End-fire Array Research Articles

Refined acoustic modeling and analysis of shotgun microphones

A shotgun microphone is a highly directional pickup device widely used in noisy environments. The key element that leads to its superior directivity is a tube with multiple slot openings along its length. One traditional way to model the directional response of a shotgun is to assume plane waves traveling in the tube as if it is in the free field. However, the frequency response and directivity predicted by this traveling wave model can differ drastically from practical measurements. In this paper, an in-depth electroacoustic analysis was conducted to examine the problem by considering the standing waves inside the tube with an analogous circuit containing phased pressure sources and T-networks of tube segments. A further refinement is to model the housing diffraction effect with the aid of the equivalent source method (ESM). The on-axis frequency response and directivity pattern predicted by the proposed model are in close agreement with the measurements. From the results, a peculiar bifurcation phenomenon of directivity pattern at the Helmholtz frequency was also noted. While the shotgun behaves like an endfire array above the Helmholtz frequency, it becomes a broadside array below the Helmholtz frequency. The standing wave effect can be mitigated by covering the slot openings with mesh screen, which was found to alter the shotgun response to be closer to that of the traveling wave model above a critical frequency predicted by the half-wavelength rule. A mode-switching model was developed to predict the directional responses of mesh-treated shotguns.

ARRAY OF MULTIPLE LF TRANSDUCERS WITH ULTRAHIGH CARDIOID SOUND PATTERN GENERATION

Differential spacing of loudspeakers in an end fire array is employed to reduce beam spread and, where a ground barrier is present, to suppress any resultant back wave.

Multi-Band Rectangular Patch End-Fire Antenna Array

A novel multi-band end-fire antenna array was designed, fabricated, and characterized. Analytical calculations were carried out to determine the critical antenna dimensions and the design was optimized using a 3D electromagnetic finite-element solver. The measured results were in good agreement with the designed results. The proposed antenna array exhibits multi-band capabilities which can be potentially used for applications that require a multi-band end-fire radiation pattern.

A Bidirectional Array of the Same Left-Handed Circular Polarization Using a Special Substrate

A novel bidirectional circularly polarized array is presented. The type of circular polarization is the same in both axial directions. The array consists of eight dipoles spaced by λ0/4(λ0 is the wavelength in free space), which combines endfire array, crossed dipoles, and substrate loading. For the bidirectional circular polarization of the same rotation, the adjacent elements need to be excited in phase, and the adjacent +45°/ -45° dipoles need to be excited with 180° phase difference. To satisfy the special phase distribution, a substrate with dielectric constant of 6.0 is used, which makes the guided wavelength λg equal to λ0/2. A prototype is developed at 2.5 GHz. With this array, the measured 3-dB axial-ratio (AR) bandwidth is 770 MHz (2.05 ~ 2.82 GHz), and the 10-dB return-loss bandwidth achieved is 580 MHz (2.32 ~ 2.9 GHz). The maximum bidirectional left-handed circular polarization (LHCP) gain is about 7.3 dBic achieved at 2.45 GHz.

Application of Parametric Acoustic Receiving Array to Underwater Acoustic Measurement

The Parametric Acoustic Receiving Array achieves the directional reception characteristics through an end-fire array of virtual sources by the use of only two transducers (pump and hydrophone). With a radial size much smaller than linear receiving array whose radial size is too wide to fit in laboratory measurement of low frequency transducers, and with the ability of totally suppressing the inversely traveling signals, parametric acoustic receiving array can be used to measure low frequency complex waveform signals with large pulse length and broad bandwidth in reverberant tank. This paper gives a detail analysis and setups for this application.

Surface Micromachined Millimeter-Wave Log-Periodic Dipole Array Antennas

Two wideband millimeter-wave end-fire log-periodic dipole array (LPDA) antennas are designed and fabricated using a sequential surface micromachining. The antennas are excited by monolithically integrating a micro-coaxial line feed in the antenna's lower boom designed to provide band rejection at 60 GHz. Antennas 1 and 2 are designed to operate in 18-110 GHz and 18 GHz-50 GHz bands, and measured up to 40 GHz and 50 GHz, respectively. VSWR <; 2.5 is obtained for both antennas throughout the measured bandwidth. Measured directivity around 10 dBi and consistent E and H-plane 3 dB beamwidths around 56° and 72°, respectively, are obtained for Antenna 2 throughout its bandwidth. Good pattern correlation between measured and simulated results is also achieved. This paper clearly demonstrates that the wideband high-quality millimeter-wave end-fire LPDAs are now possible.

A comparison of directional robustness for endfire versus Blumlein microphone arrays used in hearing aids

An endfire microphone array uses two omnidirectional microphones in a delay-and-sum configuration. A Blumlein array mixes one omnidirectional and one (bi)directional mic. Each can be engineered to provide any 1st order directional pattern. The three critical factors for providing good directionality include the relative sensitivity and phase between the microphones in addition to the placement of the hearing instrument on the user’s head. In this context, a directional system is robust if its factors can operate over a wide range of levels without degrading the directional performance. In this study, each array was engineered to have the same aperture spacing and tuned to the same freefield polar pattern; this tuning provided the nominal operating levels. Both arrays were placed in-situ on a measurement manikin and 614 impulse responses were acquired in ten degree resolution on all four microphones for different in-situ positions. The data for each array were combined as described above, and the aforementioned factors were varied around the nominal range of levels in a simple central composite design of experiments. The results of the in-situ directional response show improved robustness for a Blumlein mix.

Optimal two-layer directive microphone array with application in near-field acoustical holography

Conventional near-field acoustical holography (NAH) is generally based on the free-field assumption, which can cause errors when interfering sources are present in practical environments. To cope with this problem, previous research suggested a combined pressure-velocity approach for NAH that provides certain advantages to rejection of interferences. This paper revisits this idea in a broader context of optimal array design and examines the feasibility of using unidirectional microphones in each channel of the array such that the robustness of inverse reconstruction is enhanced against interfering sources. As indicated in the simulation, the numerical noise in finite difference estimation of particle velocity can nullify the advantage of the well-conditioned velocity-based reconstruction. In the proposed approach, the characteristics of each array channel consisting of two microphones are tailored by taking into account not only the directivity, but also the robustness against self-noise. An objective function based on directivity index and white noise gain is exploited in a linear quadratic optimization of a two-element end-fire array. The proposed optimal array is validated in conjunction with the equivalent source model (ESM) -based NAH through numerical simulations, with an interfering source positioned behind the array. The results have shown the directive optimal array has yielded improved quality of images in comparison with conventional approaches in the presence of an interfering source and sensor noise.

A Planar End-fire Array in S-band for Airborne Applications

Planar end-fire arrays can be used in airborne radar applications for forward and rear vision. The purpose of this work is to demonstrate the viability of constructing end-fire arrays with good bandwidth and front-to-back (f/b) ratio. The array so constructed must be extendable to a large number (possibly >1 000) of elements depending on the platform. In this work, starting from a basic S-band 2×2 array, arrays with higher number of elements have been studied through simulations such as 4×4, 4×8, and 2×16 configurations. An end-fire array comprising 4×8 elements (uniformly excited) has been successfully fabricated and tested giving a f/b ratio of 15 dB, and directivity 15 dB and an array comprising 4×8 elements excited by coefficients of Chebyshev polynomials has been simulated which gives a f/b ratio of 25 dB and similar directivity. Beam steering simulations show that the 4×8 array can be steered to ±25 degrees with acceptable side lobe levels.

A Bidirectional Endfire Array With Compact Antenna Elements for Coal Mine/Tunnel Communication

A six-element endfire array is proposed for coal mine or tunnel communication in this letter. To further decrease the end cross section, which has adverse effect on air ventilation, meander-line folded dipoles are used as antenna elements. By the antenna itself, the cross section toward the ventilation direction is only 100 × 1 mm2. A prototype array is fabricated and tested. Bidirectional endfire radiation patterns with low cross-polarization levels in the azimuth and elevation plane are achieved. The measured - 10-dB bandwidth is approximately 7.5% (834-899 MHz), and the corresponding peak gain is 8.05-9.05 dBi.

Electrically Small Superdirective Endfire Arrays of Metamaterial-Inspired Low-Profile Monopoles

This letter presents the design and experimental characterization of subwavelength, superdirective, endfire antenna arrays operating in the 2.45-GHz band. Microstrip-fed, low-profile folded monopoles are designed using metamaterial-inspired phase-shifting lines composed of inductively coupled spiral resonators and employed for the synthesis of three different superdirective, two-element antenna arrays with variable interelement spacing. High directivity values are obtained experimentally. The experimental results show that the radiation efficiency of the reported arrays decreases as the interelement spacing of the arrays is reduced. Nevertheless, the directivity increase that is achieved with the interelement spacing reduction compensates the efficiency reduction, enabling even the more compact arrays to deliver reasonable gain values.

Design of a wideband rasorber with a polarisation-sensitive transparent window

The present thesis considers two different subjects in the research area of electromagnetics. The first part is concerned with antenna design and the second with radar absorbers and rasorber.In the first part, a novel excitation technique for cylindrical dielectric resonator antennas is introduced to produce circular polarization. The exciter is a tape helix that is wound around the dielectric resonator and is fed by a coaxial probe. The helix excites the HE11σ modes in phase quadrature in the cylindrical dielectric resonator antenna. The height of the helix is determined using the Hansen-Woodyard condition for an end-fire array based on the phase velocity of the surface wave traveling along the dielectric resonator side wall. This phase velocity is estimated from the phase velocity in an infinitely long dielectric rod with the same permittivity and radius as the dielectric resonator antenna. The helical exciter is required to operate in the helix axial mode. The height of the helix is usually taller than the height of the dielectric resonator core. Using this type of excitation, a 3 dB axial-ratio bandwidth of 6.4% was achieved for a sample design with dielectric constant er ~ 11. The achieved 3 dB axial-ratio bandwidth is greater than that typical of other reported single feed cylindrical dielectric resonator antennas. A prototype of the sample design is fabricated and measured and a good agreement between simulation and measurement is observed. Furthermore, two approaches for the enhancement of the 3 dB axial ratio bandwidth are proposed: removing the central portion of the cylindrical dielectric resonator and using stacked cylinders. The advantages and limitations of each approach are discussed. Another perspective on the proposed design is to consider the antenna as a helix with a dielectric resonator core. In this perspective, the effects of the dielectric core on the helix antenna are discussed.The second part of the thesis is concerned with the design of thin wideband electromagnetic planar absorber for X- and KKu-band which also has a polarization sensitive transparent window at frequencies lower than L-band. The design is based on a two layer capacitive circuit absorber with the backmetal layer replaced with a polarization sensitive frequency selective surface. The structure is studied for normally incident waves with two orthogonal linear polarizations. The structure is optimized to have high transparency at low frequencies for one of the polarizations and at the same time good absorption efficiency for both polarizations at the high-frequency band. For one of the polarizations a -1.9 dB transmission with a transmission loss of less than 10% at 1 GHz as well as a 2.25:1 (75%) bandwidth of -20 dB reflection reduction are achieved. For the other polarization we obtained more than 3:1 (100%) bandwidth of -19 dB absorption. Compared with our earlier design based on a Jaumann absorber, we succeed in significantly reducing the transmission loss at the transparent window. Furthermore, the module of absorption quality is extensively improved. The improvements are based on using periodic arrangements of resistive patches in the structure design. The investigation of the structure for oblique angles of incidence and non-ideal materials is also accomplished.

2채널 마이크로폰을 이용한 청각 기기에서의 빔포밍에 대한 객관적 검증

Hearing devices like cochlear implant, vibrant soundbridge, etc. try to offer better sound for people. In hearing devices, several beamformers including conventional directional microphone are applicable to noise reduction. Each beamformer has different directional response and it could change sound intelligibility or quality for listeners. Therefore, we investigated the performance of three beamformers, which are first and second order directional microphone, and broadband beamformer(BBF) with a computer simulation assuming hearing device microphone configuration. We also calculated objective measurements which have been used to evaluate speech enhancement algorithms. In the simulation, a single speech and a single babble noisewere propagated from the front and azimuth degrees respectively. Microphones were configured in an end-fire array and the spacing was varied in comparison. With 3 cm spacing, BBF had about 3 dB higher enhanced SNR than that of directional microphones. However, enhancement of segmental SNR and frequency weighted segmental SNR were similar between the first order directional microphone and broadband beamformer. In addition when steady state noise was used, broadband beamformer showed the increased performance and had the highest enhanced SNR, and segmental SNR.

Using Bayesian Inference for Linear Antenna Array Design

Based on the observation that design and inference are both generalized inverse problems, we devise a new approach that uses the Bayesian inference framework for the automated design of linear antenna arrays. Compared to the optimization-based techniques that are widely used for automated antenna design, this newly-developed method has a prominent advantage, which is the capability to determine automatically the number of antenna elements required to satisfy design requirements and specifications. Three broadside array design problems, which include the null-controlled pattern, the sector beam pattern, and the Chebyshev pattern, along with an end-fire array design problem, are presented as examples. The obtained results demonstrate the advantages of using the Bayesian inference framework for the design of linear antenna arrays.

Implementation of wideband digital beam forming in the E-band

This paper reports the test results of a small-scale prototype that implements a digitally beam-formed phased antenna array in the E-band. A four-channel dual-conversion receive RF module for 71–76 GHz frequency band has been developed and integrated with a linear end-fire antenna array. Wideband frequency-domain angle-of-arrival estimation and beam-forming algorithms were developed and implemented using Orthogonal Frequency Division Multiplexing (OFDM) with Quadrature Phase-Shift Keying (QPSK) at 1 Gbps. Measured performance is very close to the simulated results and experimental data for an analogue-beam-formed array. This work is a stepping stone toward practical realization of larger hybrid arrays in the E-band.

Evaluating small end-fire loudspeaker array under various reverberations

A compact end-fire loudspeaker array based on the linearly constrained minimum variance has been investigated in various reverberation rooms. In the linearly constrained minimum variance beamformer, the coherent matrix that determines the suppression direction of the radiation sound is important. Therefore, we evaluated the directivities of a loudspeaker array for four different coherent matrices that were calculated using the following methods in which the array will not radiate the sound: (I) for any direction except the reproduction direction, (II) in the direction of several measured room transfer functions, (III) in the direction of several ideal (free field) room transfer functions, and (IV) in the direction of the weighted ideal room transfer functions. To evaluate performance, a 4-element line array of 28-mm-diameter loudspeakers with equal spacing of 48 mm was implemented. The array used loudspeaker units without enclosures to shorten the spaces between the units. The directivities of the prototype loudspeaker array were measured in an anechoic room and three reverberant rooms by using the above four methods. The prototype arrays achieved higher directivity of 6 to 15 dB for wideband frequencies (300 Hz to 3.4 kHz) than the single loudspeaker for all methods. Moreover, the experimental results clarified that the array with method (IV), which did not use prior measured room transfer functions, had efficient performance in comparison with the array with method (II), especially when the reverberation time was 120 ms or less.

Distributed Control of Antenna Array with Formation of UAVs

AbstractThis paper studies the feasibility and the advantages of a distributed control strategy for a linear end-fire antenna array formation with UAVs. We first analyze the sensitivity of different interaction topologies to a low frequency sinusoidal disturbance affecting just one single vehicle, for antenna array sizes of up to 30 elements. The ETH Zurich Flying Machine Arena (FMA) is used as a test case. Then we consider a more realistic case of wind gust acting on all of the antennas. We show that under such conditions the simplified analysis does well at predicting the formation behavior under different distributed and decentralized control strategies.

Biologically inspired coupled antenna beampattern design

We propose to design a small-size transmission-coupled antenna array, and corresponding radiation pattern, having high performance inspired by the female Ormia ochracea's coupled ears. For reproduction purposes, the female Ormia is able to locate male crickets’ call accurately despite the small distance between its ears compared with the incoming wavelength. This phenomenon has been explained by the mechanical coupling between the Ormia's ears, which has been modeled by a pair of differential equations. In this paper, we first solve these differential equations governing the Ormia ochracea's ear response, and convert the response to the pre-specified radio frequencies. We then apply the converted response of the biological coupling in the array factor of a uniform linear array composed of finite-length dipole antennas, and also include the undesired electromagnetic coupling due to the proximity of the elements. Moreover, we propose an algorithm to optimally choose the biologically inspired coupling for maximum array performance. In our numerical examples, we compute the radiation intensity of the designed system for binomial and uniform ordinary end-fire arrays, and demonstrate the improvement in the half-power beamwidth, sidelobe suppression and directivity of the radiation pattern due to the biologically inspired coupling.

Three‐dimensional polar analysis of a directional endfire microphone array on a measurement manikin due to microphone drift.

A dual microphone endfire array is a directional system commonly used in hearing aids. The directional performance of such systems are sensitive to sensor mismatch and drift. In this study, a pair of matched omnidirectional microphones in a delay‐and‐sum configuration are mounted in a hearing aid and their directional response is measured in‐situ on KEMAR at 10 resolution in azimuth and elevation. The resulting 3‐D polar balloons, directivity indices, and unidirectional indices are computed as a function of frequency. The measured transfer functions are then perturbed with sensor mismatch responses acquired empirically from typical lots of hearing‐aid microphones. The resulting polar benchmarks are evaluated and compared to the original benchmarks. A detailed analysis, both visual and numerical, will be presented.

Demonstration of Adaptive Analogue Beam Forming in the E-Band

In this paper, we report the test results of a small-scale prototype that implements an analogue-beam-formed phased antenna array in the E-band. A four-channel dual-conversion receive RF module for 71~76 ㎓ frequency band has been developed and integrated with a linear end-fire antenna array. Measured performance is very close to the simulated results. An ad-hoc wireless communication system has also been demonstrated. Low BER was measured for an 8PSK data stream at 1.5 Gbps with the receive array beam formed in the direction of arrival of the transmitted signal. To our knowledge this is the first steerable antenna array reported to date in the E-band.