Conformal Array Research Articles

1-D and 2-D Direction of Arrival Estimation in a Conical Conformal Array: Design and Implementation.

Direction of arrival (DOA) estimation for conformal arrays is challenging due to non-omnidirectional element patterns and shadow effects. Conical conformal array (CCA) can avoid the shadow effect at small elevation angles. So CCA is suitable for DOA estimation on both azimuth and elevation angles at small elevation angles. However, the element pattern in CCA cannot be obtained by conventional directional element coordinate transformation. Its local element pattern also has connection with the cone angle. The paper establishes the CCA radiation pattern in local coordinate system using 2-D coordinate transformation. In addition, in the case of large elevation angle, only half elements of the CCA can receive signal due to the shadow effect. The array degrees of freedom (DOF) are reduced by halves. We introduce the difference coarray method, which increases the DOF. Moreover, we propose a more accurate propagator method for 2-D cases. This method constructs a new propagation matrix and reduces the estimation error. In addition, this method reduces computational complexity by using linear computations instead of eigenvalue decomposition (EVD) and avoids spectral search. Simulation and experiment verify the estimation performance of the CCA. Both demonstrate the CCA model established in this paper is corresponding to the designed CCA antenna, and the proposed algorithms meet the needs of CCA angle detection. When the number of array elements is 12, the estimation accuracy is about 5 degrees.

Beam Stabilization of Deformed Conformal Array Antenna Based on Physical- Method -Driven Deep Learning

The array antenna will be inevitably deformed affected by the motion of the communication system or other special requirements, which will reduce the original good radiation performance, such as the gain dropping, the side lobes increasing, and even the beam splitting. In order to cope with the beam deterioration caused by the array antenna’s deformation, it is necessary to quickly manipulate the amplitude and phase distribution according to the current conformal array shape to stabilize the beam. This paper proposes a physical-method-driven deep-learning-based (PMDL-based) fast beam stabilization algorithm for the antenna array deformation. Firstly, we theoretically analyze the radiation pattern synthesis of the conformal array with the arbitrary surface to design the corresponding physical method, and we verify the accuracy of the method by the calculated and simulated results. Then, the deep neural network driven by the physical method is designed integrated with the aforementioned radiation pattern synthesis, whose training process is given. Finally, a 1×16 array antenna is taken as an example to verify the validity of the beam stabilization algorithm when the designed array is deformed. The simulated and measured results both indicate that the gain dropping and the sidelobe level rising are respectively less than 1.5 dB and 2 dB by applying the proposed PMDL-based fast beam stabilization when the array antenna is deformed within the range of 100° of the normal vector of each array element, while the beam stabilization time is less than 1.0 ms (testing time).

Modeling the effect of bend radius on the performance of a conformal dual-band mm-wave patch antenna array

There is growing interest in the use of additive manufacturing for the fabrication of RF devices due to fast prototyping capabilities and the use of less material as opposed to traditional fabrication techniques. In addition to the previously mentioned advantages, the aerosol jet printing method in particular allows for the conformal printing of RF components. This work models the effect of the bend radius on the return loss and radiation pattern of a dual-band (28 GHz and 39 GHz) mm wave patch antenna array design that will be aerosol jet printed. The antenna is modeled on a 5-mil thick flexible substrate (PET) and the simulation is run from 27 GHz to 40 GHz using HFSS. The return loss and radiation pattern results of the antenna are compared for various bend radii (0.25 in to 2 in) of the conformal surface the antenna is to be printed upon. Simulation results show a maximum frequency deviation of about 400 MHz on the return loss for the low (28 GHz) frequency band of the antenna when it is bent from a bend radius of 0 in to 2 in.

Robust Mainlobe Interference Suppression Based on Joint Oblique Projection for Dual Polarization Conformal Array

In this paper, a robust mainlobe interference suppression algorithm is proposed in a space-polarization joint domain based on a dual-polarized conformal array. An oblique projection filter can eliminate the interference while retaining the desired signal information. However, little deviation in signal parameters will lead to significant performance degradation. To solve such problems, the joint steering vector of the desired signal is corrected by solving the quadratic convex optimization problem. Parameters of interferences are first estimated by using multiple signal classification (MUSIC) joint spectrum, and then, the interference covariance matrix can be reconstructed individually to further estimate the interference steering vector. After that, the joint oblique projection operator can be established by the estimated steering vectors. Consequently, an antijamming beamformer can be obtained by the joint oblique projection filter to achieve robustness on the interference parameter estimation deviation and desired signal mismatch. Simulation experiments verify the effectiveness of the proposed method.

Adaptive Beamforming with Hydrophone Arrays Based on Oblique Projection in the Presence of the Steering Vector Mismatch

In sonar systems, the performance of adaptive beamformers severely degrades when mismatches occur between the actual and presumed steering vectors of the desired signal, mainly due to hydrophone position errors, amplitude-phase errors, and the scattered effect of arrays. Similarly, an inadequate number of “training” samples can lead to performance degradations similar to those caused by mismatches. In this paper, an adaptive beamforming algorithm based on oblique projection (OP-ABF) mismatch compensation is proposed to remove the degradation caused by the arbitrary-type steering vector mismatch of the desired signal. The proposed algorithm is motivated by the fact that the weight vector of adaptive beamforming can be represented as a linear combination of the optimal one and the oblique projection (OP) vector, which is generated by the steering vector mismatch and does not exist without this. Our algorithm was developed by constructing the oblique projection mismatch compensation vector (OPMCV) to provide the minimum variance distortionless response (MVDR) beamformer. Then, the algorithm could be implemented by the solution of the OP matrix with the formulation of the covariance matrix loading (CML). The simulation results of a uniform linear array (ULA) and a half-cylindrical conformal array (HCCA) show that the OP-ABF can optimize the original weight vector as much as possible without sacrificing the output signal-to-interference-plus-noise ratio (SINR) under different conditions. Experimental results for the HCCA also confirm the effectiveness of this algorithm.

Target polarization scattering matrix estimation with conformal MIMO radar

Conformal multiple-input multiple-output (MIMO) radar is a wide generation of conventional MIMO radar and it uses multiple antennas of different polarization transmit beampatterns for a larger aperture and better performance of target scattering measurement. In this paper, we develop a signal processing algorithm for conformal MIMO radar. According to Least Square (LS) criterion and Maximum Likelihood (ML) criterion, the target Sinclair matrix is estimated. Both unbiased but the ML estimator can achieve the Cramer–Rao bound. The radar polarization measurement capability is evaluated by the rank of Fisher information matrix (FIM). Numerical results with a three-dimensional transmit beampattern of a conformal antennas array show the estimation performance in theory measured by the root mean square error (RMSE).

Low profile conformal phased array with wide scanning range

AbstractA linearly polarized conformal phased array antenna with a low profile and a large scanning range is proposed, based on stacked patch antenna element fed by a coupled microstrip line. A backed compact cavity is designed in the antenna element, to reduce the mutual coupling between the elements. The prototype of the proposed conformal array, a conformal array composed of 4 × 4 elements which are axially mounted onto a cylindrical surface, is fabricated and measured, whose profile is as low as 0.072 λh (λh is the free‐space wavelength at the highest operating frequency). The experimental results demonstrate that the array achieves active VSWR below than 2.5 over the frequency band of 1.52–1.77 GHz (15%), within the wide‐scanning ranges of ±70° in the E plane and ±60° in the H plane, and the measured results are in good accordance with the calculation.

Activatable Conformal Electrode Arrays for Highly Stable and Breathable 12‐Lead Clinical Electrocardiographic Monitoring

AbstractIn clinical practices, ambulatory electrocardiography (ECG) signals are a significant index in early diagnosis of heart diseases. Commercial 12‐lead Holter monitors are widely used to evaluate the heartbeat over time. However, poor sweat‐proof adhesion, low water vapor transmittance, and wired connectivity of Holter monitors limit their popularization in wearable electronics. Herein, a facile, low‐cost, and scalable stretchable supramolecular material with activatable high adhesion is reported, which can be used to fabricate breathable on‐skin electrode arrays for stable 12‐lead ECG monitoring applications. In the proposed strategy, a supramolecular polymer network with various dynamic chemical bonds is obtained, resulting in the material exhibiting good stretchability. The introduction of tannic acid (TA) ensure high intrinsic adhesion at wet/dry skin surfaces. Moreover, the combination of poly (lipoic acid) and TA can realize alcohol‐activated high adhesion. Subsequently, electrode arrays are assembled using adhesive layers comprising supramolecular materials and a stretchable conductive Styrene Ethylene Butylene Styrene (SEBS)/Au layer. In addition, the micromesh structures obtained by laser scribing endowed the electrodes with good water/air permeability. Finally, the breathable on‐skin electrode arrays are combined with flexible ECG sensors for stable 12‐lead ECG monitoring.

Millimeter-Wave 1-D Wide-Angle Scanning Pseudo-Curved Surface Conformal Phased Array Antennas Based on Elongated Planar Aperture Element

To perform wide-angle scanning while maintaining a planar low profile, we propose a new kind of millimeter-wave (mmWave) planar phased array antenna using inclined beam elements to imitate a curved surface conformal (CSC) array, called a pseudo-curved surface conformal (PCSC) phased array antenna. We present the concept of our PCSC phased array and then design an elongated planar aperture antenna (EPAA) element, achieving stable high broadside gain and wide beam on the scanning plane within a wide bandwidth. Then, by introducing asymmetries to the EPAA element, the beam inclination of the EPAA element can be adjusted. Accordingly, we use EPAA elements with different beam inclinations to form a 1 × 4-element 1-D wide-angle scanning PCSC phased array antenna. We design, fabricate, and measure a prototype of our proposal, with measured results showing that the proposed design has a 3 dB gain variation across a scanning range of more than ±62° (and as high as ±68°) within the 5G mmWave band (24.25 to 29.5 GHz), which is about ±10° better than its traditional counterpart.

Direction finding by sparse‐based approach for dual‐polarised conformal array in the presence of gain and phase uncertainties

AbstractThis article investigates the problem of robust direction‐of‐arrival (DOA) estimation with unknown sensor gain and phase uncertainties. A Dual‐Polarised Regularised Multiple FOCal Underdetermined System Solver is proposed to estimate the 2‐D DOA for the dual‐polarised conformal array. Each polarisation signal is decomposed into two orthogonal components, which are regarded as a pair of coherent signals, and the polarisation parameters are merged into the signal waveform. A sparse‐based method based on the coherent signals block is presented, implementing weight sharing and grid pruning within the block to alleviate the negative influence of the errors. Meanwhile, gain and phase errors are modelled as the equivalent noise, which can be suppressed with the adaptive adjustment for the regularisation parameter. As the insensitivity of the sparse‐based algorithm to coherent signals, the proposed algorithm can realise the blind polarisation 2‐D DOA estimation. The Cramer–Rao bound is derived, and simulations exhibit the effectiveness and robustness of the proposed method with the unknown sensor gain and phase uncertainties.

Millimeter-Wave Wideband Wide-Angle Scanning Proactive Conformal Phased Array Antenna

Though conformal phased array antennas have better wide-angle scanning capability than their traditional planar counterparts do, their fabrication is generally complex. Therefore, this letter presents a millimeter-wave (mmWave) proactive conformal (PC) phased array antenna (PAA) formed by high-low alternate elements/subarrays to proactively modulate the array aperture, achieving wide-angle scanning and easy fabrication. A 1×6-element endfire high-gain low sidelobe level subarray is designed, based on which a 4×6-element PC PAA is implemented. The measured results show that the reflection coefficient of the fabricated prototype is mostly <−10 dB, the port isolation is >18 dB, the endfire scanning gain is >15.5 dBi, and the 3-dB gain variation scanning range is >±60° with a peak up to ±68° within 24.25–29.5 GHz (19.5%).

Wing Conformal Load-Bearing Endfire Phased Array Antenna Skin Technology

In this article, a phased array antenna skin is proposed for conformally assembling scenarios and wide scanning applications. Planar inverted F antenna (PIFA) is used as a radiation element of the proposed array due to its low profile and omnidirectional radiation pattern. The PIFA element is conformally assembled in a wing of an aircraft and is designed and fabricated with the technology of composite material, which makes the PIFA element load bearing. The endfire beam synthesis is also investigated, and the proposed array supports wide-angle beam scanning in both forward and backward regions. An array of 240 PIFA elements are designed, simulated, fabricated, and experimentally tested. The fabricated sample is able to bear an overload of 1 g in the mechanical test, and all the electronic experiment results agree well with the simulation data.

Antenna Arrangement in UWB Helmet Brain Applicators for Deep Microwave Hyperthermia.

Deep microwave hyperthermia applicators are typically designed as narrow-band conformal antenna arrays with equally spaced elements, arranged in one or more rings. This solution, while adequate for most body regions, might be sub-optimal for brain treatments. The introduction of ultra-wide-band semi-spherical applicators, with elements arranged around the head and not necessarily aligned, has the potential to enhance the selective thermal dose delivery in this challenging anatomical region. However, the additional degrees of freedom in this design make the problem non-trivial. We address this by treating the antenna arrangement as a global SAR-based optimization process aiming at maximizing target coverage and hot-spot suppression in a given patient. To enable the quick evaluation of a certain arrangement, we propose a novel E-field interpolation technique which calculates the field generated by an antenna at any location around the scalp from a limited number of initial simulations. We evaluate the approximation error against full array simulations. We demonstrate the design technique in the optimization of a helmet applicator for the treatment of a medulloblastoma in a paediatric patient. The optimized applicator achieves 0.3 °C higher T90 than a conventional ring applicator with the same number of elements.

Medical Antennas for Microwave Radiothermometry of Biological Objects.

The results of an analytical review of various antennas used in medicine are presented. Issues of modern microwave radiothermometry related to the development of new antennas are discussed, as well as possible ways to solve them. The tasks of further research aimed at creating new designs for conformal antennas and antenna arrays providing a significant expansion of the functionality and improvements in the characteristics of medical radiothermographs are formulated. The study was supported by the Russian Science Foundation (Grant No. 22-19-00113), https://rscf.ru/project/22-19-00113/.

An Active Electromagnetic 3-D Surface Cloak

An active interior cloak is composed of a conformal array of radiating sources surrounding a target object. With proper configuration, this array radiates a field discontinuity which cancels out any scattering which occurs when the object is illuminated by an impinging electromagnetic wave. Unlike many other cloaking methods, an interior cloak can be completely constructed using conventional components while its active nature allows it to circumvent passivity-based constraints. This enables an interior cloak to hide an object of virtually any size, shape, and material composition as well as accommodate multiple frequencies and polarizations. Despite its potential, this approach has only been demonstrated on 2-D and quasi-2-D targets to date. As such, this article presents the first 3-D implementation of an interior cloak. Here, the design of a cloak array capable of hiding a conductive, circular cylinder in free space is first presented. Cloaking performance is then validated via full-wave simulations and an experimental demonstration in which the device successfully suppresses scattering when the target is illuminated by a ridged horn antenna at 1.2 GHz.

Pattern synthesis of dual‐polarised conformal array based on adaptive iterative convex optimisation

Pattern synthesis of dual‐polarised conformal array based on adaptive iterative convex optimisation

Design of Miniaturized Wideband Beam Deflection Conformal Array Antenna

Antenna beam deflection, along with miniaturization and wideband of the antenna is in demand for practical applications. In this paper, a cylindrical conformal array antenna with a small-tilt forward beam was designed. The microstrip antenna unit was loaded with the artificial electromagnetic structure, which reduced the size of the antenna unit. As a result, the center spacing of the array elements can be shortened with the same array element spacing. The beam deflection angle can be increased in this way without increasing the coupling effect between the parts. Changing the number of line array elements and the number of line arrays can regulate the beam width of E-field and H-field, respectively. The bandwidth of the antenna can be significantly extended by slotting the ground plane. This work implemented a cylindrical conformal array of the antenna's forward beam with a small dip angle using a cylindrical carrier as an example. The measurement results showed that the angle between the main beam and the carrier axis of the conformal antenna was less than 30°, the bandwidth was more than 30%, and the antenna volume decreased by 40.4%.

A Composite Right/Left-Handed Phase Shifter-Based Cylindrical Phased Array with Reinforced Particles Responsive to Magneto-Static Fields

A conformal cylindrical phased array antenna excited with composite right/left-handed (CRLH) phase shifters is proposed. The phase tuning of the CRLH phase shifter is achieved by embedding novel magneto-static field-responsive micron-sized particles in its structure. It is shown that through the tiny magnet activation of these novel magneto-static particles at appropriate locations along the length of CRLH stub and inter-digital fingers, variable phase shifts are obtained. The proposed particle-based CRLH phase shifter operates in C-band (5–6) GHz with a low insertion loss and phase error. The 1 × 4 cylindrical phased array is excited with the four unit cells of the proposed particle-embedded CRLH transmission line phase shifters to scan the main beam at desired scan angles. A prototype of a 1 × 4 cylindrical phased array excited with the particle-based CRLH phase shifters was fabricated, and the results show that the simulated results are in close agreement with the measured results. The conformal cylindrical array with the proposed particle-based CRLH phase shifters has great potential for use in printed and flexible electronics design where commercially available phase shifters have a definite drawback.

High Aperture Efficiency Arced Conformal Array With Phasor Beam Steering Antenna

A phasor beam steering stacked patch antenna with abilities of both the amplitude and phase pattern reconfigurations is proposed and used for improving the aperture efficiency of arced conformal array. The phasor beam steering antenna is composed of two parasitic patches on the top integrated with four PIN diodes and a driven patch on the lower layer. By switching the bias states of the PIN diodes, the induced complex current of the top patch is alterable and the phasor beam steering can be delivered. Thus, in the conformal array design, the phase compensation can be realized by the phase pattern reconfiguration of the phasor beam steering antenna instead of the conventionally used phase delay lines. Meanwhile, the amplitude pattern reconfiguration of the phasor beam steering antenna can also be used for enhancing the contribution of side elements to the gain of axial beam. A four-element arced conformal array with the phasor beam steering antenna is designed and measured for verification. Both the simulated and measured results indicate that the realized gain of the proposed conformal array is 12.0 dBi, and the corresponding aperture efficiencies are improved up to 51.48%, which is better than the results of the arced conformal array with the phase compensation network.

Cognitive Conformal Subaperturing FDA-MIMO Radar for Power Allocation Strategy

Recently, the conformal array radar has attracted wide attention compared with the conventional array radar due to its flexible element configuration and more diverse signal processing methods. During the development of the jamming equipment, the electromagnetic environment becomes more complex, which brings more challenges to the radar system. To overcome the variable interference environment's negative influence on moving target tracking, we propose two adaptive power allocation methods based on two criteria to improve target tracking accuracy. Firstly, we build the data model of the conformal subaperturing frequency diverse array multi-input and multi-output (CSFDA-MIMO) radar and the interference signal. Moreover, the Bayesian Cramer-Rao bound (BCRB), signal-to-interference-plus-noise ratio (SINR) are derived and taken as the objective functions in proposed optimization problems. Based on it, we build two different power allocation optimization problems under BCRB minimization criteria and SINR maximization criteria, respectively, and two different power constraints are considered. Finally, simulation results verify the effectiveness of the proposed adaptive power allocation methods. The power allocation strategies under two criteria can obtain better tracking accuracy compared with uniform allocation, and the tracking accuracy of the adaptive power allocation method can be influenced by different power constraint conditions.