Half Beamwidth Research Articles

Target Fast Reconstruction of Real Aperture Radar Using Data Extrapolation-Based Parallel Iterative Adaptive Approach

Real aperture radar (RAR) usually sweeps a wide sector to continuously observe the scenario of interest. Because its angular resolution is limited by the size of the antenna aperture, target reconstruction methods are widely applied to obtain super-resolution radar images. However, the wide-sector processing mode suffers from high operational complexity because of the high-dimensional matrix inversion. Even worse, for the targets located at the scene edge, its echo data are received less than half beamwidth. The incomplete echo data will lead to the deformation of the reconstructed targets by existing reconstruction methods. To overcome the two problems, a data extrapolation-based parallel iterative adaptive approach is proposed to fast reconstruct the targets in the whole sector without the distortion at the scene edge. First, the echo model of RAR is repaired to remedy the model error. Then, based on the correlation of the echo data within one beamwidth, an autoregressive model is adopted to extrapolate the data of the missing half beamwidth. Finally, a parallel iterative adaptive approach method is proposed to efficiently recover the targets by exploiting the regular characteristics of the repaired steering matrix. Simulations and experimental data are applied to verify the proposed method.

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.

Flexure strengthening of lightweight reinforced concrete beams using carbon fibre-reinforced polymers

A series of 40 lightweight reinforced concrete (LWRC) beams of 1400 mm length and a rectangular cross section of 150 × 200 mm were cast, strengthened and then tested under four-point bending test to study the effectiveness of using externally applied carbon fibre-reinforced polymer (CFRP) composites as a method of increasing the flexural strength of under-reinforced LWRC beams. Parameters investigated include reinforcement ratio, ρ; ρ = 0.55ρb and ρ = 0.27ρb, CFRP sheet length; 600, 800 and 1000 mm, CFRP sheet width; beam width and half-beam width. Three types of strengthening schemes were used: jacketing covers the beam from bottom and two sides with total width of 500 mm, sheets at the tension side with width equal to beam width and sheets with width equal to half-beam width. Test results showed a limited increase in ultimate load-carrying capacity accompanied with some reduction in mid-span deflection for the strengthened beams. Among the strengthening schemes investigated, jacketing was the most effective for strength enhancement (about 41%) with respect to control beam; however, it reduced ductility significantly. An analytical model was proposed for predicting the ultimate load-carrying capacity of LWRC beams strengthened with CFRP composites.

Wide range beam steering capability of a 1-D MEMS transducer imager array with directional beam pattern

A linear array of capacitive micromachined ultrasonic transducer (CMUT) has been designed as an alternative to conventional piezoelectric transducers. The proposed array is favorable for imaging applications where there is a need for high resolution detection in directions with an angle other than that perpendicular to the axis of the transducer array. The transducer array cells include a thin silicon nitride membrane that is suspended over a bottom electrode, fabricated on a silicon wafer. In immersion mode, the transducer cell shape and dimensions are optimized for an operating frequency of 10MHz. The proposed imager array is shown to be able to generate a pencil shape beam with a ∼1.5° half beam width for enhancing the detector resolution. A phased array technique is employed to excite multiple cells using time-delayed signals to steer the acoustic beam toward the object. Therefore, the need for mechanically moving the transducer array or the cell membrane is eliminated, which results in simplifying the transducer driving system. Moreover, unlike conventional transducers, the pencil beam can be effectively steered over a wide range of angles without producing grating lobes or degrading the power difference between the main lobe and side lobes. Consequently, in transmitting mode the power loss in any undesired directions is minimized and in receiving mode the noise level from other angles interfering with the data collection is suppressed, which improves the signal to noise ratio of the imager CMUT array.

Capacitive Micromachined Ultrasonic Transducer Array with Pencil Beam Shape and Wide Range Beam Steering

Abstract A capacitive micromachined ultrasonic transducer (CMUT) array is designed as an alternative to conventional piezoelectric transducers. A thin silicon nitride membrane is suspended over a bottom electrode on a silicon wafer. In the immersion mode, the transducer cell shape and dimensions are optimized for an operating frequency of 10 MHz. We show that the proposed imager array can generate a pencil shape beam with a ∼1.5° half beam width, enhancing the detector resolution. A phased array technique is employed to excite multiple cells using time-delayed signals to steer the acoustic beam toward the object. This eliminates the need to mechanically move the detector, simplifying the transducer driving system. Moreover, unlike conventional transducers, the pencil beam can be effectively steered over a wide range of angles without producing grating lobes, which minimizes power loss in undesired directions. This can also improve the signal to noise ratio of the imager CMUT array.

Non-Planar Quasi-Self-Complementary Ultra-Wideband Feed Antenna

We are presenting a novel ultra-wideband, dual polarized, antenna design based on a non-planar quasi self-complementary (QSC) approach. An important characteristic of this QSC antenna is an input match better than -10 dB over a decade of bandwidth. We have demonstrated the concept by fabricating a QSC prototype antenna that works from 400 MHz to 4.0 GHz. We have measured the prototype performance in terms of input impedance and radiation patterns, from which the antenna directivity, beamwidth, polarization, and aperture efficiency for a 65° illumination angle were calculated. From these measurements we confirmed the ultra-wideband characteristics of the QSC feed prototype. In particular, we looked for the presence of resonances by measuring the radiation patterns with 3 MHz resolution, over the decade of bandwidth, in both polarizations, and none was found. The measured directivity of the QSC antenna feed is 10 dBi. The half beam-width, at -10 dB level, varies between 60° and 70°, with a frequency average value of 65°. The measured beam-integrated cross-polarization inside the -10 dB level of the patterns is better than -10 dB. The phase center location does not move with frequency.

A polarization diversity antenna using a printed dipole and patch with a hole

AbstractThis paper proposes a polarization diversity antenna that has a half beam width of 60° in the horizontal plane for use in wireless telephony base stations. Specifically, a printed dipole antenna is used as a vertical polarization element and a patch antenna with a hole is used as a horizontal polarization element. With these two combined, a small diversity antenna is constructed. Also, this antenna is analyzed by means of FDTD, confirming the experimental characteristics and elucidating the effect of the antenna parameters on the half beam width in the horizontal plane. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(9): 57–66, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10072

Evanescent wave coupling in light scattering of an off-axis normally incident Gaussian beam by two parallel nonabsorbing cylinders.

A theoretical and numerical investigation is devised for resonant light scattering of an off-axis normally incident Gaussian beam by two parallel nonabsorbing cylinders based on the related beam theory developed in J. Opt. Soc. Am. A 14, 640 (1997). By varying the half-beam width, we show that the multireflection process between the two scatterers can be minimized. Moreover, the study is an attempt to understand the underlying physics present in the process of resonance excitation by evanescent wave coupling.

Broadband compact horn feed for prime-focus reflectors

A compact flare angle controlled corrugated horn antenna with a 60° half beamwidth is optimised by using a moment method for bodies of revolution. The horn has corrugations with simple rectangular cross-sections and is excited by a smooth wall circular waveguide. The resulting 15 dB beamwidth varies by <18% and the crosspolar sidelobe levels are more than 26 dB below the mainlobe, both over a 1.8:1 bandwidth. All results are confirmed by measurements. The horn is suited for broadband feeding of e.g. prime-focus reflectors in radio astronomy applications.

Cartesian diver observations of double frequency pressure fluctuations in the upper levels of the ocean

Pressure oscillations having a narrow spectrum centered at 0.175 Hz have been measured in the deep sea at depths between 100 and 300 m. At these depths the nonlinear interaction between oppositely directed sea surface gravity waves causes pressure fluctuations, the intensity of which varies inversely with the square of the depth, uninfluenced by microseismic Rayleigh waves propagating on the seafloor below. The observed pressure fluctuations are consistent with the interaction between waves having a unimodel beam pattern whose half beam width is slightly larger than 90 degrees or with a shoreward propagating pencil beam of swells interacting with a small component reflected from the shore.

Density and size distribution of seamounts in the eastern Pacific inferred from wide‐beam sounding data

Theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow‐beam and wide‐beam echo sounders. The shapes of seamounts are approximated as truncated right‐circular cones of constant slope ε, height‐to‐radius ratio ξ, and flatness f; i.e., a seamount of basal radius r has a height h = ξr, a flat top of radius fr, and a slope angle of ϕ = arctan ε. The set of all seamounts in a given physiographic province is assumed to be a Poissonian ensemble specified by a function v(R), the average number of seamounts per unit area for which r ≥ R. A simple choice for the size distribution of seamounts, and one shown to be consistent with data in the eastern Pacific, is to specify that it decays exponentially with R: v(R) = vo exp (−αR). In the case of a profile of length L taken in water of depth D with a wide‐beam echo sounder (half beam width > ϕ), the shallow‐slope, deep‐water approximation (ε << 1, ε/α << D) yields that the expected number of seamounts with apparent heights is . We apply this sampling theory to apparent‐height data measured from 44,020 km of wide‐beam profiles on Pacific crust of post‐Mesozoic age. Although the spatial distribution of seamounts in the eastern Pacific is certainly not Poissonian, the abundance statistics do not involve spatial correlations, and we show they are robust with respect to deviations from the Poisson model, provided the statistics are interpreted as averages over the area actually sampled by the track lines. The observed falloff of apparent heights in the eastern Pacific is very nearly exponential, justifying our use of an exponential model of seamount size distribution. The shape parameters are estimated from multibeam surveys of Pacific seamounts; an analysis of 30 features in the height range 200–4000 m yields ξ = 0.21±0.011 and f = 0.21±0.028. Using these parameters, we obtain vo = (4.0±1.1) × 10−9 m−2 and α = (6.3±0.8) × 10−4 m−1. These values imply that per 106 km2 in the eastern Pacific there are 1600±400 seamounts with summit heights greater than 300 m and 200±60 greater than 1000 m; integrated over the entire size distribution, seamounts occupy about 6% of the seafloor area and constitute about 0.4% of the oceanic crustal volume. Our estimates of seamount abundance are appreciably greater than those of most other authors, a discrepancy we attribute primarily to the sampling bias of previous studies, which generally used uncorrected map counts. This hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central Pacific from the Scripps charts; above 3000 m the map counts are consistent with the predictions of the exponential model, but below this height they are biased to low values.

The field distribution in the focal plane of a paraboloidal reflector

The electromagnetic field in the focal plane of a finite axially-symmetrical paraboloidal reflector illuminated by a plane wave of arbitrary polarization incident nearly normally at the vertex, has been investigated. This paper describes the investigation, explains the method, and summarizes some of its results. The reflector is assumed to be of a focal length which is large compared with the wavelength of the incident wave, and the plane of the aperture is assumed to be about halfway between the focus and the vertex. The purpose is to view the diffraction pattern in the focal plane by treating the currents induced on the concave surface of the reflector so that the electromagnetic effect due to them on one part of the surface is taken into account at another by means of waves of current and charge on the paraboloid. The wave equation is derived by approximation from Maxwell's equations; this is feasible because the radii of curvature are large compared with the wavelength. The calculation proceeds by finding approximate solutions of the wave equation by the WKB methods and by noting the turning point of the waves at the same distance from the vertex as that of the geodesic corresponding to each pair of characteristic modes of the same harmonic order. The modes are determined and normalized, and thereon the coupling of the incident field is computed. From this representation of the current and charge distribution the electromagnetic field in the focal plane is derived. The integrations required are made by asymptotic approximation using the method of stationary phase. It is shown that in the vicinity of the focus the contribution from these secondary currents is very small compared with the field due to the primary currents 2(\overrightarrow{n} \times H^{inc} ). The primary field is represented by Fourier-Bessel series the number of terms in which is substantially [ m_{1} \sin \alpha +1/2 ] where m_{1}= (4\pi/\lambda) \times focal length, and \alpha =angle of incidence at the vertex. The representation that approximates to geometrical optics when m_{1} is large enough does not emerge until the parameter m_{1}^{1/2} \sin \alpha is greater than unity and involves m_{1}^{1/2} times as many terms in the series. This does not occur until \alpha is many times the angular half beamwidth of the main lobe of the reflector.

Hydrogen Line Observations of M33.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hydrogen Line Observations of M33. Turner, K. C. ; Tuve, M. A. ; Burke, B. F. Abstract A series of observations spaced at intervals of a half beamwidth (every 5 min of arc) was made along the major and minor axes of M33 with the NRAO 300-ft radio telescope and the Carnegie 90-channel hydrogen-line receiver. No evidence of expansion was found along the minor axis, although some asymmetry in velocity was seen at the center. The recession velocity of the nebula was found to be 179+1 km/sec. At least two hydrogen spiral arms could be distinguished on either side of the center along the major axis. Marked departure from solid-body rotation occurs at about 10 min of arc from the center. After this point the rotation laws for the two halves of the nebula differ from one another. No hydrogen was observed with rotational velocity greater than 150 km/sec with respect to the center. A rotation curve for the object would have a maximum velocity below 100 km/sec. Publication: The Astronomical Journal Pub Date: September 1963 DOI: 10.1086/109109 Bibcode: 1963AJ.....68R.295T full text sources ADS |