM AUTHOR]

An integrated automatic optical inspection (iAOI) system with a procedure was proposed for a printed circuit board (PCB) production line, in which pattern distortions and performance deviations appear with process variations. The iAOI system was demonstrated in a module comprising a camera and lens, showing improved supportiveness for commercially available hardware. The iAOI procedure was realized in a serial workflow of image registration, threshold setting, image gradient, marker alignment, and geometric transformation; furthermore, five operations with numerous functions were prepared for image processing. In addition to the system and procedure, a graphical user interface (GUI) that displays sequential image operation results with analyzed characteristics was established for simplicity. To demonstrate its effectiveness, self-complementary Archimedean spiral antenna (SCASA) samples fabricated via standard PCB fabrication and intentional pattern distortions were demonstrated. The results indicated that, compared with other existing methods, the proposed iAOI system and procedure provide unified and standard operations with efficiency, which result in scientific and unambiguous judgments on pattern quality. Furthermore, we showed that when an appropriate artificial intelligence model is ready, the electromagnetic characteristic projection for SCASAs can be simply obtained through the GUI. [ABSTRACT FROM AUTHOR]

This article introduces an ultrawideband antenna with stable phase center and circular polarization. The proposed antenna is a low-profile cavity-backed spiral antenna measuring $58\times 58\times 50$ mm3, where the cavity is partially filled with an absorber. Throughout the desired frequency range of 2 GHz to 18 GHz, the antenna maintains a return loss better than 10 dB; the phase center offset remains within 10 mm in the direction of radiation and 4 mm in the plane of the spiral arms, the phase center variation is less than 6 degrees; and the axial ratio of less than 3 dB. Fabrication and measurements of the antenna serve to validate its performance.

Suppression of Enemy Air Defence (SEAD) is a fundamental element of Air Power application by means of in protecting friendly air attackers and destroying the enemy's ability to defend against air attack. Most of the SEAD operation even today relies on Anti-radiation missile (ARM) which is an air-to-surface tactical missile designed to detect, seek, attack and destroy opponent's radar. Passive seeker of ARM is a miniaturized ESM receiver which is capable of extracting the necessary angular data from the enemy radar emissions. Single head passive seeker covering wide frequency range from L to Ku band is the preferred choice. Wideband antennas have been designed and utilized for Direction Finding applications of ESM/ELINT receivers for ground, air and ship borne platforms. Unlike these platforms, there are several restrictions for passive seeker based compact ESM receiver for missile borne platform specially air to surface missile where lesser diameter is one of the preferred design parameter. This review paper mainly discusses the existing wideband antennas such as spiral, log-periodic, printed circuit vivaldi and all-metal vivaldi antennas and the comparison of their various parameters for passive seeker. The paper also suggests their suitability with respect to their placement on the missile for three configurations: concealed inside the radome, flush-mounted and conformal antenna based. The paper also brought about the specific test facility required for testing and evaluation of passive seeker to characterize it with missile radome which is the most challenging and time consuming task. Among the three passive seeker configuration discussed, conformal antenna based passive seeker using all-metal Vivaldi is the best option avoiding radome aberration correction which is being utilized in the present third generations of ARM. The second commonly and established passive seeker configuration is concealed inside the radome using spiral antennas where handling radome aberration correction is a limitation. [ABSTRACT FROM AUTHOR]

Numerous designs and methods have been examined to improve penetration depth (PD), but there is a need for research to explore the potential increase in PD through uniform heating, a compact applicator, and low input power. This paper presents metasurface based hyperthermia lens applicator with water bolus for uniform heating of cancerous tissues. The proposed applicator consists of a stacked spiral antenna and a spiral-shaped frequency selective surface as a superstrate. The spiral antenna and superstrate are optimized on a low cost FR4 substrate having a size of 32 × 32 × 3.27mm3 and 10 × 10 × 1.6mm3 (size of the unit cell), respectively. The proposed applicator is simulated with heterogeneous phantom (skin, fat, and muscle layers) and with the Gustav voxel model with and without a water bolus layer. The number of unit cells in the superstrate is optimized to direct the maximum energy toward the tumor location. The performance study of the applicator is carried out in terms of specific absorption rate, PD, and effective field size. Further, thermal analysis is carried out with 1.9 W of input power at the antenna port, and the highest 44.7 °C temperature rise is obtained. The cancerous tissue's (tumor) surrounding temperature is between 41 and 45 °C, which is adequate for efficient hyperthermia treatment. Finally, the proposed metasurface hyperthermia lens applicator is fabricated and experimentally validated in a mimicked phantom's presence. [ABSTRACT FROM AUTHOR]

The article is dedicated to the analysis of electrodynamic properties of the cylindrical spiral structure. Internal problem for the considered structure in the framework of the thin-wire approximation is reduced to a pair of independent integral Fredholm equations of the first kind, written with respect to even and odd components of the unknown current function. A study of spectral characteristics of the integral operator of the integral equation for various electrical lengths and ratios cylinder radius to its height for a fixed number of turns. Using corresponding systems of projection functions in the framework of the method of moments two independent systems of linear algebraic equations are formed with respect to the unknown expansion coefficients of the even and odd components current function. Certain features of the behavior of eigenfunctions in depending on the wave dimensions and geometrical parameters of the spiral. Features of the frequency dependence of the eigenvalues integral operator were revealed. The conclusion is made about the resonant nature of these dependencies, what makes a cylindrical spiral structure in many respects similar to the previously considered tubular vibrator, a spherical spiral particle and an elliptical frame structure. A hypothesis is put forward about the relationship between the quality factor of resonances of eigenvalues and the emergence of a traveling wave mode in the spiral. Presented results contribute to the formation of an in-depth understanding of emerging in the structure internal processes and serve as a guideline in the construction of approximation models for solving an internal problem. [ABSTRACT FROM AUTHOR]

An integrated automatic optical inspection (iAOI) system with a procedure was proposed for a printed circuit board (PCB) production line, in which pattern distortions and performance deviations appear with process variations. The iAOI system was demonstrated in a module comprising a camera and lens, showing improved supportiveness for commercially available hardware. The iAOI procedure was realized in a serial workflow of image registration, threshold setting, image gradient, marker alignment, and geometric transformation; furthermore, five operations with numerous functions were prepared for image processing. In addition to the system and procedure, a graphical user interface (GUI) that displays sequential image operation results with analyzed characteristics was established for simplicity. To demonstrate its effectiveness, self-complementary Archimedean spiral antenna (SCASA) samples fabricated via standard PCB fabrication and intentional pattern distortions were demonstrated. The results indicated that, compared with other existing methods, the proposed iAOI system and procedure provide unified and standard operations with efficiency, which result in scientific and unambiguous judgments on pattern quality. Furthermore, we showed that when an appropriate artificial intelligence model is ready, the electromagnetic characteristic projection for SCASAs can be simply obtained through the GUI.

This paper presents the design and fabrication of a wideband array antenna module for a two-dimensional (2-D) interferometer direction-finding system. The array antenna module comprises an installation panel, spiral antennas, absorbers, and a radome. The absorber was configured to minimize the phase difference between vertical and horizontal polarizations to reduce the degradation of the direction-finding performance. Rubber and foam absorbers were applied in a hybrid form to minimize diffuse reflection. The radome was applied using an A-sandwich structure and manufacturing a laminated structure with a small vertical- and horizontal-polarization phase difference. The average phase difference between the vertical and horizontal polarizations of the array antenna was measured as an azimuth of 8.09° and elevation of 7.89°. Therefore, the fabricated array antenna can be applied to the 2-D interferometer direction-finding system. [ABSTRACT FROM AUTHOR]

A compact focused metamaterial‐based applicator is investigated for hyperthermia cancer treatment in this letter. The metamaterial‐based applicator consists of a double spiral antenna (DSA), an artificial magnetic conductor (AMC) as a reflector, and a frequency selective surface (FSS) as a lens. An AMC structure (64 × 64 mm2) is placed in the back of the antenna at a 5 mm distance that converges the bidirectional field pattern of the antenna into the directional field pattern. Further, to increase penetration inside the tissue, an FSS structure (64 × 64 mm2) is placed at the top of DSA at a distance of 4 mm. The FSS directs the energy toward the tumor location and provides a uniform heating pattern. The field and temperature distribution are numerically obtained for the proposed applicator in the heterogeneous phantom (layers of skin, fat, and muscles), and the performance of the applicator is evaluated in terms of the specific absorption rate, penetration depth, and effective field size. The performance of the applicator is also analyzed for a deeply placed (at 16 mm from the skin surface) irregular‐shaped tumor. The temperature profile calculates 44°C at an input power of 2.5 W. The results show that the applicator is capable of heating sufficiently deep‐seated tumors. [ABSTRACT FROM AUTHOR]

The analysis of Archimedean spiral slots using the theory of characteristic modes is presented in this article. Purely magnetic characteristic modes are used to study the behavior of a spiral slot etched in an infinite ground plane. The effect of the spiral geometry parameters (slot width, separation between spiral slot arms, number of spiral turns, and inner spiral spacing) on the characteristic modes are investigated using the theory of characteristic modes. Based on the results of the characteristic mode analysis, guidelines are presented for selecting the appropriate slot dimensions for realizing a compact spiral slot assembly (miniaturization), multimode behavior and for improving the bandwidth of the purely magnetic characteristic modes. Additionally, analytical equations are presented for accurately predicting the resonant frequencies of the purely magnetic characteristic mode of the spiral slot antenna.

Design and experimental verification of an ultrawideband planar circularly polarized ultrahigh frequency (UHF) sensor is presented for detection of partial discharge (PD) activities in high voltage (HV) equipment. The proposed sensor is cavity-backed cosine slot Archimedean spiral (CSAS) antenna with an electromagnetic (EM) transparent radome. The dual-arm slot spiral was meandered as cosine wave for size reduction and optimized for broadband operation with circular polarization (CP) over 0.5-5 GHz. Unidirectional radiation pattern with CP was achieved over 0.5-5 GHz using an optimized cavity loaded with EM absorber. The fabricated UHF CSAS sensor has the widest operating bandwidth of 0.5-5 GHz with circular polarization and the smallest aperture of $0.26\lambda {}_{L}$ when compared to the existing UHF PD sensors, where $\lambda {}_{L}$ is the wavelength at the lowest operating frequency. Time domain incipient discharge signals and phase resolved PD patterns gathered for three commonly occurring defects in test cells mimicking gas insulated switchgear (GIS) indicate better sensitivity and detection capability for the fabricated CSAS UHF sensor than the reference PD sensor. PD signal measurements and comparison with other UHF PD sensors indicate that the proposed UHF CSAS sensor could be used for PD detection in HV equipment.

This paper presents the conceptual stages of the simulation and development of a modified transceiver antenna for a high-power pulsed nuclear quadrupole resonance (NQR) detector of explosives containing the 14N isotope. At a frequency of 4.645 MHz, better characteristics are obtained using a nine-turn coil shaped as half of a Fermat spiral with an outer radius of 75 mm. Using a COMSOL Multiphysics numerical parametric simulation and a materials browser, it was possible to calculate a physical system with parameters as close to reality as possible. According to the results of the experimental studies of the radio frequency (RF) energy, the proposed antenna features an increase in the working area compared to a similar antenna, the topology of the conductive coil of which has the form of an Archimedean spiral. The resulting diagrams of the distribution of the magnetic induction also indicate that the topology of the electromagnetic (EM) field does not depend on the orientation of the sample under study relative to the axis of the radial symmetry observed in square–rectangular planar antennas. [ABSTRACT FROM AUTHOR]

A compact, omnidirectional, dual-polarized and electrically small spiral antenna with improved isolation, bandwidth and gain characteristics for 5G-WLAN applications is discussed in this article. The proposed MIMO spiral radiator is composed of two asymmetric CPW trapezoidal spiral-shaped (TSS) structures loaded with an aperiodic (2 × 1) array of SRRs for achieving better isolation characteristics. The TSS antenna operates at TM11 mode which resonates at 5.2 GHz, 5.8 GHz, and TM21 mode resonating at 5.5 GHz, for radiators 1 and 2, respectively. The experimentally validated gain of the TSS antenna is found to be 5.2 dB, 7.2 dB and 6.5 dB at the resonating bands, respectively. The proposed MIMO antenna radiating performances, such as TARC, ECC, DG and isolation characteristics, are evaluated using S-parameters. The modeled TSS–MIMO antenna has a compact size of 35 × 30 × 1.6 mm3, with triple-band resonances at 5.2 GHz, 5.5 GHz, and 5.8 GHz. With these attributes of the proposed TSS configuration, it can be considered a good candidate for WLAN, 5G-WiFi and future multi-band 5G applications. [ABSTRACT FROM AUTHOR]

This letter work develops a liquid metal-based spiral antenna for frequency reconfiguration over an ultrawide band and polarization reconfiguration at a partial operating band. Dual-layer polymethylmethacrylate microfluidic is glued on the dielectric substrate to provide channels for liquid metal flow. The syringe could smoothly control the liquid metal to change the lengths of the spiral antenna arms and result in effective frequency reconfiguration. Moreover, polarization reconfiguration between left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) is achieved by redistributing the liquid metal from the upper layer microfluidic channels to the lower layer microfluidic channels. Experimental results demonstrate that the proposed antenna supports continuous frequency reconfiguration from 0.6 to 13.5 GHz and realizes polarization reconfiguration between RHCP and LHCP from 1 to 2.5 GHz. The presented design enabled robust communications over a broad frequency spectrum. [ABSTRACT FROM AUTHOR]

Low-profile wideband spiral antennas suffer from poor impedance matching and cross polarization at lower frequencies or high variation in gain at higher frequencies. These significant differences, primarily caused by electromagnetic coupling with a conventional conductive backing, and the resulting interference are addressed in this letter by suitably designing the profile of the conductor. The proposed geometry has a maximum height of 0.08 $\lambda _{m}$ at the minimum operating frequency and has wideband circular polarization performance with a good boresight gain response over 2–18 GHz range. After numerical investigations, a prototype antenna is fabricated and characterized. An excellent impedance matching and circular polarization behavior are measured close to 2 GHz while the variation in gain is limited between 5.8 and 8.6 dBic above 10 GHz. [ABSTRACT FROM AUTHOR]

We report here that spiral and curl antennas are essentially the same when using the moment method. This aids in the understanding and design of the two antennas, and serves as the groundwork for new ideas regarding antennas. First, the spiral antenna height above the ground plane was gradually reduced, and at each height, the spiral configuration parameters were optimized for an axial ratio of less than 0.1 dB. It was found that the spiral antenna had configuration parameters almost the same as those for a curl antenna at a height of 0.15 wavelengths. Next, the curl antenna was analyzed with a reduced height. The antenna, at a height of 0.10 wavelengths, showed a 3 dB axial ratio bandwidth of 3%, with a VSWR of less than two. The analysis results are verified with experimental results. [ABSTRACT FROM AUTHOR]

Spiral antennas are often used for communication applications. However, this antenna has a two-way radiation pattern. The back-lobe antenna can reduce the main lobe transmit power. This article discusses the design of a spiral antenna with a directional radiation pattern. The antenna substrate uses RO4350B with a thickness of 1.524mm and a dielectric constant of 3.66. On the antenna arms, there is a resistive load to extend the bandwidth and reduce the dimensions of the antenna. A balun is used to feed the Archimedean spiral antenna as a balanced antenna. The absorbed cavity is used to reflect the beam in an undesired direction. The simulation results show that the antenna's operating frequency is in the range of 2.5-12.5GHz for a return loss of less than-10 dB. The absorber helps suppress undesired radiation fields and reduces the back lobe of the antenna. In addition, the use of an absorber can reduce antenna gain. Therefore, the thickness of the absorber needs to be considered. The simulation results show the performance of this antenna. [ABSTRACT FROM AUTHOR]

This paper presents a new design for a wideband circularly polarized (CP) spiral antenna array fed by a differential aperture coupling. The differential excitation is realized by two vias located at the lateral sides of a transverse slot etched on the lower substrate. The proposed two-arm spiral antenna has a smaller size compared to conventional spiral antennas with several turns. The presented single radiating element provides the impedance bandwidth of (|S11|

This paper presents a conformal spiral antenna that is miniaturized and with dual-resonant for the wireless implantable capsule system. The spiral antenna conforms to a swallowable capsule with a radius of 3 mm and a length of 26 mm without occupying the internal space of the capsule. The compact antenna adopts two spiral arms to extend the effective current path for miniaturization. Biocompatible flexible polyimide was used as the dielectric substrate and capsule shell, achieving conformal properties of the antenna as well as compatibility with human tissue. The antenna has been simulated in different environmental models. The bandwidth of the antenna can reach 39.16 % (1.82 GHz-2.76 GHz) and 12.06 % (5.36 GHz-6.06 GHz) at 2.4 GHz and 5.8 GHz. The maximum gains of −35.2 dBi and −28.1 dBi can be achieved at 2.4 GHz and 5.8 GHz, respectively. In addition, the transmission characteristics of the antenna were experimentally verified in the minced pork and pig intestine. By analyzing the communication link, the communication distance between transceivers at 2.4 GHz and 5.8 GHz can meet 14 m and 5 m. These results show that the proposed antenna is suitable for wireless implantable capsule systems.

The problems of electrodynamic analysis of thin-wire radiating structures with double symmetry are considered. New generalized integral representations of the electromagnetic field are obtained for the case of structures with single and double symmetries. Based on the obtained expressions, mathematical models of two - and four-way elliptical spiral antennas are constructed. It is shown that taking into account double symmetry in solving the internal electrodynamic problem leads to a set of independent Fredholm integral equations of the first kind written with respect to the distributions of normal current waves, which significantly simplifies the solution of the internal electrodynamic problem. Comparisons of current distributions along conductors, their input resistance dependences on the radius of the structure, and normalized radiation patterns for two- and four-way spiral emitters are presented.

A compact circularly polarized (CP) spiral antenna with enhanced impedance and 3-dB axial ratio (AR) bandwidths that is fed by a tapered coax balun is presented. The antenna is composed of a two-arm nonself-complementarity planar equiangular spiral, a z-directional helix, absorbing material with a stepped cylindrical honeycomb structure, and a ground plane. The z-directional helix is used to extend the current path and to improve the impedance matching and AR in the low-frequency band. The coupling between the nonself-complementarity planar spiral arms is increased, which improves the impedance matching in the low-frequency band and the AR at approximately 5.55 GHz. The stepped cylindrical absorbing material can reduce the reverse current and improve the AR in the high-frequency band. The measured results show that the impedance bandwidth and 3-dB AR bandwidths range from 0.5 to 6 GHz and from 0.5 to 5.8 GHz, respectively. The overall size of the proposed antenna is 0.189λL × 0.065λL (diameter × height, where λL is the free-space wavelength at the starting frequency).

This paper presents the conceptual stages of the simulation and development of a modified transceiver antenna for a high-power pulsed nuclear quadrupole resonance (NQR) detector of explosives containing the 14N isotope. At a frequency of 4.645 MHz, better characteristics are obtained using a nine-turn coil shaped as half of a Fermat spiral with an outer radius of 75 mm. Using a COMSOL Multiphysics numerical parametric simulation and a materials browser, it was possible to calculate a physical system with parameters as close to reality as possible. According to the results of the experimental studies of the radio frequency (RF) energy, the proposed antenna features an increase in the working area compared to a similar antenna, the topology of the conductive coil of which has the form of an Archimedean spiral. The resulting diagrams of the distribution of the magnetic induction also indicate that the topology of the electromagnetic (EM) field does not depend on the orientation of the sample under study relative to the axis of the radial symmetry observed in square–rectangular planar antennas.

In this paper, a nested dual-arm spiral antenna (NDASA) is proposed to generate multiple orbital angular momentum (OAM) modes at the same operating frequency. The NDASA consists of the feeding network and three nested dual-arm spiral structures with different sizes. Meanwhile, a metal reflector is located between the spiral structures and the feeding network to enhance the radiation directivity, which can also reduce the impact of the radio frequency (RF) coaxial cables. Both the experimental and simulated results show OAM waves with topological charges of l = 1, l = 3 and l = 5 can be independently generated at f = 3 GHz, and their maximum gains are 6.94dBi, 6.76dBi and 5.49dBi, respectively. In addition, the OAM modes multiplexing and demultiplexing technology are investigated in simulations and experiments. The isolations between the OAM modes of l = 1, l = 3 and l = 5 and the other different OAM modes are greater than 13.8dB, 13.2dB and 25.1dB, respectively, which proves the potential ability of the proposed NDASA for multiplexing and demultiplexing multiple OAM modes.

A compact wideband four‐arm circularly polarized (CP) spiral antenna with both sum mode (1) and difference mode (2) is presented. The antenna is composed of a modified four‐arm equiangular spiral, a four‐arm z‐directional helix, a ring‐shaped absorber, and a ground plane. By using the modified equiangular spiral and the z‐directional helix, the impedance matching of mode 1 and mode 2 is improved, and the cross‐polarization discrimination of mode 2 is also improved in the low frequency band. The z‐directional helix is also applied to achieve a compact size of the antenna and an enhanced AR bandwidth of mode 2. A prototype of the four‐arm spiral antenna is fabricated and measured with a beamformer operating at 0.5 to 2 GHz. It is shown that both sum and difference radiation patterns are generated in the whole frequency band of 0.5 to 2 GHz. The overall size is 0.486λ0.5 GHz × 0.097λ0.5 GHz (diameter × height, λ0.5 GHz is the free‐space wavelength at 0.5 GHz). [ABSTRACT FROM AUTHOR]

We report here that spiral and curl antennas are essentially the same when using the moment method. This aids in the understanding and design of the two antennas, and serves as the groundwork for new ideas regarding antennas. First, the spiral antenna height above the ground plane was gradually reduced, and at each height, the spiral configuration parameters were optimized for an axial ratio of less than 0.1 dB. It was found that the spiral antenna had configuration parameters almost the same as those for a curl antenna at a height of 0.15 wavelengths. Next, the curl antenna was analyzed with a reduced height. The antenna, at a height of 0.10 wavelengths, showed a 3 dB axial ratio bandwidth of 3%, with a VSWR of less than two. The analysis results are verified with experimental results.

A low-profile spiral antenna backed by a circular high-impedance surface (HIS) reflector composed of homogeneous (CirHISRhomo) fan-shaped patch elements is investigated. The total antenna height from the bottom of the CirHISRhomo to the spiral antenna is very small: 0.09 wavelength at a lowest design frequency of 3 GHz. It is revealed that the CirHISRhomo contributes to a constant input impedance and a small axial ratio (AR) across the design frequency range of 3–10 GHz (the fractional bandwidth is approximately 108%). For comparison, the AR for the spiral antenna backed by a conventional perfect electric conducting reflector is also presented. The analysis results are verified by the measured results of the fabricated antenna. [ABSTRACT FROM AUTHOR]

A novel artificial magnetic conductor (AMC) ground plane with spherical curvature is proposed. The metallic patches used to form the high impedance surface (HIS) ground plane are of circular periodicity. A detailed description on the design procedure and reflection phase characterization of such ground planes is provided. The impact of the curvature on the reflection characteristics and the maximum curvature which can be applied to the flat HIS are also examined. Two curvilinear antenna elements (loop and spiral) are located above the designed ground plane, and their performances are compared when the two are placed on a flat HIS. Measurements are performed to validate the simulations. An excellent agreement is attained. [ABSTRACT FROM AUTHOR]

This paper studies a unidirectional ultra‐wideband circularly polarized (CP) spiral antenna array. The ultra‐wideband high‐gain CP operation is realized by implementing four‐element sequentially rotated spiral array antennas fed with an ultra‐wideband feeding network. Tapered multisections power dividers and broadband microstrip‐slot phase shifters are utilized to provide accurately equal power division and constant phase shifts of 90°. In order to further improve the CP operating bandwidth, foam of absorber is adopted between the radiating aperture and the feeding network substrate. The proposed CP array is designed, fabricated, and measured. Test results show that the impendence bandwidth of the proposed array covers 2.2 to 11.6 GHz (5.3:1) and 3 dB axial‐ratio bandwidth covers 2.3 to 13 GHz (5.65:1), demonstrating a stable CP array performance within an ultra‐wide working bandwidth. [ABSTRACT FROM AUTHOR]

Stepped frequency continuous wave (SFCW) radar is a promising noncontact method to detection of human vital signs. Antenna as a key component of radar directly related to the performance of the radar. In this letter, we present a 0.5‐1.2 GHz compact multiple cut‐corners square spiral antenna for SFCW radar system. Compared with conventional planar spiral antennas, the improved antenna possesses the advantages of miniaturization and excellent axial ratio. The gradient‐change of the cut‐corners along the radiation determines its operation range: large cut‐corner corresponds to high frequency, while small cut‐corner affects low frequency. This design successfully leads to a small geometric size of 0.282 × 0.282 × 0.133 m. We measured this improved antenna in an anechoic chamber, and the results demonstrated that the circular polarization bandwidth for axial ratio ≤3 dB approached 100%, and a right‐hand circularly polarization (RHCP) gain of 4‐8.98 dBic in end‐fire direction is achieved in its operational range. Finally, we designed the experiment, experimental results show that the proposed antenna can work well with the SFCW radar in the detection of human vital sign (respiratory signals), indicating a great potential for detected life in practice. [ABSTRACT FROM AUTHOR]

This paper proposes a stepped-width spiral antenna for gain enhancement at a low operating frequency. The frequency-dependent characteristic of the stepped-width transmission line is applied to the spiral arm to enhance the gain at the low end of the operating frequency, and a cavity-backed ground is added to the bottom side of the antenna to further improve the gain at the bore-sight direction. To verify the gain enhancement at the low operating frequency and impedance matching characteristic, we observe the antenna performances such as bore-sight gains and reflection coefficients according to the shape of the spiral arm. The results confirm that the proposed stepped-width structure can enhance the gain at the low operating frequency by improving the impedance matching characteristic. [ABSTRACT FROM AUTHOR]

Design of a conical-disc-backed circular-polarized Archimedean single-arm spiral antenna is presented in this paper. The antenna operation covers the X-band frequencies ranging from 8 to 12 GHz. The antenna makes use of a very simple structure having the single-arm spiral backed by a cone-shaped metallic disc to achieve high gain, circular polarization, and unidirectional symmetric radiation near the boresight. The diameter of the antenna only measures to 40 mm. The simulated and measured results show that the antenna has a very good impedance matching (better than -10 dB), good right-hand circular polarization (with an axial ratio of ≤3 dB) throughout the frequency range of interest, and offers a maximum peak gain of 11.4 dBiC. The presented S11 response and radiation pattern results also show that the antenna offers excellent performance in the X-band with no need of a balun. Antenna usefulness is also established through a detailed parametric study and comparison with a traditional flat disc structure. Compact size, simple design, wide range, and high gain make the proposed antenna design a good choice for radar, terrestrial communications, and satellite/aerospace communications applications.

This paper presents an ultrawideband (UWB) circularly polarized (CP) two-arm Archimedean spiral antenna with unidirectional radiation patterns. The antenna is directly fed by a 50-Ω coaxial cable and reflected by a circular cavity. The proposed design is able to exhibit high gain, high radiation efficiency as well as UWB CP operation by properly choosing the reflector's dimensions and the spiral's growth rate. This overcomes the critical drawback of the other reported UWB CP spiral antennas in literature, which always suffer from poor performance in the lower operating frequencies. The measured reflection coefficient and axial-ratio show a good performance over a frequency band 6–16 GHz. Furthermore, the values for gain and radiation efficiency of the proposed design are always better than 5.2 dBic and 85%, respectively. [ABSTRACT FROM AUTHOR]

The paper presents design, simulation and experimental results for the reconfigurable square spiral antenna. This antenna is reconfigurable for two bands: band-I is 0.4–3 GHz which covers GSM, CDMA, Wi-Fi, multipoint communication system and band-II is 4.4–5.9 GHz which covers Wi-Max, 802.11Y, WLAN and 802.11a/h/j/n/ac standards. The antenna can be tuned to other aerospace and electronic warfare applications. This antenna structure offers a good radiation efficiency as the cavity is filled with low dielectric constant material and has high gain and directivity. PIN diodes have been used to achieve the reconfigurability. [ABSTRACT FROM AUTHOR]

A flexible low-impedance wideband antenna with a hybrid structure of Archimedean spiral and equiangular spiral is presented in this letter. The input impedance of the antenna is reduced to about 75 Ω by the attenuation effect of the equiangular spiral connected to the Archimedean spiral. The prototypes of the designed flexible antennas with and without polydimethylsiloxane encapsulation were fabricated with the copper-clad polyimide film and the transfer-printing technique. The simulation results of impedance and voltage standing-wave ratio show the fabricated antenna can be operated over the frequency range of 2.5–4.5 GHz. The S-parameters of the antennas were measured before and after applying uniaxially tensile strain and complex strain induced by balloon. The results show that S11 parameter of the antennas maintains less than –10 dB in the operating frequency band with deformation. The mechanical reliability of the antennas was also confirmed by the finite-element analysis of the strain in the copper layer. [ABSTRACT FROM AUTHOR]

This paper reports on the development and testing of a wideband spiral-shaped coupler for a partial discharge (PD) detection in the UHF spectrum. The coupler design, which is based on the log-periodic spiral slot antenna, has been optimized using finite element simulation. The following parameters have been optimized: feed location, arm termination shape, width of arm root, and termination. The unique feature of the coupler is the inclusion of tuning nodules on the spiral arms, which widen its bandwidth without the need for additional components. Tests in an anechoic chamber have shown a close agreement with the simulation results. The coupler exhibits a wide bandwidth (S11 < −9.55 dB) between 0.5–2.6 GHz and a peak gain of 4.8 dBi (at 1.35 GHz), while using an infinite balun coaxial feed, which allows for an input impedance of 50 Ω. The PD testing in the laboratory has shown that the phase resolved PD patterns acquired with the coupler are very similar to those captured with conventional PD sensors. An application of the coupler as an internal sensor in a gas-insulated busduct is detailed and a bandwidth comparison with other UHD PD sensors is provided. [ABSTRACT FROM AUTHOR]

A novel method of radiating circularly polarized (CP) impulse through a dispersive spiral antenna is presented. The proposed method utilizes time‐reversal‐based pulse compression to radiate a CP impulse without any modification to the existing antenna design, providing flexibility where the antenna can be used as a normal dispersive spiral antenna when TR is not applied. Using an Archimedean spiral antenna, the proposed method is validated with numerical simulation and measurement. [ABSTRACT FROM AUTHOR]

This article presents a high‐efficiency dual‐band rectenna, for RF energy harvesting in GSM bands (900‐1800 MHz). The rectifier used is a dual‐diode rectifier topology based on a Delon voltage doubler. To ensure, the highest efficiency as possible, source‐pull simulations are used to determine the optimal diodes input impedance. The antenna is a spiral planar structure, where two slots ring resonators have been added to expand bandwidth at low frequencies without increasing its initial size. It is associated with a broadband balun to transform its impedance into the desired 50‐Ω rectifier input impedance. The antenna and balun are sized for broadband operation to cover the main wireless communication bands. Measurement results show high RF‐DC conversion efficiencies of 28% and 20% at 890 MHz and 1780 MHz, respectively, for a low RF input power of −20 dBm and with a 10 kΩ DC load. Efficiency reaches 43.5% with a dual‐tone signal at −20 dBm. [ABSTRACT FROM AUTHOR]

We use a genetic algorithm to minimize the relative sidelobe level (RSLL) of a dual polarized concentric ring array. The optimization variables are the placement of uniformly weight spiral antennas in an aperture with a small area. We optimized a cost function with isotropic sources instated of spiral elements to make the cost function reasonable to evaluate. We developed approaches to calculate a faster cost function and reduce the number of function calls in the optimization. We then calculated the array with an approximation of the element pattern to evaluate its performances. [ABSTRACT FROM AUTHOR]

Objective: This numerical study was designed to evaluate the feasibility of using an inductive coil for monitoring the changes in the volume of a hematoma in the head in situ and to compare the inductive coil performance to that of a spiral antenna based on the radar principle. Methods: Numerical analysis was used to solve the complete set of Maxwell's equations in full three-dimensional anatomical model of a head and brain with data on clinical occurrence of hematomas from the clinical literature, for frequencies of 100 MHz, 500 MHz, and 1 GHz. Results: 1) The analysis shows that the spiral radar antenna provides a better resolution when the antenna can be placed exactly facing the center of the volume of blood. Under any other circumstance, the inductive coil has a better resolution at both 500 MHz and 1 GHz. 2) The induction coil is more sensitive to rotation artifacts than the spiral antenna. 3) Single frequency measurements do not provide conclusive results. Conclusion: The inductive coil has the ability to monitor small changes in the volume of a hematoma in the head. However, multifrequency measurements are required for correct diagnostic. Significance: This study provides a new, low-cost alternative to the conventional medical imaging for monitoring the hematoma increase. [ABSTRACT FROM AUTHOR]