Center Operating Frequency Research Articles

A Wideband Low-Profile All-Metal Cavity Slot Antenna With Filtering Performance for Space-Borne SAR Applications

This letter presents a wideband all-metal cavity slot antenna with a filtering performance for space-borne synthetic aperture radar applications. The proposed antenna element consists of a metal cavity and several radiation slots. It is fabricated by aluminium through milling and welding process, the techniques of which are reliable for the space environment. Benefiting from multiresonances of the cavity and slots, the antenna is able to obtain a wide operating bandwidth. Additionally, the cavity has a very low profile of 2.8 mm, equivalent to 0.093 wavelength at the central operating frequency. An 8 × 12 antenna array is designed, fabricated, and measured for performance verification. The measured impedance bandwidth with S 11<−10 dB is achieved from 7.2 to 12.2 GHz, giving a relative bandwidth of 51%. Furthermore, it shows good filtering characteristic with more than 20 dB out-of-band rejections, which is capable to suppress unwanted interferences and contribute to a good electromagnetic compatibility design.

Design of Ultraflat Phase Shifters Using Multiple Quarter-Wavelength Short-Ended Stubs

This letter presents a kind of ultraflat phase shifters using multiple quarter-wavelength ( $\lambda /4$ ) short-ended stubs. The proposed topology consists of a uniform line with multi- $\lambda /4$ short-ended stubs and a reference line. Inspired by the high-pass filter design, the phase difference can be analytically determined by the impedance and the number of the short-ended stubs. In order to obtain a wider phase bandwidth with ultraflat phase difference around the center operating frequency, a generalized design equation is newly derived over a wide achievable range from 15° to 360°. As a validation, the third-order proposed phase shifter with 135° phase difference is designed and implemented at 3 GHz. The measured results show a good agreement with those obtained from simulation. The measured 10-dB impedance bandwidth achieves 102%, and the bandwidth of 135° ±5° phase difference is about 78%.

Analysis of a fractal ultrasonic transducer with a range of piezoelectric length scales

Abstract The transmission and reception sensitivities of most piezoelectric ultrasonic transducers are enhanced by their geometrical structures. This structure is normally a regular, periodic one with one principal length scale, which, due to the resonant nature of the devices, determines the central operating frequency. There is engineering interest in building wide-bandwidth devices, and so it follows that, in their design, resonators that have a range of length scales should be used. This paper describes a mathematical model of a fractal ultrasound transducer whose piezoelectric components span a range of length scales. There have been many previous studies of wave propagation in the Sierpinski gasket but this paper is the first to study its complement. This is a critically important mathematical development as the complement is formed from a broad distribution of triangle sizes, whereas the Sierpinski gasket is formed from triangles of equal size. Within this structure, the electrical and mechanical fields fluctuate in tune with the time-dependent displacement of these substructures. A new set of basis functions is developed that allow us to express this displacement as part of a finite element methodology. A renormalization approach is then used to develop a recursion scheme that analytically describes the key components from the discrete matrices that arise. Expressions for the transducer’s operational characteristics are then derived and analysed as a function of the driving frequency. It transpires that the fractal device has a significantly higher reception sensitivity (18 dB) and a significantly wider bandwidth (3 MHz) than an equivalent Euclidean (standard) device.

The Design of Optical Circuit-Analog Absorbers through Electrically Small Nanoparticles

In the last few years, the perfect absorption of light has become an important research topic due to its dramatic impact in photovoltaics, photodetectors, color filters and thermal emitters. While broadband optical absorption is relatively easy to achieve using bulky devices, today there is a strong need and interest in achieving the same effects by employing nanometric structures that are compatible with modern nanophotonic components. In this paper, we propose a general procedure to design broadband nanometer-scale absorbers working in the optical spectrum. The proposed devices, which can be considered an extension to optics of microwave circuit-analog absorbers, consist of several layers containing arrays of elongated nanoparticles, whose dimensions are engineered to control both the absorption level and the operational bandwidth. By combining a surface-impedance homogenization and an equivalent transmission-line formalism, we define a general analytical procedure that can be employed to achieve a final working design. As a relevant example, we show that the proposed approach allows designing an optical absorber exhibiting a 20% fractional bandwidth on a thickness of λ/4 at the central frequency of operation. Full-wave results confirming the effectiveness of the analytical findings, as well as some considerations about the experimental realization of the proposed devices are provided.

Wideband Slot Antenna Backed by Cylindrical Substrate-Integrated Waveguide Cavity

A cylindrical substrate-integrated waveguide cavity-backed slot antenna is presented in this paper, which is devised for wideband operation. The fundamental and critical approach to generate a wide antenna bandwidth performance, in this case, is to merge three basic resonant modes (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">010</sub> , TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">110</sub> , and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">111</sub> modes) of the cylindrical cavity. In this paper, a fractional impedance bandwidth of 60% with S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> < −10 dB from 8 to 14.8 GHz has been achieved through this scheme. This antenna has a compact size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.33\lambda $ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> denotes the space wavelength corresponding to the operating 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">$f =11.25$ </tex-math></inline-formula> GHz in this demonstration), which is found suitable for many wireless applications. Its resulting simple structure is definitely of benefit to the fabrication and integration with other front-end parts. The proposed antenna exhibits a broadside radiation pattern, which is stable within the operating frequency band of interest. The simulated radiation efficiency is above 85%. The antenna has been fabricated and experimentally verified. Measured results show a good agreement with simulated counterparts.

Design and Optimization of Nonuniform Helical Antennas With Linearly Varying Geometrical Parameters

Nonuniform helical antennas have many degrees of freedom, which makes the search space for the optimal design very challenging. The objective of this paper is to systematically analyze nonuniform helical antennas with linearly varying geometrical parameters and to provide analytical equations that approximate the optimal design and the gain of the designed antennas. Using various optimization algorithms, we made a large database of the optimal nonuniform helical antennas with linearly varying geometrical parameters. Based on these results, we made analytical equations that approximate the optimal design and the gain of the designed antennas. These equations allow for a fast design procedure yielding all necessary parameters needed for the design and fabrication of nonuniform helical antennas that meet specified characteristics. Special attention is devoted to antenna losses. Antennas designed following the presented procedure achieve around 2.5 dB higher gain than uniform helical antennas of the same axial length, while maintaining the bandwidth and axial ratio. As a verification of the proposed design procedure, a helical antenna with the central operating frequency of 1 GHz was designed, simulated, fabricated, and measured. The comparison between measured and simulated results confirms the validity of the presented design procedure.

Planar In-Phase Filtering Power Divider With Tunable Power Division and Controllable Band for Wireless Communication Systems

A wideband in-phase power divider with tunable power division ratio (PDR) and filtering response is presented. The proposed design is quite compact and consists of a three-line coupled structure and loaded with a pair of varactor-loaded short-ended stubs at two output terminations. A variable power division is realized by tuning the coupling factors between the centerline and sidelines of the three-line coupled structure using a pair of varactors. A varactor at the center of the loaded short-ended stub and another one connecting the end of the stub to the ground is used to control the center frequency and band of operation. A prototype is modeled using the microstrip technique and then simulated and tested. The experimental result indicates a tunability of PDR from 0.5:1 to 2:1 across wideband range of 60% and a controllable band from 35% to 63.2%, with 13% center frequency tunability, sharp passband cutoff selectivity, as well as harmonic suppression to more than 6 GHz (4 times the center frequency). Moreover, the overall loss is around 1–1.2 dB; the isolation is more than 14 dB, whereas the differential phase is less than 7° in all the investigated cases.

Miniaturization of Broadband Wilkinson Power Dividers

This paper proposed three modified Wilkinson power dividers in order to achieve a size reduction and a wide bandwidth. The first structure presented the power divider using compact folded step impedance transmission lines rather than the uniform microstrip line design for operating center frequency of 3 GHz. The second structure showed the power divider with delta-stub for 2.4 GHz. Finally, the third modified structure introduced the two-section Wilkinson power divider using series-delta stub for center frequency of 2.4 GHz as well. The study managed to get an overall dimension of 15 mm × 9.5 mm for the first proposed design achieving a reduction of 75.6 % and fractional bandwidth of 133 %. For the second proposed structure, the size was 15 mm × 15 mm with a reduction of 56 % and fractional bandwidth of 56 %. While the third design size was 17 mm × 15 mm with a reduction of 63.6 % and the structure achieved a broadband bandwidth with fractional bandwidth of 220 %. The proposed power dividers used RT/duroid 5880 substrate with a thickness of 0.38 mm. Simulation and measurement results indicated that the modified power dividers showed equal power division, good phase balance, high isolation between output ports, and good return loss better than -12 dB covering the operating frequency range&lt;strong&gt;.&lt;/strong&gt;

Wide Bandwidth and Enhanced Gain of a Low-Profile Dipole Antenna Achieved by Integrated Suspended Metasurface

A low-profile metasurface-based dipole antenna is proposed, analyzed, and implemented for the wideband operation with improved gain. The metasurface is suspended by separating it from the ground plane with an air gap and positioning it right beneath a horizontally oriented dipole. The wide operating bandwidth is attributed to the simultaneous excitation of the transverse electric surface wave resonance of the suspended metasurface (SMS) and the adjacent metasurface-based dipole resonance. The small propagation constant of transverse magnetic surface wave in the operating band enlarges the SMS aperture along the dipole direction for higher gain compared to a conventional low-profile antenna. The proposed antenna design with an overall thickness of $0.07\lambda $ ( $\lambda $ is the wavelength in free space at the center operating frequency) shows the realized boresight gain of 8.5–11.5 dBi over $\vert \text{S}_{11}\vert \le -10$ dB impedance bandwidth of 33.6% (or 4.15–5.85 GHz).

Performance Analysis of a Low Profile Hybrid Antenna for Broadband Applications

In this paper, a low profile dielectric resonator antenna (DRA) is proposed and investigated. To achieve the broad impedance bandwidth the proposed antenna geometry combines the dielectric resonator antenna and an underlying microstrip-fed slot with a narrow rectangular notch, which effectively broadens the impedance bandwidth by merging the resonances of slot and DRA. The physical insight gained by the detailed parametric study has led to find out a set of guidelines for designing the antennas for any particular frequency band. The design guidelines have been verified by simulating a set of antennas designed for different frequency bands. For validation, a prototype antenna is fabricated and tested experimentally. The measured results show that the proposed DRA offers an impedance bandwidth of about \(125.34\%\) from 1.17 to 5.1 GHz with reasonable gain between 3.5 and 5.7 dBi. The volume of the proposed DRA is \(0.16\lambda _{dr}^{3}\), where \(\lambda _{dr}\) is the wavelength at center operating frequency of the DR. A comprehensive study on bandwidth shows that the proposed DRA provides maximum bandwidth in terms of the DR volume (\(\hbox {BW}/V_{dr}\)) and the DR height (\(\hbox {BW}/h_{dr}\)) than the other similar reported work on hybrid wideband DRA designs.

A variable-frequency structural health monitoring system based on omnidirectional shear horizontal wave piezoelectric transducers

Structural health monitoring (SHM) is of great importance for engineering structures as it may detect the early degradation and thus avoid life and financial loss. Guided wave based inspection is very useful in SHM due to its capability for long distance and wide range monitoring. The fundamental shear horizontal (SH0) wave based method should be most promising since SH0 is the unique non-dispersive wave mode in plate-like structures. In this work, a sparse array SHM system based on omnidirectional SH wave piezoelectric transducers (OSH-PT) was proposed and the multi data fusion method was used for defect inspection in a 2 mm thick aluminum plate. Firstly, the performances of three types OSH-PTs was comprehensively compared and the thickness-poled d15 mode OSH-PT used in this work was demonstrated obviously superior to the other two. Then, the signal processing method and imaging algorithm for this SHM system was presented. Finally, experiments were carried out to examine the performance of the proposed SHM system in defect localization and imaging. Results indicated that this SHM system can locate a through hole as small as 0.12λ (4 mm) in diameter (where λ is the wavelength corresponding to the central operation frequency) under frequencies from 90 to 150 kHz. It can also locate multiple defects accurately based on the baseline subtraction method. Obviously, this SHM system can detect larger areas with sparse sensors because of the adopted single mode, non-dispersive and low frequency SH0 wave which can propagate long distance with small attenuation. Considering its good performances, simple data processing and sparse array, this SH0 wave-based SHM system is expected to greatly promote the applications of guided wave inspection.

Dynamic modulation of wideband slow light with continuous group index in polymer-filled photonic crystal waveguide.

The dynamic modulation of wide bandwidth and low-dispersion slow light with continuous variation of group index ng is realized in a polymer-filled photonic crystal waveguide (PF-PCW) with optimal structure. By adjusting the unified radius of air holes under a different refractive index of polymer in the first two rows of holes adjacent to the defect, the structure optimization of PF-PCW is first studied, then the fixed optimal structure is obtained. In the optimal photonic crystal waveguide with hole radius r0=0.328a, a fixed refractive index n1=1.74 of polymer in the first-row holes, and by adjusting refractive index n2, the flattened wideband slow light with large normalized delay bandwidth product of group index from 17.15 to 55.65 has been demonstrated. Then, by filling polymer with electro-optic effect into the second-row holes, the dynamic modulation of the optimized slow light in PF-PCW is investigated. The simulation shows that the center operating frequency slightly shifts linearly to a higher one, and the average group index increases exponentially from 33.943 to 75.546 with a normalized delay bandwidth product larger than 0.3089 as the applied voltage increases. The modulation sensitivity of the average group index is about 0.3467/V when applied voltages vary from 0V to 120V. These results open the possibility for the dynamic control of slow light according to the practical requirements of flexibility and tunability.

A method for the design of ultrasonic devices for scanning acoustic microscopy using impulsive signals

Scanning acoustic microscopy (SAM) using impulsive signals is useful for characterization of biological tissues and cells. The operating center frequency of an ultrasonic device strongly depends on the performance characteristics of the device if the measurement is conducted by using impulsive signals. In this paper, a method for the design of ultrasonic devices for SAM using impulsive signals was developed. A new plane-wave model was introduced to calculate frequency characteristics of loss of ultrasonic devices by taking into account the conversion loss at the ultrasonic transducer, the transmission loss at the acoustic anti-reflection coating, and the propagation loss in the couplant. Ultrasonic devices were fabricated with a ZnO ultrasonic transducer using two acoustic lenses with aperture radii of 1.0 mm and 0.5 mm, respectively. The frequencies at which measured losses became minima corresponded to the calculation results by the plane-wave model. This numerical calculation method is useful for designing ultrasonic devices for acoustic microscopy using impulsive signals.

Comparison between broadband Bessel beam launchers based on either Bessel or Hankel aperture distribution for millimeter wave short pulse generation.

In this paper, a comparison is presented between Bessel beam launchers at millimeter waves based on either a cylindrical standing wave (CSW) or a cylindrical inward traveling wave (CITW) aperture distribution. It is theoretically shown that CITW launchers are better suited for the generation of electromagnetic short pulses because they maintain their performances over a larger bandwidth than those realizing a CSW aperture distribution. Moreover, the wavenumber dispersion of both the launchers is evaluated both theoretically and numerically. To this end, two planar Bessel beam launchers, one enforcing a CSW and the other enforcing a CITW aperture distribution, are designed at millimeter waves with a center operating frequency of f¯=60GHz and analyzed in the bandwidth 50 - 70 GHz by using an in-house developed numerical code to solve Maxwell's equations based on the method of moments. It is shown that a monochromatic Bessel beam can be efficiently generated by both the launchers over a wide fractional bandwidth. Finally, we investigate the generation of limited-diffractive electromagnetic pulses at millimeter waves, up to a certain non-diffractive range. Namely, it is shown that by feeding the launcher with a Gaussian short pulse, a spatially confined electromagnetic pulse can be efficiently generated in front of the launcher.

Wideband TEM-TE11 mode convertor for HPM applications.

The mode convertor design of fundamental coaxial TEM to the lowest asymmetric TE11-mode of a circular waveguide was proposed and optimized with ANSYS HFSS software. It includes axially aligned parts: the input coaxial line with the high voltage insulator, conical coaxial matching line, wave-coax transition section and output circular waveguide. The most losses in this type of convertor caused by the wave of coaxial TE11-mode running back to the microwave source. To minimize these losses, there is the matching conical coaxial line with the cut-off insertion for coaxial TE11-mode. Characteristics of the convertor are as follows: the maximum input peak power – 3GW, the input impedance – 28Ohm, the central operating frequency – 1.14GHz. The power conversion efficiency to the output mode is from 90% upto 100% in the frequency band of 20%.

Wide-Angle Scanning Low Profile Phased Array Antenna Based on a Novel Magnetic Dipole

A novel microstrip magnetic dipole antenna that can be applied to wide-angle scanning phased array antennas in both E- and H-planes is proposed. The proposed antenna has a low profile of $0.03 {\lambda }_{0}$ ( $ {\lambda }_{0}$ denotes the free-space wavelength corresponding to the center operating frequency), and a compact size of $0.32 {\lambda }_{0} \times 0.31 {\lambda }_{0}$ . The half-power beamwidth of the proposed antenna with an infinite ground is about 180° in the E-plane and 85° in the H-plane, which has a large potential for wide-angle scanning phased array. When it is applied to phased array antennas, the scanning coverage is wide. The proposed planar phased arrays are fabricated and experimentally verified. The measured results show that the main beam can scan from −76° to 76° with 3-dB gain fluctuation in the E-plane and from −64° to 64° in the H-plane.

Ultra-broadband and high-efficiency reflective linear polarization convertor based on planar anisotropic metamaterial in microwave region

A simple design of an ultra-broadband and high-efficiency reflective linear polarization convertor with 90° rotation is presented in the microwave region. This convertor is formed from the planar anisotropic metamaterial (MM) using cut-wire (CW) structure. Three resonance modes are generated by electric and magnetic fields of incident wave, which lead to bandwidth expansion extremely of cross-polarization reflection. The simulation result shows that a linearly polarized wave can be converted into its orthogonal polarization after reflection, the polarization conversion ratio is above 90% in the frequency range of 5.1–12.1GHz and the relative bandwidth is up to 78.6%, which agree well with the experimental results. Both simulation and experiment results demonstrate the capability of linear polarization conversion by 90° with high polarization purity after reflection in the above frequency range, which can be confirmed by ellipticity η and the polarization rotation azimuth angle θ. The designed structure that utilized to build the thin MM slab is about λ0/8.72 evaluated at operating center frequency. It has application values in polarization control with high performances.

A Miniaturized UWB Biplanar Yagi-Like MIMO Antenna System

A novel 2-element ultrawideband Yagi-based multiple-input-multiple-output (MIMO) antenna system is presented based on loop excitation. Half of the driven loop element is implemented on each side of the substrate, which not only reduces the overall size of the antenna by 45%, but also increases its bandwidth significantly. The center frequency of operation is 5.8 GHz targeting WLAN applications. The proposed design has a wide measured bandwidth of 2.401 GHz covering 4.183-6.584 GHz. The proposed compact MIMO antenna system has an overall size of 50 × 80 × 0.76 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> with a single element size of 50 × 40 × 0.76 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The proposed Yagi antenna has high directional radiation characteristics with a minimum measured front-to-back ratio of 18 dB, minimum measured gain of 6 dBi, and directivity of 6.6 dB using a single director element only, at 5.8 GHz. It has a minimum measured isolation of 17 dB, measured maximum envelope correlation coefficient value of 0.0568, and measured total efficiency above 80% across the band of operation.

A Hybrid Miniaturized-Element Frequency Selective Surface With a Third-Order Bandpass Response

A new architecture for a miniaturized-element frequency selective surface (MEFSS) capable of providing a third-order bandpass response is presented and experimentally verified. The proposed MEFSS consists of three metal layers separated from one another by two dielectric substrates. The exterior metal layers are nonresonant inductive wire grids, while the center layer is a hybrid resonator composed of an inductive wire and an I-shaped resonator. The exterior metal layers and the dielectric substrates act as two inductively coupled series resonators sandwiching the hybrid resonator layer in the middle. This results in a filter with a third-order bandpass response that can also provide an out-of-band transmission null at a frequency higher than that of the main passband. An equivalent-circuit-based synthesis procedure for this MEFSS is also presented in this letter that allows for designing the FSS from its desired system-level performance indicators (e.g., center frequency of operation, bandwidth, etc.). The validity of this design procedure is verified by presenting a design example and conducting full-wave electromagnetic simulations. Finally, an experimental proof-of-concept demonstration is performed by characterizing the unit cell of this MEFSS designed to operate in a WR-112 waveguide environment.

Development of a Resistor-Loaded Ultrawideband Absorber With Antenna Reciprocity

A novel method of developing an electromagnetic absorber is proposed through the design of an ultrawideband resistor-loaded planar dipole absorber. With the method, an antenna array structure is first designed for its best transmission performance and configured as an absorber with the application of the reciprocal principle. The designed planar dipole antenna has achieved a fractional bandwidth of 105% for an absorbing rate of more than 90% under the normal incident. In addition, the thickness of the proposed structure is $0.128\lambda _{0} $ with $\lambda _{0} $ being the wavelength at the center operating frequency. Good agreement between the simulated and measured results demonstrates the validity of the proposed method.