Large Fractional Bandwidth Research Articles

A low‐profile circularly polarized cavity‐based waveguide aperture antenna

AbstractThis article proposes a new design of a circularly polarized (CP) aperture antenna based on a dual‐mode rectangular waveguide cavity‐resonator. A capacitive iris is employed to attain a strong coupling between the cavity and the input port, resulting in a larger fractional bandwidth (FBW) than a conventional aperture antenna. Another capacitive iris is embedded at the corner of the radiating aperture and extended into the cavity to maintain equal amplitude and a phase difference of 90° between the radiated electric (E) fields of the TE101 and TE110 modes, and thus yielding CP radiation. The proposed antenna has been fabricated using a computer numerical control machine. The antenna has an FBW of 18% at center frequency (f0) f0 = 10 GHz when the reflection coefficient (S11) S11 = −10 dB. The 3‐dB axial ratio bandwidth is 7%. A gain of 8.0 dBic is found at 10.06 GHz. The low profile and competitive performance of the antenna make it a good candidate in radar applications.

Propagation effects at low frequencies seen in the LOFAR long-term monitoring of the periodically active FRB 20180916B

ABSTRACT LOFAR (LOw Frequency ARray) has previously detected bursts from the periodically active, repeating fast radio burst (FRB) source FRB 20180916B down to unprecedentedly low radio frequencies of 110 MHz. Here, we present 11 new bursts in 223 more hours of continued monitoring of FRB 20180916B in the 110–188 MHz band with LOFAR. We place new constraints on the source’s activity window $w =4.3^{+0.7}_{-0.2}$ d and phase centre $\phi _{\mathrm{c}}^{\mathrm{LOFAR}} = 0.67^{+0.03}_{-0.02}$ in its 16.33-d activity cycle, strengthening evidence for its frequency-dependent activity cycle. Propagation effects like Faraday rotation and scattering are especially pronounced at low frequencies and constrain properties of FRB 20180916B’s local environment. We track variations in scattering and time–frequency drift rates, and find no evidence for trends in time or activity phase. Faraday rotation measure (RM) variations seen between June 2021 and August 2022 show a fractional change >50 per cent with hints of flattening of the gradient of the previously reported secular trend seen at 600 MHz. The frequency-dependent window of activity at LOFAR appears stable despite the significant changes in RM, leading us to deduce that these two effects have different causes. Depolarization of and within individual bursts towards lower radio frequencies is quantified using LOFAR’s large fractional bandwidth, with some bursts showing no detectable polarization. However, the degree of depolarization seems uncorrelated to the scattering time-scales, allowing us to evaluate different depolarization models. We discuss these results in the context of models that invoke rotation, precession, or binary orbital motion to explain the periodic activity of FRB 20180916B.

Suppresion of Log‐Periodic Dipole Antenna Spurious Radiation by Lumped Element Loading for Radioastronomical Application

AbstractLog‐Periodic Dipole Antennas have been known to provide good performance over large fractional bandwidths. However, optimized designs of log‐periodic antennas, in particular for radioastronomy and remote sensing, need to tackle the spurious radiation of secondary dipole harmonics that can compromise the gain smoothness and therefore the ability to detect natural spectral features emitted by the probed sources. A new, low‐cost method applicable also at a post‐design stage is proposed in this paper, which relies on loading some of the dipoles with appropriate lumped circuit elements in order to avoid this unwanted feature while preserving the desired performance characteristics. Application is found on the log‐periodic dipole antenna selected for the low‐frequency instrument of the SKA (Square Kilometer Array), and general guidelines are described for other designs. An experimental test using a transmitting VHF antenna and the antenna under test with new RLC loaded dipoles confirms our solution.

Design and optimization of a high-energy optical parametric amplifier for broadband, spectrally incoherent pulses.

Spectrally incoherent laser pulses with sufficiently large fractional bandwidth are in demand for the mitigation of laser-plasma instabilities occurring in high-energy laser-target interactions. Here, we modeled, implemented, and optimized a dual-stage high-energy optical parametric amplifier for broadband, spectrally incoherent pulses in the near-infrared. The amplifier delivers close to 400 mJ of signal energy through noncollinear parametric interaction of 100-nJ-scale broadband, spectrally incoherent seed pulses near 1053 nm with a narrowband high-energy pump operating at 526.5 nm. Mitigation strategies for high-frequency spatial modulations in the amplified signal caused by index inhomogeneities in the Nd:YLF rods of the pump laser are explored and discussed in detail.

Wideband Surface Acoustic Wave Filter at 3.7 GHz With Spurious Mode Mitigation

As an essential functional block in radio frequency (RF) front-ends, surface acoustic wave (SAW) filters with large fractional bandwidth (FBW), high center frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{c}$ </tex-math></inline-formula> ), and low loss are highly sought-after in 5G new radio (NR). To address this end, 32°Y-X LiNbO3/SiO2/Si heterostructure with high electromechanical coupling coefficient of 23% was constructed, while the propagation characteristics as well as spurious resonance were analyzed through finite element method (FEM) and one-port resonators measurement. It is revealed that by using thin electrodes in the series resonator and thick electrodes in the parallel resonator, spurious modes are successfully eliminated out of the passband. As a result, the SAW filters formed by the above resonators are capable of several superb RF performances: a large bandwidth of 730 MHz (FBW of 19.5%) at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{c}$ </tex-math></inline-formula> of 3.74 GHz, a small insertion loss (IL) of 3.03 dB, and a flat passband. Furthermore, since our devices were fabricated by standard photolithography process, this work advances commercial implementation of SAW filter in 5G NR.

Wide-band Timing of the Parkes Pulsar Timing Array UWL Data

In 2018 an ultra–wide-bandwidth low-frequency (UWL) receiver was installed on the 64 m Parkes Radio Telescope, enabling observations with an instantaneous frequency coverage from 704 to 4032 MHz. Here we present the analysis of a 3 yr data set of 35 ms pulsars observed with the UWL by the Parkes Pulsar Timing Array, using wide-band timing methods. The two key differences compared to typical narrowband methods are (1) generation of two-dimensional templates accounting for pulse shape evolution with frequency and (2) simultaneous measurements of the pulse time of arrival (TOA) and dispersion measure (DM). This is the first time that wide-band timing has been applied to a uniform data set collected with a single large fractional bandwidth receiver, for which such techniques were originally developed. As a result of our study, we present a set of profile evolution models and new timing solutions, including initial noise analysis. Precision of our TOA and DM measurements is in the range of 0.005–2.08 μs and (0.043–14.24) × 10−4 cm−3 pc, respectively, with 94% of the pulsars achieving a median TOA uncertainty of less than 1 μs.

A single-layer, wideband and angularly stable metasurface based polarization converter for linear-to-linear cross-polarization conversion.

In this article, a single-layer metasurface based reflector design is proposed for linear-to-linear cross-polarization conversion in microwave frequency range. The unit-cell of the proposed design consists of triple-arrow resonant design printed on a grounded FR4 substrate. Excellent cross-conversion is achieved over a broad frequency range (8.0-18.50 GHz) with polarization conversion efficiency higher than 90%. The proposed design has a large fractional bandwidth (FBW) of 80% due to three resonances occurring in the band. The polarization response is angularly stable with respect to oblique incidences with incidence angles up to 45°. The proposed design has been fabricated and experimentally validated. The measurement results are in good agreement with the simulation results.

Highly efficient tunable broadband terahertz polarizers based on a graphene metasurface

Ultrathin tunable broadband terahertz transmission mode linear-to-circular polarizers are proposed and numerically validated. Each structure consists of a dielectric substrate with slotted graphene patches on one or both sides of the substrate. Both polarizers yield tunable broadband conversion with large transmission coefficients in terahertz frequency range. Compared to the previous polarizers, the proposed structure with a single-layer of graphene patches exhibits a 120% larger fractional bandwidth and 60% larger transmittance. The polarizer with two layers of graphene patches features a 138% larger fractional bandwidth and 25% larger transmittance, compared to the previous polarizers, with excellent axial ratio. The tuning frequency range for the proposed single- and double-layer structures is (2.4 to 4.9) and (1.8 to 5.8) THz, respectively. The tuning is achieved by varying the graphene Fermi energy (chemical potential) from 0.2 to 1 eV, by the application of a DC bias voltage. Moreover, the polarization conversion performance is well maintained under oblique incidence condition, over a wide range of incident angles up to 70°. The size of the square-shaped unit cell is λ0/10, much smaller than the free-space wavelength of the incident waves, λ 0 , while the thickness is λ0/22. With the above features, the present work offers a further step in developing controllable polarization converters with potential applications in imaging, sensing, and communications.

Wide-band Timing of GMRT-discovered Millisecond Pulsars

Modeling of frequency-dependent effects, contributed by the turbulence in the free electron density of interstellar plasma, is required to enable the detection of the expected imprints from the stochastic gravitational-wave (GW) background in pulsar timing data. In this work, we present an investigation of temporal variations of interstellar medium for a set of millisecond pulsars (MSPs) with the upgraded Giant Metrewave Radio Telescope (GMRT) aided by large fractional bandwidth at lower observing frequencies. Contrary to the conventional narrowband analysis using a frequency-invariant template profile, we applied PulsePortraiture-based wide-band timing analysis while correcting for the evolution of the pulsar profile with frequency. Implementation of the PulsePortraiture-based wide-band timing method for the GMRT-discovered MSPs to probe the dispersion measure (DM) variations resulted in a DM precision of 10−4 pc cm−3. In general, we achieve similar DM and timing precision from wide-band timing compared to the narrowband timing with matching temporal variations of DMs. This wide-band timing study of newly discovered MSPs over a wide frequency range highlights the effectiveness of profile modeling at low frequencies and probes the potential of using them in a pulsar timing array.

Pulsar scintillation studies with LOFAR

Context. Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionized interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision. Aims. The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA). Methods. We use observations from five international LOFAR stations and the LOFAR core in the Netherlands. We analyze the 2D auto-covariance function of the dynamic spectra of these observations to determine the characteristic bandwidth and timescale of the ISS toward the pulsars in our sample and investigate the 2D power spectra of the dynamic spectra to determine the presence of scintillation arcs. Results. In this initial set of 31 sources, 15 allow for the full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120–180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight do not display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and a dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations. Conclusions. Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters such as pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of ISS effects at low frequencies.

Lead-Free Sodium Potassium Niobate-Based Multilayer Structures for Ultrasound Transducer Applications

Thick films with nominal composition (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) on porous ceramics with identical nominal composition were investigated as potential candidates for environmentally benign ultrasonic transducers composed entirely of inorganic materials. In this paper, the processing of the multilayer structure, namely, the thick film by screen printing and the porous ceramic by sacrificial template method, is related to their phase composition, microstructure, electromechanical, and acoustic properties to understand the performance of the devices. The ceramic with a homogeneous distribution of 8 μm pores had a sufficiently high attenuation coefficient of 0.5 dB/mm/MHz and served as an effective backing. The KNNSr thick films sintered at 1100 °C exhibited a homogeneous microstructure and a relative density of 97%, contributing to a large dielectric permittivity and elastic constant and yielding a thickness coupling factor kt of ~30%. The electroacoustic response of the multilayer structure in water provides a centre frequency of 15 MHz and a very large fractional bandwidth (BW) of 127% at −6 dB. The multilayer structure is a candidate for imaging applications operating above 15 MHz, especially by realising focused-beam structure through lenses to further increase the sensitivity in the focal zone.

A compact SWB MIMO antenna with 45° clock-wise square patch inclusions for polarization diversity applications

AbstractIn this paper, a compact super wideband annular ring antenna using 45° clock-wise square patch inclusions for super high frequency and polarization diversity applications is proposed. The inclusions consist of a combination of squares and circles into one another in the inner area of a main annular ring radiator. The antenna uses a partial ground plane having a stair-type defected ground structure, is designed on an FR-4 substrate, and has a total size of 25 × 26 × 1.6 mm3 (0.17λ × 0.18λ). The design was fabricated and experimental results fairly agreed with simulations and resulted in an antenna with an operating frequency from 2.07 to 30 GHz; that is, a large fractional bandwidth of 174.2$\%$ with a bandwidth (BW) ratio of 14.5:1 and a high BW dimension ratio, BW per unit electrical length of 5693, and a measured peak gain of 8 dBi with an average gain of 5 dBi for the overall operating frequency. For the polarization diversity, a 4 × 4 multiple input-multiple output configuration is additionally presented, offering an effective isolation of ≥ 22.5 dB between ports and corroborated by measurements.

A novel wideband multi‐mode coupled resonator filter based on bulk acoustic wave technology

This paper proposes a novel multi-mode bulk acoustic wave (BAW) coupled resonator filter (CRF) for wideband applications. Three stepped-impedance coupling layers of CRF are used to generate multi-mode properties, and external inductors are added for port match. To facilitate the calculation of resonance frequencies, a simplified Mason model is derived and applied in the even- and odd-mode analysis of CRF. Based on different acoustical length of CRF coupling layers, four cases that are applicable for different filter bandwidth are proposed. From these cases, fractional bandwidth (FBW) of the multi-mode CRF can be realized in a wide range from 12.8% to 33% when the coupling layers adopt the commonly used SiO2/Mo/SiO2 materials. Moreover, the tuning range of FBW can be further enlarged by changing the materials of coupling layers. To verify the proposed multi-mode CRF, a triple-mode n77 band filter with center frequency of 3.723 GHz and FBW of 24.2%, and a penta-mode filter with a larger FBW up to 33% are designed and simulated. Simulation results of the original Mason model, the simplified Mason model, and the three-dimensional (3D) model based on the finite element method are consistent well with each other.

Bandwidth enhancement of low-profile slot antennas using theory of characteristic modes

In this article, we have presented a novel design of wideband slot antennas using theory of characteristic modes and theory of mode coupling. First, the narrow slot antenna, etched in a finite ground, is investigated with combined characteristic modes. While most of resonant slot antennas show narrowband operation, the modification of modal properties using structural manipulations is studied in detail and this analysis helps in the design of low-profile wideband slot antennas. Characteristic mode analysis explains the physical insight of resonance and radiation mechanism of proposed wideband slot antennas. Bandwidth enhancement techniques of these slot antennas are presented and validated with measured results of wideband slot antennas. This approach gives a large fractional bandwidth of >35% and high gain of >3 dB.

Wideband and Low-Loss Surface Acoustic Wave Filter Based on 15° YX-LiNbO₃/SiO₂/Si Structure

With the development of the radio frequency technique, the explosive growth of transmitted data in 5G era puts higher requirements on surface acoustic wave (SAW) filter bandwidth. In this work, 15°YX-LiNbO3(LN)/SiO2/Si multilayer structure was proposed, where both LN and SiO2 films possess uniform thickness and the interfaces between films are quite clear. With hierarchical cascading algorithm, the spurious resonance that resulted from Rayleigh mode was minimized through the modulation of Cu electrode thickness. One-port resonators measurement results confirm a high electromechanical coupling coefficient of shear horizontal mode ranging from 22.5% to 25.2%. Furthermore, a ladder-type filter with a center frequency of 1279 MHz and a large fractional bandwidth of 20.2% was successfully fabricated. Excellent bandpass filtering properties were achievable with minimum insertion loss of 0.8 dB and in-band fluctuation less than 0.9 dB. Multilayer structure SAW filters also exhibited a temperature coefficient of frequency of −57.7 ppm/°C and a power durability of 33 dBm, which are both significant improvements compared with that of devices built on bulk 15°YX-LN substrate. This work provides an effective solution for wideband and low-loss radio frequency filters in 5G communication systems, suitable for large-scale application and commercial promotion.

Design and comparative analysis of compact flexible UWB antenna using different substrate materials for WBAN applications

In the present scenario, smart devices are using flexible wearable wireless technologies to enable interaction among people and their surrounding equipments. Due to the commercialization of these flexible electronic devices and systems, these systems require the integration of flexible antennas. The compactness and flexibility of the antenna are the major objectives of the current research. This paper highlights the comparative study of a small-sized swastika-shaped slotted patch ultra-wide-band (UWB) antenna. The designed microstrip patch antenna is simulated using Rogers RT 5880 as a substrate mounted on the partial ground structure. A rectangular substrate with a size of 24.56 × 22.48 mm × 1.575 mm is placed between the partial ground and patch. The dimensions of the radiating patch are 13 mm × 10.5 mm × 0.035 mm. The resonant frequencies of the designed antenna are 4.43 GHz and 8.84 GHz and have a large fractional bandwidth of 110% in the range of 3.1 to 10.7 GHz. The maximum observed gain and directivity of the proposed antenna are 5.9 dB and 6.8 dBi at 4.43 GHz. The simulation of the presented antenna design is performed in CST Microwave Studio 2018. The proposed antenna performance metrics are comparatively analyzed using different substrate materials like Rogers RT 5880, Jeans, and rubber substrates. The developed dual-band antenna can be useful for WBAN communication applications in the UWB band.

Fast algorithms to approximate the position-dependent point spread function responses in radio interferometric wide-field imaging

ABSTRACT The desire for wide field of view, large fractional bandwidth, high sensitivity, high spectral and temporal resolution has driven radio interferometry to the point of big data revolution where the data are represented in at least three dimensions with an axis for spectral windows, baselines, sources, etc., where each axis has its own set of subdimensions. The cost associated with storing and handling these data is very large, and therefore several techniques to compress interferometric data and/or speed up processing have been investigated. Unfortunately, averaging-based methods for visibility data compression are detrimental to the data fidelity, since the point spread function (PSF) is position-dependent, that is, distorted and attenuated as a function of distance from the phase centre. The position dependence of the PSF becomes more severe, requiring more PSF computations for wide-field imaging. Deconvolution algorithms must take the distortion into account in the major and minor cycles to properly subtract the PSF and recover the fidelity of the image. This approach is expensive in computation since at each deconvolution iteration a distorted PSF must be computed. We present two algorithms that approximate these position-dependent PSFs with fewer computations. The first algorithm approximates the position-dependent PSFs in the uv-plane and the second algorithm approximates the position-dependent PSFs in the image plane. The proposed algorithms are validated using simulated data from the MeerKAT telescope.

AlN MEMS filters with extremely high bandwidth widening capability

This paper presents radio frequency (RF) microelectromechanical system (MEMS) filters with extremely high bandwidth widening capability. The proposed filtering topologies include hybrid configurations consisting of piezoelectric MEMS resonators and surface-mounted lumped elements. The MEMS resonators set the center frequency and provide electromechanical coupling to construct the filters, while the lumped-element-based matching networks help widen the bandwidth (BW) and enhance the out-of-band rejection. Aluminum nitride (AlN) S0 Lamb wave resonators are then applied to the proposed filtering topologies. AlN S0 first- and second-order wideband filters are studied and have shown prominent performance. Finally, the AlN S0 first-order wideband filter is experimentally implemented and characterized. The demonstrated first-order filter shows a large fractional bandwidth (FBW) of 5.6% (achieved with a resonator coupling of 0.94%) and a low insertion loss (IL) of 1.84 dB. The extracted bandwidth widening factor (BWF) is 6, which is approximately 12 times higher than those of the current ladder or lattice filtering topologies. This impressive bandwidth widening capability holds great potential for satisfying the stringent BW requirements of bands n77, n78, and n79 of 5G new radio (NR) and will overcome an outstanding technology hurdle in placing 5G NR into the marketplace.

On the Frequency Dispersion in DBF SAR and Digital Scalloped Beamforming

Digital beamforming (DBF) with Scan-On-Receive (SCORE) in elevation is a powerful technique for spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide-swath (HRWS) images. DBF in spaceborne SAR should be implemented in real-time to reduce the volume of the downloaded data. Current digital beamformers in SAR systems usually use phase shift instead of a time delay network to ensure real-time implementation. However, conventional phase shift DBF SAR with narrow array bandwidth is not applicable for broadband signals. The beam steering deviation as a function of frequency is obvious for signals with large fractional bandwidth, which appears as a phenomenon of frequency dispersion. If the signal bandwidth is wider than the array bandwidth, frequency dispersion can significantly deteriorate the signal-to-noise ratio (SNR) and must be well relieved. This article analyzes the frequency dispersion in DBF SAR and proposes an innovative DBF-SCORE scheme with specially designed scalloped beam. The proposed digital scalloped beamforming (DSBF) scheme is effective in relieving the influence of frequency dispersion in DBF SAR. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme. The proposed DSBF scheme gives practical solution to the problem of frequency dispersion in broadband DBF SAR, which makes DBF-SCORE a more practical technique for future spaceborne HRWS SAR.

Searching for the largest bound atoms in space

Context. Radio recombination lines (RRLs) at frequencies ν &lt; 250 MHz trace the cold, diffuse phase of the interstellar medium, and yet, RRLs have been largely unexplored outside of our Galaxy. Next-generation low-frequency interferometers such as LOFAR, MWA, and the future SKA will, with unprecedented sensitivity, resolution, and large fractional bandwidths, enable the exploration of the extragalactic RRL universe. Aims. We describe methods used to (1) process LOFAR high band antenna (HBA) observations for RRL analysis, and (2) search spectra for RRLs blindly in redshift space. Methods. We observed the radio quasar 3C 190 (z ≈ 1.2) with the LOFAR HBA. In reducing these data for spectroscopic analysis, we placed special emphasis on bandpass calibration. We devised cross-correlation techniques that utilize the unique frequency spacing between RRLs to significantly identify RRLs in a low-frequency spectrum. We demonstrate the utility of this method by applying it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new observations of 3C 190. Results. Radio recombination lines have been detected in the foreground of 3C 190 at z = 1.12355 (assuming a carbon origin) owing to the first detection of RRLs outside of the local universe (first reported in A&amp;A, 622, A7). Toward the Galactic supernova remnant Cassiopeia A, we uncover three new detections: (1) stimulated Cϵ transitions (Δn = 5) for the first time at low radio frequencies, (2) Hα transitions at 64 MHz with a full width at half-maximum of 3.1 km s−1 the most narrow and one of the lowest frequency detections of hydrogen to date, and (3) Cα at vLSR ≈ 0 km s−1 in the frequency range 55–78 MHz for the first time. Additionally, we recover Cα, Cβ, Cγ, and Cδ from the −47 km s−1 and −38 km s−1 components. In the nearby starburst galaxy M 82, we do not find a significant feature. With previously used techniques, we reproduce the previously reported line properties. Conclusions. RRLs have been blindly searched and successfully identified in Galactic (to high-order transitions) and extragalactic (to high redshift) observations with our spectral searching method. Our current searches for RRLs in LOFAR observations are limited to narrow (&lt;100 km s−1) features, owing to the relatively small number of channels available for continuum estimation. Future strategies making use of a wider band (covering multiple LOFAR subbands) or designs with larger contiguous frequency chunks would aid calibration to deeper sensitivities and broader features.