Antenna Pattern Distortion Research Articles

Antenna Pattern Calibration Method for Phased Array of High-Frequency Surface Wave Radar Based on First-Order Sea Clutter

The problem of accurate source localization has been an area of focus in high-frequency surface wave radar (HFSWR) applications. However, antenna pattern distortion (APD) decreases the direction-of-arrival (DOA) estimation performance of the multiple signal classification (MUSIC) algorithm. Up to now, limited studies have been conducted on the calibration of antenna pattern distortion for phased arrays in HFSWR. In this paper, we first analyze the effect of APD on the performance of the MUSIC algorithm through estimation of accuracy and angular resolution. We demonstrate that using the actual pattern (or say APD) can improve DOA estimation performance. Based on this proposition, we propose a novel iterative calibration method that employs the first-order sea clutter data and can jointly estimate DOA and APD in an iterative way. To obtain available calibration points, we introduce the extraction methods of the first-order sea clutter spectrum and single-DOA spectrum points. Meanwhile, in each iteration, the Beamspace MUSIC algorithm and artificial hummingbird algorithm (AHA) are utilized to estimate the DOA and APD, respectively. Numerical results reveal a good coincidence between the actual pattern and the estimated APD. We also apply this method to process the experimental data of HFSWR. We obtain the APD vector of the real phased array and improve the direction-finding performance of several real ship targets using this vector. Both numerical and experimental results prove the correctness of our proposed calibration method.

Measurement and Analysis of Antenna Pattern for MIMO HF Surface Wave Radar

The Multiple Input Multiple Output (MIMO) is a popular technology applied in radar field for its ability to generate virtual elements, which ensures radar miniaturization in the case of obtaining a large array aperture especially for high frequency surface wave radar (HFSWR). Antenna pattern distortions caused by environmental obstacles seriously conta-minate the manifold of MIMO synthesized array, resulting in inaccurate direction-of-arrival (DOA) estimation. In this letter, MIMO antenna patterns including transmitters and receivers are initially measured by shipborne transponder and their effect on static beam patterns are analyzed. The comparison between ideal beam patterns and actual beam patterns reveals that the transmit antenna pattern distortion has an obvious impact on weakening the effect of MIMO synthesized array. Moreover, the expe-rimental results using multiple signal classification (MUSIC) algorithm demonstrate that the measured antenna patterns (MAPs) are effective for MIMO DOA estimation improvement.

Ultrawideband TEM horn with inhomogeneous dielectric medium

AbstractThis article presents a novel design of ultrawideband TEM horn with dielectric filling of the space between its leaves. The distribution of dielectric permittivity in the medium is obtained according to the laws of geometric optics. The electromagnetic simulation and experimental measurements are performed; voltage standing wave ratio and radiation patterns at a set of frequencies up to 20 GHz are obtained. The described method allowed to prevent antenna pattern distortion at frequencies from 7.5 to 20 GHz.

Relationship Between DOA Estimation Error and Antenna Pattern Distortion in Direction-Finding High-Frequency Radar

The direction-finding high-frequency radars (HFRs) are widely used for remotely sensing the ocean surface currents. However, the performance of the direction-finding HFR is limited by the inevitable distortion of the actual antenna pattern of the compact monopole-cross-loop antenna, because the distortion of the antenna pattern leads to errors on the direction-of-arrival (DOA) estimation, which results in correctly determined radial velocities being placed into the incorrect bearing sectors. In this letter, a model determining the DOA estimation error (DOAEE) stemming from the antenna pattern distortion (APD) is presented with a detailed analytical derivation. This model suggests that the DOAEE only depends on the relative distortion of the two orthogonal loops. To validate the correctness of the proposed model, simulations are carried out. In addition, the simulation results show that the model well captures the actual relationship between the APD and the DOAEE. Moreover, a data set collected by a direction-finding HFR system and a synchronized automatic identification system receiver is used to further validate the proposed model. The precessing result of this data set demonstrates that the model-calculated DOAEE and the actual DOAEE are in good agreement with a correlation coefficient of 0.92 and a root-mean-square error of 9°, which provides solid evidence to verify the validity of the proposed model in determining the relationship between the DOAEE and the APD.

Analysis and Validation of an Improved Method for Measuring HF Surface Wave Radar Antenna Pattern

Antenna pattern distortion incorporating the systematic differences among array elements is one of the most important factors that decrease the accuracy of radar direction of arrival estimation. In this letter, a new antenna pattern measurement (APM) method based on a fully-digital radar receiver and a portable signal source is presented. Compared with previous methods, the proposed method not only improves the measurement accuracy, but also reduces the complexity and time cost of measurement. APM results of a compact high-frequency (HF) antenna measured in two modes are depicted, both showing that two independent measurements at the same site are highly consistent. Moreover, the method is further verified by a comparison of radar measured radial current velocity with acoustic Doppler current profiler data.

Accuracy Assessment of Surface Current Velocities Observed by OSMAR-S High-Frequency Radar System

High-frequency (HF) radar receives the sea echo backscattered from the fluctuating ocean surface to remotely sense ocean surface currents over a large area with a high space-time resolution. To advance our understanding on this technology, we carried out an experiment for verifying the performance of ocean state monitoring and analyzing radar, model S (OSMAR-S) in ocean surface current measurement. The hourly radial currents measured by this radar system show a good agreement with acoustic Doppler current profilers (ADCPs) with correlation coefficients ranging in 0.40–0.93 and root-mean-square-error (rmse) values in 0.11–0.18 m/s. The u/v components of the radar-derived vector currents show a similar deviation from the ADCPs with correlation coefficients varying from 0.56 to 0.93 and rmse values varying in 0.13–0.18 m/s. Also, the bearing offset of this radar system in current extraction is in the range from $-$ 11 $^{\circ }$ to 18 $^{\circ }$ . In consideration of the fact that the ideal antenna pattern was used in determining the bearing of the radials, we assume the main source resulting in the bearing offset is the antenna pattern distortion. Thus, a quantitative relation between the antenna pattern distortion factor and the bearing offset has been deduced. With this relation, we estimated the bearing error in the moorings’ bearing with the aid of the current information recorded by ADCPs. As expected, the result indicates that the antenna pattern distortion dominates the error of the bearing determination.

Electromagnetic Compatibility Effects on Antenna Pattern of Spaceborne SAR

Due to the limitation of satellite size, electromagnetic compatibility (EMC) effects on the antenna pattern should be taken into account, especially in the case of small satellite platform. In this paper, the property of SAR antenna pattern is discussed firstly. Then, the effects on antenna pattern is studied and analyzed, which results in antenna pattern distortion. Moreover, considering the antenna pattern distortion, the influence on SAR image quality is present as well. Finally, simulations are performed to validate the effects caused by EMC.

Phased Array Antenna Calibration with Probe Positioning Errors [Measurements Corner

Calibration is often performed inservice with a calibration probe (CP) located in the far field (FF) of the antenna under test (AUT). Nevertheless, in some cases, it is suitable to work in the near-field (NF) environment. NF calibration is especially useful for large-aperture phased-array antennas (PAAs), when it is often complicated to arrange the CP in the FF of the antenna array. Recalculating complex initial coefficients from the NF to the FF requires taking into account distances between the phase centers of the emitters of the channels under test and the phase center of the CP. In this article, the influence of errors in the determination of coordinates of a CP on the calibration results, together with antenna pattern distortions, are considered. The derived formulas could be used to produce new requirements on the accuracy of an AUT in a CP survey.

Calibration of antenna pattern and phase errors of a cross‐loop/monopole antenna array in high‐frequency surface wave radars

High-frequency surface wave radar has now been widely used as a regular instrument in remote sensing of sea states. To solve the contradiction of the array pattern beamwidth and array aperture, multiple cross-loop/monopole (CM) antennas are used as sub-arrays to form a uniform linear array, which is called the CM-uniform linear array (ULA). Adaptive beamforming via virtual interferences is used to synthesize the desired pattern and a half beamwidth of about 20° is achieved by a two-unit CM-ULA with a spacing of 0.75λ (λ is the radar wavelength). A novel iterative method is proposed to calibrate the array errors, including the individual antenna pattern distortions and the channel gain/phase errors, without resorting to any known signals such as a beacon or transponder signal. Simulations are executed and the results show the validity of the proposed method.

평면형 능동위상 배열안테나 빔 패턴 보상에 관한 연구

본 논문에서는 평면형 능동 위상 배열 안테나에서 일부 모듈의 고장이 발생할 경우, 발생되는 빔 패턴 왜곡을 여러가지 경우로 비교하여 검토하고, 기존에 연구된 빔 패턴 보상 방법을 실제 안테나에 적용하여 빔 패턴 왜곡 개선 가능성을 분석하였다. 빔 패턴의 왜곡은 주로 부엽 수준의 증가로 나타나게 되는데, 부엽 수준이 기준치를 넘어서게 되면 고장모듈을 제외한 나머지 모듈의 크기/위상값을 재조정하여 부엽을 낮출 수가 있으며, 본 논문에서는 이 방법을 실제 제작된 안테나에 적용하기 위한 방안 및 근접 전계 챔버에서 측정한 결과를 시뮬레이션 결과와 비교하여 나타내었다. 고장모듈이 발생하는 경우는 무작위로 여러 곳에서 발생하는 경우와 여러 개의 배열을 포함하는 단위 모듈이 고장 날 경우로 나누어서, 시뮬레이션 결과와 측정 결과를 비교 분석하였다. 측정 결과는 시뮬레이션 결과와 유사한 경향을 나타내며, 고장 모듈이 발생할 경우 남아있는 정상 모듈의 크기/위상 조정을 통해서 부엽을 낮추기 위한 빔 패턴 보상이 가능함을 확인할 수 있다. This paper discusses about the beam pattern distortion caused by the failures of some antenna modules in the active array antenna and analyses the possibility of improvement through applying the beam pattern compensation method previously studied. The beam pattern distortion which is mostly represented as an increase of the sidelobe level, can be suppressed through re-synthesizing each module's magnitude and phase. This method was applied to the prototype of active array antenna system, and the results of antenna pattern distortion and compensation were analyzed and measured in the Near Field Chamber. Array failures are generally divided into random TR module failures and TRU(TR Unit: combination of TR modules, Beam Computation module, Power supply module) failures. The results of beam pattern compensation were analyzed in each failure and compared to the results of the simulation. The beam pattern compensation results applied to the real active antenna array system showed the similar to the simulation results. Consequently, it was verified the beam pattern could be compensated with the magnitude and phase adjustment of other normal antenna modules.

Navigation antenna performance degradation caused by plasma sheath

A plasma sheath generated around a hypersonic vehicle can be a serious obstacle to fixed-frequency communications, especially navigation systems. This paper studies the effects of a plasma sheath on a Beidou navigation circular microstrip patch antenna. The dynamic stratified medium model method, which is based on the gradient of electron density profile curve, is employed to model the nonuniform plasma sheath. Using the algorithm calculation of the time-domain finite integration, the navigation antenna radiation pattern property is obtained, along with the changes in the reflection coefficient and input impedance properties. Results show that antenna pattern distortion, resonant frequency shift, and impedance mismatch caused by plasma are proportional to the electron density. The process of reentry blackout is also reproduced from the perspective of the antenna performance degradation. The inductor L and capacitor C (LC tuning circuit) impedance mismatch compensation scheme proposed is demonstrated to be effective and it can decrease return loss by 19.4 dB under the most serious plasma environment, thus reducing signal reflection and enhancing navigation system performance.

Estimation and Assessment of Errors Related to Antenna Pattern Distortion in CODAR SeaSonde High-Frequency Radar Ocean Current Measurements

Abstract A simulation-based investigation of errors in HF radar–derived, near-surface ocean current measurements is presented. The simulation model is specific to Coastal Ocean Dynamics Application Radar (CODAR) SeaSonde radar systems that employ a compact, collocated antenna geometry. In this study, radial current retrievals are obtained by processing simulated data using unmodified CODAR data processing software. To avoid limiting the results to specific ocean current and wind wave scenarios, the analyses employ large ensembles of randomly varying simulated environmental conditions. The effect of antenna pattern distortion on the accuracy of retrievals is investigated using 40 different antenna sensitivity patterns of varying levels of distortion. A single parameter is derived to describe the level of the antenna pattern distortion. This parameter is found to be highly correlated with the rms error of the simulated radial currents (r = 0.94) and therefore can be used as a basis for evaluating the severity of site-specific antenna pattern distortions. Ensemble averages of the subperiod simulated current retrieval standard deviations are found to be highly correlated with the antenna pattern distortion parameter (r = 0.92). Simulations without distortions of the antenna pattern indicate that an rms radial current error of 2.9 cm s−1 is a minimum bound on the error of a SeaSonde ocean radar system, given a typical set of operating parameters and a generalized ensemble of ocean conditions.

Calibration and Validation of Direction-Finding High-Frequency Radar Ocean Surface Current Observations

This paper focuses on the validation of remotely sensed ocean surface currents from SeaSonde-type high-frequency (HF) radar systems. Hourly observations during the period July 22, 2003 through September 9, 2003 are used from four separate radar sites deployed around the shores of Monterey Bay, CA. Calibration of direction-finding techniques is addressed through the comparisons of results obtained using measured and ideal (i.e., perfect) antenna patterns. Radial currents are compared with observations from a moored current meter and from 16 surface drifter trajectories. In addition, four overwater baselines are used for radar-to-radar comparisons. Use of measured antenna patterns improves system performance in almost all cases. Antenna-pattern measurements repeated one year later at three of the four radar locations exhibit only minor changes indicating that pattern distortions are stable. Calibrated results show root-mean-square (rms) radial velocity differences in the range of 9.8-13.0 cm/s, which suggest radar observation error levels in the range of 6.9-9.2 cm/s. In most cases, clear evidence of bearing errors can be seen, which range up to 30deg for uncalibrated radar-derived radial currents and up to 15deg for currents obtained using measured antenna patterns. Bearing errors are not, however, constant with angle. The results recommend use of measured antenna patterns in all SeaSonde-type applications. They also recommend an expanded simulation effort to better describe the effects of antenna-pattern distortions on bearing determination under a variety of ocean conditions

Factors Affecting the Accuracy of SHOWEX HF Radar Wave Measurements

Abstract Ocean Surface Current Radar (OSCR) HF radar measurements of ocean waves and currents were made during the Shoaling Waves Experiment (SHOWEX) in the fall of 1999. During some periods, at some locations, good quality wave measurements were obtained. Limitations in the wave measurement capability due to OSCR hardware, deployment configuration, signal-to-noise ratio, and antenna sidelobes are identified and discussed. A short period of large currents in the presence of antenna pattern distortion is identified as the source of the main errors in the wave measurements.

Two-dimensional diffraction by half-planes and wide slits near radiating apertures

With the complex source-point method used to produce the basis elements of an array of linearly and directionally equispaced two-dimensional (2-D) beams, the fields of any aperture distribution at any range to any degree of accuracy can be obtained. For efficiency a limited number of significant beams and beam directions is required. Approximately twice as many beams as the aperture width in wavelengths, with all beam directions normal to the aperture, is found to be sufficient here for simple uniform and cosinusoidal distributions in apertures of moderate size at ranges outside the evanescent field zone of the aperture. Now the exact solution for the far field of a line source, or here a beam source in the presence of a conducting half-plane, is used as our basis element to give the solution for antenna pattern diffraction by a local half-plane. Antenna pattern diffraction by an aperture near a wide slit is presented as simply a superposition of the solutions for two coplanar half-planes with separated parallel edges. Antenna pattern distortion by various other local obstacles can be obtained similarly.

Comparision of Measured and Fdtd Calculated Distortions of Antenna Radiation Patterns Due To Nearby Boards

This paper presents comparisons of measured and calculated antenna pattern distortions caused by two perpendicular boards in the nearfield region of an antenna at frequency 1.96 GHz. Two different antennas were used: an array of two dipoles above a ground plane, and a λ/4 patch antenna. The calculations were carried out using commercial software based on the FDTD (Finite Difference Time Domain) method. The results illustrate the accuracy of the FDTD models needed with the two antennas and give knowledge of the distortions caused by nearby walls on the radiation patterns of the two antennas.

Antenna pattern distortion by an isotropic plasma slab

A quantitative laboratory experimental investigation has been conducted on the distortion of the main far-field radiation pattern of a horn antenna situated behind a layer of isotropic plasma. The power radiation pattern and phase for orthogonal components of the radiation transmitted through the plasma were investigated for various plasma properties. The experimental results were compared to theory based on the reciprocity theorem and good agreement was obtained. The major features are a gradual attenuation and distortion of the radiation pattern for plasma electron densities below the critical density and a rapid change in the radiation pattern with severe attenuation in the region of the critical electron density followed by "cut-off" at still larger electron densities. No appreciable polarization change of the energy radiated by the antenna is introduced by the plasma.

An experimental investigation of antenna pattern distortion due to a plasma layer

Abstract : Measured radiation patterns of a plasma-covered antenna are compared with an approximate leaky wave analysis due to Oliner and Tamir. The results clearly show a shift in the optics ''critical angle'' and the appearance of several side lobes are predicted by theory. The plasma is produced in a He - Xe admixture at 0.4 torr by a capacitively-coupled 10-Mcps cw source and contained in a rectangular pyrex vessel of interior dimensions 8 in. by 4 in. by 5 in. Electron densities in excess of 5 x 10 to the 11th power per cc are measured by a standard microwave interferometer by manually balancing a bridge or automatically by displaying a set of interference ''fringes'' after a pattern is recorded. (Author)

Antenna pattern distortion by dielectric sheets

The radiation pattern of an antenna covered with a radome can be determined from the following data: The antenna Fresnel-Zone fields in the absence of the radome, and the fields inside the radome produced by a plane-wave incident from an external source. In certain cases the pattern calculations may be simplified without excessive loss of accuracy by using optical theory to describe approximately the plane-wave transmission through the radome, and neglecting scattering by the antenna in determining the fields inside the radome from the external plane-wave source.