Field Of Antenna Research Articles

Active VLF Transmission Experiments Between the DSX and VPM Spacecraft

AbstractThis study presents results from magnetic field line conjunctions between the medium‐Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low‐Earth orbiting (LEO) very low frequencies (VLF) Propagation Mapper (VPM) satellite. DSX transmitted at VLF toward VPM, which was equipped with a single‐axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include (a) the predicted power at LEO from DSX transmissions was too low for VPM to observe; (b) VPM's trajectory missed the “spot” of highest intensity due to the focused ray paths reaching LEO; or (c) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray‐tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low‐frequency (below 4 kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid‐ and high‐frequency transmissions (∼8 and 28 kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5 μV/m and 1 μV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations.

An Appraisal Among Wired, Hybrid and Wireless Smart Homes to Mitigate Electromagnetic Radiation

The global Covid-19 pandemic caused a rapid transitioning to remote work settings, one likely to linger post-pandemic, resulting on people spending more time at home for work or study. The globalpandemic defined a new normal that is expected to be digital and heavily relying on technology. Smart buildings which are envisioned to be the next paradigm shift in the built environment are also foreseen as a response solution to aid in situations like pandemic. However, such a move yields benefits as well as risks, prompting wide debates on the priority to safeguard building occupants health, safety and well-being. Researchers, designers and engineers are seeking solutions to incorporate or modify design features in the indoor environment that prioritize the dwellers’ health and wellness. Though benefits of smart and IoT devices aid in monitoring health and wellness, radiation from these wireless devices may cause harm to human health, especially those with weaker health, as indicated by several research findings. Some of the negative impacts from wireless radiation include cell damage, cancer, tumor, change in hormonal levels, and neurological damage. Thus, this study seeks to determine the difference in radiation level inside a wired, hybrid and a wireless smart home through Computer Simulation Technology (CST) simulation. Such a quantification can help designers develop strategies to design smart buildings that cause low radiation for its occupants. Antenna field source was imported to CST to create the wireless and hybrid design scenario. The measurement for wired and hybrid were evaluated keeping the wired design as baseline. The results revealed that wireless produced 26.55% more radiation than wired scenario at 2.45 GHz, taken as baseline measurement. Further, the total Electromagnetic Radiation (EMR) and radiation patterns are dependent on several factors like proximity of IoT and smart devices to building walls and interior furnishings, frequency of operation. In order to create a safer indoor environment, this study recommends the use of both wired and hybrid design in lieu of totally wireless smart buildings.

Co-substituted CuO–ZrO2–Nb2O5 composite ceramics with low-temperature sintering and low-loss for high-performance patch antenna

In the field of microwave dielectric ceramics for patch antennas, it has always been a thorny issue to simultaneously satisfy high dielectric constant, low sintering temperature and low dielectric loss. Here, a CCZN (x = 0.04) composite ceramic with excellent microwave dielectric properties was developed. It can be found that an appropriate amount of Co2+ substitution can enhance the compactness, increase the dielectric constant (εr), reduce the dielectric loss (tanδ) and enhance the temperature coefficient (τf). The composite ceramic with excellent dielectric properties (εr = 22.6, Q × f = 37,624 GHz, τf = −64.9 ppm/°C) shows good sintering compatibility with Ag. Based on this composite ceramic, a high-performance patch antenna was designed for Beidou satellite navigation system (BDS). It presents a small size of 30 × 30*3.8 mm3 and excellent radiation performance (return loss = −32.9 dB, VSWR = 1.047, impedance bandwidth = 60.6 MHz, radiation efficiency = 94.6% and realized gain = 7.704 dB). This work promotes the application of microwave dielectric ceramics in the field of patch antennas.

Polarization manipulation associated with electromagnetically induced transparency based on metamaterials

The polarization of electromagnetic waves is a key feature in modern optics and information science research fields. How to effectively control the polarization state of electromagnetic waves in electromagnetically induced transparency (EIT) is still a challenge. Herein, the EIT behavior associated with transmissive linear-to-circular (LTC) polarization conversion is theoretically proposed in the terahertz (THz) regime by employing a metamaterial. Four asymmetric H-shaped resonators are arranged in a cross-shaped arrangement on the metamaterial. It has numerically demonstrated that the electric and magnetic resonances are mutually coupled, leading to the famous EIT phenomenon. Because of the strong dispersion properties and high transmission dominated by the EIT behavior and the anisotropy of metamaterial, the transmissive LTC polarization conversion is conspicuously realized while the x- and y-polarized waves are incidents. Furthermore, two new transparent windows are located in 1.396–1.654 THz and 1.258–1.721THz, which are higher than 0.9 can be gained from 1.409 (1.323) THz to 1.467 (1.594) THz, and the relative bandwidths are 4.0% and 18.6%, respectively. Moreover, the values of the maximum group delay and group index under the x- and y-polarized waves respectively reach 504 ps, 122 ps, and 18861, 4575, displaying a slow-light effect. In addition, the LTC polarization conversion is well obtained at 1.685 THz, where the transmission coefficient highly reaches 0.605. Based on the Stokes parameters, the corresponding ellipticity η, the band of the maximum 3 dB axial ratio, and the polarization azimuth ψ respectively are 0.961, 2.1%, and −42.5°. It is worth noting that not only the thickness of the reported metamaterial is subwavelength, but also the proposed structure with low loss is great transparency for the electromagnetic waves. Surpassing conventional EIT metamaterials and polarization converters, the proposed structure synchronously equipped with those two characteristics has numerous potential applications in the integration to the existing terahertz devices, future polarization controls, and the antenna fields.

Combining Pancharatnam–Berry Phase and Conformal Coding Metasurface for Dual-Band RCS Reduction

A flexible, dual-broadband, and polarization-insensitive coding metasurface is proposed to manipulate electromagnetic (EM) scattering in microwave frequency band. The 1 bit coding units “0” and “1” are formed by the Pancharatnam–Berry (PB) phase based on the same-sized meta-atoms with different orientations. The layout of the coding metasurface is obtained through genetic algorithm (GA). The simulation results indicate that the proposed coding metasurface in this communication can achieve more than 10 dB radar cross section (RCS) reduction in the range of 9.26–12.87 and 14.84–19.35 GHz in the planar case, which is attributed to the reorientation of the reflected energies into different directions by optimizing the coding sequence. The diffuse scattering performance in the dual-wideband is well maintained, while the coding metasurface is conformal on metallic cylinder with diverse curvature radii. Moreover, the scattering characteristics of conformal metasurfaces become better with the decrease of curvature radius under the case of the certain size of the flexible metasurface. A flexible coding metasurface prototype is prepared and measured. The experiment results coincide with the numerical simulation ones, demonstrating the outstanding capacity of RCS reduction. The proposed design has potential application value in the field of antenna and stealth of more complex objects.

Numerical analysis of RF discharge in a nonuniform magnetic field

The radio-frequency (RF) plasma thruster has been studied to overcome the issue of a reduced lifetime of plasma thrusters due to electrode erosion, and it has been reported that a nonuniform magnetic field near the RF antenna improves the thrust performance. In this study, to obtain knowledge for optimizing the configuration of the thruster and the magnetic field, the RF discharge in a nonuniform magnetic field is numerically investigated focusing on RF power absorption and plasma generation, and the effect of the positional relation between the magnetic cusp and the RF antenna on the RF discharge is discussed. We adopt the two-fluid plasma model considering ions and electrons and confirm the qualitative validity of the simulation by comparing the simulation results with the results of a laboratory experiment. The simulation reveals that the energy transport due to the Ey×B drift from the region of RF power absorption near the RF antenna determines the distributions of the plasma generation and temperature. Therefore, the plasma distribution changes depending on the RF antenna position and the phase of the RF period because the Ey×B drift direction changes depending on the directions of the electric and magnetic fields. In addition, we find that it is important to place the antenna in a strong magnetic field for effective power absorption because this increases the azimuthal diamagnetic current.

Remote Measurement of the Lightning Impulse Charge Moment Change Using the Fast Electric Field Antenna

The impulse charge moment change (iCMC) is an important electrical property of cloud-to-ground (CG) lightning. In this paper, a new method of measuring the iCMC at distances of several hundred kilometers is proposed. The method is based on the vertical electric field below 1 kHz measured by the widely used fast electric field antenna with low frequency/very low frequency (LF/VLF) band. The impulse response of Earth-ionosphere waveguide (EIWG) is modeled using a finite difference time domain (FDTD) method considering an anisotropic ionosphere. By comparing the observed waveform with the simulated impulse response, the lightning discharge is classified into the impulsive discharge and the non-impulsive discharge. For the impulsive discharge, its iCMC is obtained directly by comparing the measured ELF waveform to the modeled impulse response at the same distance. For the non-impulsive discharge, its current moment waveform is assumed to be a sum of two Heidler’s functions, and the genetic algorithm is used to search the unknown parameters in the functions. The good agreement between the measured ELF waveform and the simulated waveform implies that the extracted current moments are reasonable. This method can be used to continuously monitor the lightning iCMC in a given time and space.

“It’s a Wonderful Time to Be an Antenna Engineer”: An Interview With Prof. John Volakis [Young Professionals

In a podcast published by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IEEE Open Journal of Antennas and Propagation</i> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OJAP</i> ) in June 2021, Prof. John (Yiannis) Volakis [past president of the IEEE Antennas and Propagation Society (AP-S), and dean of the College of Engineering and Computing at Florida International University] engaged in a conversation with Prof. Asimina Kiourti ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OJAP</i> ’s senior editor and assistant professor at The Ohio State University) about his exciting career path and the incredible advances that the field of antennas and propagation is currently seeing <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> . This article provides a transcript of the key parts of the podcast with Prof. Volakis. The interview has been edited to fit the scope of the “Young Professionals” column of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IEEE Antennas and Propagation Magazine</i> . Figures are also included highlighting some of Prof. Volakis’s major research contributions. His story is truly inspiring and motivating for young professionals.

On the Near-Field Spherical Wave Formation in Resonant Leaky-Wave Antennas: Application to Small Lens Design

In this work, we show that the near field of leaky-wave resonant antennas (LWAs) radiating into a dense medium can be locally represented as a spherical wave in a certain solid angle around broadside using an accurate definition of the phase center. The near field in this solid angle can be efficiently evaluated through the integration of the spectral Green’s function along the steepest descent path (SDP). Beyond this solid angle, defined as the shadow boundary angle, a residual contribution due to the leaky-wave pole must also be added to fully describe the near field. It is found that this shadow boundary angle can be used to define the phase center and geometry of a truncated lens that couples well to leaky-wave antennas, even in electrically small-to-medium sized lenses and low-contrast cases. To demonstrate the applicability of the proposed study, we combine the SDP field calculation with a Fourier optics (FO) methodology to evaluate the aperture efficiency and radiation patterns of small-to-medium sized lenses in reception. A truncated silicon lens with a diameter of only four free-space wavelengths is presented with almost 80% aperture efficiency. Excellent agreement with full-wave simulations is achieved, which demonstrates the accuracy of the proposed design and analysis methodology.

Ultrashort-Pulse-Microwave Excited Whole-Breast Thermoacoustic Imaging With Uniform Field of Large Size Aperture Antenna for Tumor Screening.

Microwave-induced thermoacoustic imaging (MTAI) has been widely used in biomedical science, and has the potential as an auxiliary measure for clinical diagnosis and treatment. Recently, there are increasing interests in using ultrashort microwave-pumped thermoacoustic imaging techniques to obtain high-efficiency, high-resolution images. However, the traditional imaging system can only provide uniform radiation in a relatively small area, which limits their large field of view in clinical applications (such as whole-breast imaging, brain imaging). To address this problem, we propose an ultrashort pulse microwave thermoacoustic imaging device with a large size aperture antenna. The system can provide a microwave radiation area of 40 cm × 27 cm and a uniform imaging view of 14 cm × 14 cm. With 7 cm imaging depth and a 290 μm resolution. The practical feasibility of the system for breast tumor screening is tested in phantoms with different shapes and in an ex vivo human breast tumor which is embedded in the excised breast of an ewe (π × 5 cm × 5 cm). The tumor can be identified with a contrast of about 1:2. The results demonstrate that the dedicated MTAI system with the uniform large field of view, high imaging resolution, and large imaging depth have the potential for clinical routine breast screening.

A stable higher-order probe correction algorithm for spherical near field antenna measurements

ABSTRACT A novel higher-order probe correction algorithm is introduced for the spherical near field antenna measurements. The measured signal is processed by the fast Fourier transform along direction and integrated numerically along direction, thus converting the transmission formula into a series of small matrix equations. In a virtual example simulated using FEKO, a Yagi antenna is used as a higher-order probe to measure a Taylor array. It is shown that the proposed method is faster, more accurate and more stable than the FFT/matrix inverse method. Furthermore, the number of sampling points of the former is only half of that of the latter.

Radiation Pattern Reshaping of a Narrow Slot Antenna for Bandwidth Enhancement and Stable Pattern Using Characteristic Modes Analysis

A novel design concept to reshape radiated fields of a slot antenna for improved bandwidth and stable radiation pattern is proposed. Initially, the radiated fields of the traditional slot antenna are theoretically studied using characteristic modes (CMs). The results demonstrate that its CM1, CM2, and CM3 are resonated around 2.39, 4.78, and 7.17 GHz, respectively. Both CM1 and CM3 maintain the bidirectional radiation patterns, whereas CM2 generates a radiation null in the broadside direction. Then, in order to utilize the nearby CM1 and CM2 for wide bandwidth, the linear slot is properly folded so as to transform the nonbidirectional pattern of CM2 into the bidirectional one. After that, the narrow slot is reformed as the stepped scheme, thus reallocating these dual modes in proximity to each other. With these arrangements, the impedance bandwidth of the antenna is dramatically widened with two attenuation poles, while keeping a compact size and stable radiation pattern. Finally, the proposed antenna is fabricated and tested. Measured results show that the antenna without any extra feeding network has gained a wide bandwidth ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert S_{11}\vert &lt; -10$ </tex-math></inline-formula> dB) of around 31.4% ranging from 3.22 to 4.42 GHz, which is about 2.24 times wider than its traditional counterpart (14%). Besides, the overall size of the slot radiator is kept as small as about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.42\lambda _{0} \times 0.06\lambda _{0}$ </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">$\lambda _{0}$ </tex-math></inline-formula> is the free-space wavelength). Particularly, the bidirectional radiation pattern, stable gain of around 5 dBi, and low cross-polarization of below −12.3 dB are all satisfactorily generated during the operating band.

Overview of Planar Antenna Loading Metamaterials for Gain Performance Enhancement: The Two Decades of Progress

Metamaterials (MTMs) are artificially engineered materials with unique electromagnetic properties not occurring in natural materials. MTMs have gained considerable attention owing to their exotic electromagnetic characteristics such as negative permittivity and permeability, thereby a negative refraction index. These extraordinary properties enable many practical applications such as super-lenses, and cloaking technology, and are used to design different electromagnetic devices like filters, polarization converters, sensors, and absorbers. Advances in MTMs have made new application fields to emerge in communication subsystems, especially in the field of antennas. MTMs are usually arranged in front or above the radiating element, or incorporated in the same substrate to improve the performance of planar antennas, in terms of improving directivity, gain, bandwidth, and efficiency, reducing the size and mutual coupling, and deflecting the radiation characteristics. High gain antennas are demanded in modern wireless communication systems. Their importance is in improving the signal strength by reducing the interference and alleviating the free space path loss. This review paper provides a brief introduction to MTMs, with the focus on their operating principles. Furthermore, a detailed study of antenna gain enhancement based on the various properties of MTMs is carried out. MTMs with low values of constitutive parameters; zero-index material (ZIM), low-index material (LIM), epsilon-near-zero (ENZ), and mu-near-zero (MNZ) are discussed in detail in the context of their capability to enhance the gain of a broad class of planar antennas. The low impedance property and lensing property, which is achieved by three different characteristics: high refractive index (HRI), gradient refractive index (GRIN), and negative refractive index (NRI) materials, are loaded to planar antennas for gain enhancement. The scope of this review has been limited to antennas that were experimentally validated in the respective source papers.

Characteristic Modes Analysis of a Near-Field Polarization-Conversion Metasurface for the Design of a Wideband Circularly Polarized X-Band Antenna

A metasurface (MS) based on loop elements operating in the near field of a linearly-polarized microstrip antenna is employed to realize a circularly polarized radiated field. The properties of the loop unit cell are highlighted with the help of the Characteristic Mode Analysis that is employed for investigating the achievable linear to circular polarization conversion bandwidth and providing the guidelines for the design of the final antenna. A finite structure comprising 4&#x00D7;4 unit cells is tailored for achieving a circularly polarized far field within the whole X-band adopted for satellite communications (7.25 GHz-7.75 GHz, 7.9 GHz-8.4 GHz). A simple but effective single-port excitation scheme is adopted, and the overall performance are assessed by measurements on the fabricated prototype. The good agreement between simulated and measured results confirms the reliability of the proposed approach as well as the meaningful insight provided by Characteristic Modes Theory.

Influence of the Mesh Size on the Computation of the Close Near Fields of Dipole Antennas

While evaluating the near field of dipole antennas, it is noted that different software suites yield values of the electric and the magnetic fields, at the surface of antennas, that can be substantially different, especially at the tips and at the feed gap. The close near field of dipoles has not been yet analysed in detail. An asymptotic expansion method for the near fields at the surface of these antennas has been developed and compared to the computed fields. The results show that the software of the computed field should not use a uniform mesh. The mesh should be much tighter at the metal extremities than in the dipole body. The proposed technique can be used to check complex field computations, producing diverging values, with simple analytical equations.

Description of the Coupling of two Loop Antennas using Electrical Equivalent Circuit Diagrams

Abstract. EMC measurements must be carried out in standardized and defined measuring environments. The frequency range between 9 kHz and 30 MHz is a major challenge for measurement technology. The established test sites are designed with an perfect elelctrically conducting ground. For the considered lower frequency range, the metrological validation is carried out with magnetic field antennas in this frequency range. The aim is therefore to take into account the ferromagnetic properties of the ground plane in such a measurement environment and to describe them analytically or numerically with an electrical equivalent circuit diagram. In this article we simplify the model to two loopantennas in Freespace without groundplane to check if the approache with the ECD will work. Therefore we use various numerical field calculation programs in the frequency range up to 30 MHz. The results from simulations are to be checked for correctness with describing them analytically or numerically. For this purpose, a model consisting of two loop antennas was created and simulated in a numerical simulation program. In order to validate the results from the simulation, two different approaches to creating an electrical equivalent circuit (ECD) are examined. The first approach is based on the real equivalent circuit diagram of a coil and the second approach forms a parallel resonant circuit of the first resonance of an antennas input impedance. The focus here is on the mutual inductance, which represents the coupling between the two antennas.

SMAP Salinity Retrievals near the Sea-Ice Edge Using Multi-Channel AMSR2 Brightness Temperatures.

Sea-ice contamination in the antenna field of view constitutes a large error source in retrieving sea-surface salinity (SSS) with the spaceborne Soil Moisture Active Passive (SMAP) L-band radiometer. This is a major obstacle in the current NASA/Remote Sensing Systems (RSS) SMAP SSS retrieval algorithm in regards to obtaining accurate SSS measurements in the polar oceans. Our analysis finds a strong correlation between 8-day averaged SMAP L-band brightness temperature (TB) bias and TB measurements from the Advanced Microwave Scanning Radiometer (AMSR2) in the C-through Ka-band frequency range for sea-ice contaminated ocean scenes. We show how this correlation can be employed to develop: (1) a discriminant analysis that is able to reliably flag the SMAP observations for sea-ice contamination and (2) subsequently remove the sea-ice contamination from the SMAP observations, which results in significantly more accurate SMAP SSS retrievals near the sea-ice edge. We provide a case study that evaluates the performance of the proposed sea-ice flagging and correction algorithm. Our method is also able to detect drifting icebergs, which go often undetected in many available standard sea-ice products and thus result in spurious SMAP SSS retrievals.

A Review on the History and Current Literature of Metamaterials and Its Applications to Antennas &amp; Radio Frequency Identification (RFID) Devices

Metamaterials are artificially engineered novel substances (or matter) that exhibit properties not found in nature. One of the earliest examples of a metamaterial structure is the split-ring resonator. This article is a current review of metamaterials, what it is, its classification, and its applications towards the field of antennas and radio frequency identification (RFID) devices. There are two major types of metamaterial antennas: leaky-wave antennas, and resonant antennas. Antennas can be made from composite right-left handed metamaterials, or antennas can be loaded with a metamaterial screen such as a shelled electrically small antenna (ESA) fed by coax. Metamaterial resonators can aid in reducing the antenna size, help improve the gain, improve the directivity, improve the return loss, and control the radiation pattern (by loading the antenna with a tunable unit). Recent advances in printing and fabricating technologies have allowed for use of metamaterial antennas in wearable gadgets and textiles; and can be used in conjunction with RFID devices and tags to advance and aid (emergency) equipment used by first responders.

Design and Modeling of Wideband Planar Antennas Using Characteristics Modes

Origin of wideband nature and radiation behavior in many broadband antenna structures is not clearly understood. In this article, we have attempted to clarify those phenomena using characteristic mode (CM) analysis along with equivalent circuit (EC) modeling. By modeling the modal impedance of antenna for a large bandwidth, considering natural resonances and anti-resonances, the EC models are developed for various wideband antennas. It is observed that the reactive loading and resonant mode coupling effects are largely contributing in the formation of wideband and far fields of antenna. These models can be used in SPICE simulators to study the effective communication between wireless transmitter and receiver systems without an EM solver. A novel design method is proposed on the development of wideband antennas using a modal impedance model and coupled mode theory of resonant modes. This method yields accurate design to achieve desired center frequency and bandwidth using structural modifications.

A Low-Sidelobe-Level Variable Inclination Continuous Transverse Stub Antenna with a Nonlinear Slow-Wave Structure

The sidelobe level (SLL) is an essential performance factor for satellite communication antennas. A low-SLL design can effectively suppress adjacent satellite interference. A low-SLL design method for a variable inclination continuous transverse stub (VICTS) antenna is proposed in this paper. The VICTS antenna is composed of three rotatable parts: a feeding plate, a radiation plate, and a polarization plate. The radiation plate comprises two groups of stubs with different radiation ratios. Combined with the nonlinear slow-wave structure attached to the feeding plate, the radiation ratio of the unit can be adjusted. The aperture field of the VICTS antenna using this method can be tapered in order to suppress the SLL. To verify the effectiveness of this method, the antenna prototype is fabricated and measured in a microwave anechoic chamber. The simulation and the measurement are in good agreement. The reflection coefficient of the antenna is kept below −15 dB and between 13.75 GHz and 14.5 GHz. When the radiation plate and the feeding plate rotate relative to each other, the pattern beam can be scanned from 5° to 70 ° . In the scanning range, the typical SLL can reach −18 dB.