Investigation of Iterative Techniques for Synthesizing of the Array Factor of the Antenna Arrays

In this essay, several iterative techniques are investigated for synthesizing of the array factor of the antenna array. These iterative techniques include Richardson method, Jacobi method, Gauss-Seidel method, Successive-over relaxation (SOR) method. It is shown that these iterative techniques can be successfully applied to synthesize of the array factor of any arbitrary linear, planar, or even ring arrays. Traditionally, least square method (LSM) is used for this purpose. However, LSM cannot be directly applied for any practical arrays. In other words, LSM is not a general approach, and it should be mixed with other techniques to find the magnitude and phase of the excitation coefficients of each element of the under-studying arrays. Obtained results show that the proposed method, offers considerable improvements for this target

A New Strategy for Deep Wide-Field High Resolution Optical Imaging

We propose a new strategy for obtaining enhanced resolution (FWHM = 0.12 arcsec) deep optical images over a wide field of view. As is well known, this type of image quality can be obtained in principle simply by fast guiding on a small (D = 1.5m) telescope at a good site, but only for target objects which lie within a limited angular distance of a suitably bright guide star. For high altitude turbulence this 'isokinetic angle' is approximately 1 arcminute. With a 1 degree field say one would need to track and correct the motions of thousands of isokinetic patches, yet there are typically too few sufficiently bright guide stars to provide the necessary guiding information. Our proposed solution to these problems has two novel features. The first is to use orthogonal transfer charge-coupled device (OTCCD) technology to effectively implement a wide field 'rubber focal plane' detector composed of an array of cells which can be guided independently. The second is to combine measured motions of a set of guide stars made with an array of telescopes to provide the extra information needed to fully determine the deflection field. We discuss the performance, feasibility and design constraints on a system which would provide the collecting area equivalent to a single 9m telescope, a 1 degree square field and 0.12 arcsec FWHM image quality.Comment: 46 pages, 22 figures, submitted to PASP, a version with higher resolution images and other supplementary material can be found at http://www.ifa.hawaii.edu/~kaiser/wfhr

Image formation in synthetic aperture radio telescopes

Next generation radio telescopes will be much larger, more sensitive, have much larger observation bandwidth and will be capable of pointing multiple beams simultaneously. Obtaining the sensitivity, resolution and dynamic range supported by the receivers requires the development of new signal processing techniques for array and atmospheric calibration as well as new imaging techniques that are both more accurate and computationally efficient since data volumes will be much larger. This paper provides a tutorial overview of existing image formation techniques and outlines some of the future directions needed for information extraction from future radio telescopes. We describe the imaging process from measurement equation until deconvolution, both as a Fourier inversion problem and as an array processing estimation problem. The latter formulation enables the development of more advanced techniques based on state of the art array processing. We demonstrate the techniques on simulated and measured radio telescope data.Comment: 12 page

On the estimation of the directional spectrum of ocean waves from time series observations of surface elevations

A mathematical model, consistent with certain physical features of ocean waves may be constructed by superposition of long crested sinusoidal gravity waves. Such a model, as proposed by Pierson (1955) and Longuet-Higgins (1957), depends upon the random superposition of the component waves, so that the interpretation of ocean wave measurements must be regarded as a statistical problem. Barber (1958) has suggested that measurement of sea surface elevation as a function of time at several points along a line array may be used to deduce the distribution of energy with regard to frequency and direction of the component gravity waves. In fact, by preserving the time relationship among the signals from several detectors in a line array , the array need not be physically rotated to examine component gravity waves coming from various directions. After developing the physical basis and mathematical notation for a stochastic model of ocean waves the limitations and potential errors in the measurement and calculation of directional spectra from finite and discrete data are discussed. Finally, some directional spectra calculated from measurements of wind generated waves in Buzzards Bay, Massachusetts are presented without attempting interpretation.This research was supported in part by the Bureau of Ships Fundamental Hydromechanics Research Program, S-R009 01 01, administered by the David Taylor Model Basin and the Office of Naval Research Under Contract Nonr-3351(00) NR 083- 501 and Nonr 2734(00) NR 083-143

An Investigation into the Implementation and Performance of Spectrally Shaped Orthogonal Frequency Division Multiplex

Orthogonal Frequency Division Multiplex (OFDM) is a flexible, robust multi-carrier modulation scheme. The orthogonal spectral shaping and spacing of OFDM sub-carriers ensure that their spectra can be over-lapped without leading to undesirable inter-carrier interference. Conventional OFDM systems have non-band limited Sinc(x) shaped subcarrier spectra. An alternative form of OFDM, referred to hereafter as Spectrally Shaped OFDM, employs band limited Nyquist shaped sub-carrier spectra. The research described in this thesis investigates the strengths and weaknesses of Spectrally Shaped OFDM as a potential modulation scheme for future mobile radio applications. From this research a novel Digital Signal Processing architecture for modulating and demodulating Spectrally Shaped OFDM sub-carriers has been derived which exploits the combination of a complex Discrete Fourier Transform (DFT) and PolyPhase Network (PPN) filter. This architecture is shown to significantly reduce the minimum number of computations required per symbol compared to previous designs. Using a custom coded computer simulation, the effects of varying the key parameters of the novel architecture's PolyPhase Filter (PPN) filter an the overall system complexity, spectral performance and system signal-to-distortion have been extensively studied. From these studies it is shown that compared to similar conventional OFDM systems, Spectrally Shaped OFDM systems possess superior out-of-band spectral qualities but significantly worse Peak-to-Average-Power-Ratio (PAPR) envelope performance. lt is also shown that the absolute value of the end PPN filter coefficients (dependent on the roll-off factor of the sub-carrier spectral shaping) dictate the system signal-to-distortion ratio when no time-domain windowing of the PPN filter coefficients is applied. Finally the effects of a both time and frequency selective fast fading channels on the modulation scheme's uncoded Bit Error Rate (BER) versus Signal-to-Noise (SNR) performance are simulated. The results obtained indicate that Spectrally Shaped OFDM is more robust (lower BER) to frequency-selective fading than time-selective fading

A clinical system for the measurement of regional metabolic rates in the brain.

The study of the chemical events that regulate the function of the human brain is particularly difficult. The introduction by Hounsfield, in 1973, of a tomographic technique based on the attenuation of X-rays by tissues has proved invaluable in the study of the morphology of the brain. An extension of this technique, employing the concepts of computerised tomography in combination with the use of specific molecules labelled with positron emitters, is now making the direct regional measurement of metabolic rates during life possible. Although some positron tomography systems are available commercially, they do not necessarily fulfil the specific needs of all researchers. Faced with the problem of quantitating the regional distribution of the essential neurotransmitter, dopamine, in the human brain a positron tomography system, which forms the basis of this work, was designed and built based on a series of experiments aimed at optimizing spatial resolution and detection efficiency. The performance of the tomograph has been evaluated through a series of phantom studies; and the system has been used to measure the local cerebral metabolic rate of glucose and the local distribution of dopamine in the healthy and diseased brain. It is felt that the ability of this tomograph to resolve metabolic structures in the brain as small as 10[3] mm[3] will only be surpassed at the cost of unduly increasing the radiation dose to the subject. The results of positron tomographic studies performed using different positron labelled molecules and those obtained using X-ray computerized tomographic techniques and magnetic resonance techniques in the same subject have been compared. The results have been found to be complementary, each technique providing a clue to the proper understanding of the functioning of the brain

Physical limitations on antennas

"May 27, 1953."Bibliography: p. 84-85.Army Signal Corps Contract DA36-039 sc-42607 Project 132B Dept. of the Army Project 3-99-10-022John Ruze