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Antennae
Published in Le Nguyen Binh, Wireless And Guided Wave Electromagnetics, 2017
An antennae tower is a tall structure to support antennae as aerials for telecommunication and broadcasting. The first tower was constructed in Munich Olympic Park for the television broadcasting purposes of the 1972 Olympics. A dipole antenna is a simple antenna usually constructed from two wires in opposite phases positioned end to end with respect to each other. A horn antenna is a type of directional antenna shaped like a horn. An omnidirectional antenna is an antenna system that radiates power uniformly in all directions in one plane. A parabolic antenna is an antenna shaped like a parabola in one or both planes.
From launch to transmission: satellite communication theory and SNG
Published in Jonathan Higgins, Satellite Newsgathering, 2012
The parabolic antenna (Figure 2.37) is essentially an electromagnetic wave lens, which like a magnifying glass can focus sunlight energy into a narrow beam, focuses the radio frequency energy into a narrow beam. By doing so, it also has a gain characteristic as it amplifies the signal. A parabolic surface produces a parallel beam of energy if the surface is illuminated from its ‘focus’ or ‘focal point’, and the parabolic dish is commonly referred to as the ‘reflector’. The signal is transmitted via the feedhorn assembly mounted on the antenna, and the mouth of the feedhorn is coincident with the focal point of the reflector. This assembly is often loosely referred to as the ‘waveguide’, ‘feed’ or ‘launcher’ (as it ‘launches’ the signal). Flyaway antennas range in size from 0.9 to 3.7 m, though for SNG use the typical size used is 0.9–2 m.
Detecting Microwaves
Published in Iain H. Woodhouse, Introduction to Microwave Remote Sensing, 2006
A parabolic antenna is a reflecting device that focuses the incident energy from a localised direction onto the detector (or into a waveguide which then carries the signal to the detector) much like the primary mirror system on a reflecting telescope or optical scanner. The reflector is a curved surface made of a material that is highly reflective at the relevant frequencies. At short wavelengths this could be a solid metal antenna, but at longer wavelengths it can be composed of a wire mesh (as long as the holes are much smaller than the wavelength). The choice is purely a practical one, since a mesh is both lighter and has less air resistance.
Effect of the plasma sheath on the propagation of low frequency radio waves
Published in Waves in Random and Complex Media, 2021
Firstly, the following presents the basic parameter settings for the plasma-sheath simulation. The sheath data is computed by using commercial software COMSOL. In simulation, the vehicle altitude range is from 55 to 70 km, with a speed range of Mach 18–24. In this circumstance, the simulation parameters of the free stream mainly include: the temperature (233.3 K), the pressure (10.93 Pa), the air density (1.63 × 10−4 kg/m3), and the sound speed (306.19 m/s). Figure 1 shows the cross-section of the sheath model and the distribution of the observation points, in which the origin of the coordinates is at the center of the intersecting planes between the blunt head and the truncated cone. The observation points are distributed in the front, middle and tail of the vehicle, and their coordinates y and z (mm) are respectively: the front A(0, −20), B(50, −20), and C(90, −20); the middle D(90, 120); and the tail E(90, 240), F(70, 240), and G(50, 240). In the simulation, the parabolic antenna is placed at the point (0, 80), towards the negative z-axis; the FDTD grid size is with a discretization step of 1 mm in each direction; the relative permittivity of the radome is 2; and the vehicle and the antenna are both PEC.
Evolutionary system design using a generalized component–resource model
Published in Engineering Optimization, 2021
Matthew L. Marcus, Raymond J. Sedwick
A component's class determines its resource flows as functions of the remaining properties. The values of these component parameters are defined for each specific component. Parameters can either be fixed or genetically determined. Genetic component parameters allow the framework to propose and optimize its own components. A component for which all parameters are fixed is referred to henceforth as a ‘real’ component. One that also contains genetically determined parameters is referred to as a ‘notional’ component. As an example of a notional component, consider a parabolic antenna for a spacecraft. The gain of the antenna is defined as where is the antenna efficiency, A is the aperture area of the antenna [m], and λ is the wavelength of the transmitted signal [m]. The mass of the antenna is defined as where ρ is the areal density of the antenna [kg/m]. The values of and ρ will be a function of the antenna material and construction, accounting for factors that cannot be modelled analytically in a simple way. λ is determined by the incoming signal from the spacecraft's transmitter, which would be considered a resource for which an antenna of this class is a sink. A is defined by the designer, and drives the gain and mass properties given (4) and (5). The value of A would be genetically determined within some specified range.
Assessing the active-passive approach at variant incidence angles for microwave brightness temperature downscaling
Published in International Journal of Digital Earth, 2021
Peng Guo, Tianjie Zhao, Jiancheng Shi, Hongxin Xu, Xiuwei Li, Shengda Niu
TWRS uses a parabolic antenna, which is used for the 1-D synthetic aperture radiometer and the electronic scanning radar operating synchronously at the L-band. The incidence angle of synchronous active and passive observations is slated to remain the same within one measurement but would vary between days. This new feature requires careful study for applications to robust soil moisture mapping with either active or passive observations. Zhao et al. (2020b) examined soil moisture retrievals at variant incidence angles with L-band radiometry. In this study, we focused on brightness temperature downscaling using active observations. The time-series regression (TSR) algorithm has been widely evaluated as a successful brightness temperature downscaling method and has been selected as the baseline algorithm of the SMAP mission. Evaluating the active–passive downscaling approach used by SMAP requires time series observations for the regression, which will lead to difficulties when the observations are insufficient due to the orbit revisit period designed for TWRS. Spectral analysis (SA) brightness temperature downscaling algorithms that are independent of time series observations were also chosen for evaluation. In addition, what will happen in the event that the incidence angle of active and passive observations cannot remain constant or whether it is necessary to keep the incident angle consistent for active and passive observations also needs to be demonstrated. Most of the active–passive combined downscaling approaches have been developed under the condition that the incidence angles of the system's radar and radiometer are the same and constant, while few studies have focused on how to use active–passive observations at variant incidence angles for downscaling.