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Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
aperiodic convolution the convolution of two sequences. See convolution. aperiodic signal a signal that is not periodic, i.e., one for which x(t) = x(t + T ). This means that the signal x(t) has a property that is changed by a time shift T . See also periodic signal. aperiodic waveform this phrase is used to describe a waveform that does not repeat itself in a uniform, periodic manner. Compare with periodic waveform. aperture (1) an opening to a cavity, or waveguide, from which radiation is either received or transmitted. Typically used as antenna or a coupling element. (2) a physical space available for beam to occupy in a device. Aperture limitations are the physical size of the vacuum chamber, a magnetic field anomaly may deflect the beam so that the full available aperture cannot be used. aperture antenna an antenna with a physical opening, hole, or slit. Contrast with a wire antenna. aperture correction signal compensation used to correct the distortion caused by the non-zero aperture of a scanning electron beam. A standardized measure of the selectivity of a circuit or system. The -3 dB (or half-power) band width
Registration for Super-Resolution: Theory, Algorithms, and Applications in Image and Mobile Video Enhancement
Published in Peyman Milanfar, Super-Resolution Imaging, 2017
Patrick Vandewalle, Luciano Sbaiz, Martin Vetterli
In a real camera several non idealities contribute to a significant deviation from the pinhole model. The linear distortion introduced by the optics is represented by the point spread function (PSF). This is the impulse response of the imaging system, i.e. the image obtained when a point light source of infinitesimal size is placed in front of the system. Even when the system is perfectly focused, the image is not a point of infinitesimal size, but rather a disk of nonnegligible diameter. This measure describes the quality of the optical system. For example, lenses that are not ideal or are not precisely placed, result in an increase of the size of the point spread function. However, even in the ideal case, the point spread function has a non-negligible size. For an ideal lens with circular aperture, the point spread function is also called the Airy disk [8]. Its size is determined by the diffraction of the system, which is proportional to the wavelength of the light source and the aperture value (or f -number). Note that higher f -numbers correspond to a smaller aperture area, or less incident light. A large f -number corresponds to a large Airy disk and a strong low-pass effect (and at the same time a large depth of field). Conversely, a small f -number corresponds to a smaller Airy disk and sharper images.
Sound absorption and sound absorbers
Published in Heinrich Kuttruff, Room Acoustics, 2016
Now we have to discuss the losses occurring in the resonator. There are two types of losses: those which are due to the internal friction of the air oscillating in the aperture, represented by some resistance R0. If desired, R0 can be increased, for instance, by introducing some porous material in the aperture. The second sort of loss is caused by the re-radiation of sound into the free space. It is characterized by the radiation resistance Rr of the aperture. We imagine this aperture as an oscillating piston as in Figure 6.6b, mounted flush in a rigid wall of infinite extension. The lateral dimensions of the aperture are assumed as small compared with the wavelength. Then, the radiation impedance of the aperture is given by Rr≈ρ0ω2S22πc=2πρ0c(Sλ)2
Design of Asymmetrical Antenna Using Slotted Mirror Image Ground Aperture Coupling for Multiband Application
Published in IETE Journal of Research, 2022
Sanjay Vivekanand Khobragade, Sanjay Laxmikant Nalbalwar, Anil Bapusa Nandgaonkar, Abhay Eknath Wagh
In an aperture coupling feed (ACF) microstrip patch antenna, a separate dielectric substrate is used for the feed network and the patch antenna. The main difference is both the substrates are separated by a ground plane which consists of a coupling aperture or a slot between the feed and the patch antenna. The structure of this antenna is shown in Figure 1. Since all layers adhere to the conformal printed circuit technology, fabrication is thus made simple. However, alignment between layers and correct selection of aperture size and position will be critical in controlling the antenna impedance. The natural existence of small gaps between the layers of dielectric substrate can significantly change the input impedance values. The absence of abrupt current discontinuities in ACF makes it relatively easy to design the antenna accurately [1–5].
The effect of aperture shape, angle of incidence and polarization on shielding effectiveness of metallic enclosures
Published in Journal of Microwave Power and Electromagnetic Energy, 2019
Ibrahim Bahadir Basyigit, Habib Dogan, Selcuk Helhel
Aperture shape and physical structures of an enclosure, polarization and angle of incidence of impinging field are force-major parameters affecting ESE performance. The results have been presented in Figures 4–8. Observations and models has been predicted decreasing ESE performance till resonance frequencies. This is basically related to comparison of wavelength with aperture size. When the wavelength of high frequency electric field is smaller than aperture size, it has more potential to couple into an enclosure, as expected. At resonance frequencies, ESE is negative which is undesirable for shielding. Therefore it acts as if there were another source in the enclosure which leads to an increase in interference. So, at low frequencies we see high ESE values. Above resonance frequencies, ESE increases simultaneously with increased frequency.
A Reliability-Based Condition Assessment of Structural Concrete Using Synthetic Aperture Radar Imaging Techniques
Published in Research in Nondestructive Evaluation, 2020
Jones Owusu Twumasi, Paul DeStefano, John T. Christian, Yu Tzuyang
In synthetic aperture radar (SAR) imaging, the SAR system is mounted on a moving platform in order to utilize the motion of the radar antenna. During the SAR imaging of a target, consecutive time of transmission/reception of EM waves is translated into different positions due to the movement of the platform. Integration of the received EM waves leads to the formation of a virtual aperture (much longer than the actual length of the radar antenna) and an adjustable frequency bandwidth. This process produces high-resolution coherent (continuous wave) images of the target under investigation. The image formation process is formulated by a planar scattering problem in a domain Ωs containing N scattering points (Fig. 1).