This work presents an overview of rainfall fading models over satellite links in South Africa using three years of rainfall data collected by the Joss-Waldvogel RD-80 disdrometer in Durban, South Africa (29˚52'S, 30˚58'E), alongside a colocated Ku-band satellite TV link. Different drop size distribution models, such as Lognormal, Gamma, Weibull, and the Optimised drop size distribution model for Equatorial Africa, are used to formulate the rainfall attenuation models used in this study. Thereafter, the formulated attenuation models are used to convert rainfall rate time series data to predicted rainfall attenuation time series. In addition, both the ITU-R model and the Synthetic Storm Techniques are applied for comparison with the above rainfall attenuation models alongside experimental measurements over the 12.6 GHz satellite TV link from Intelsat-20 (IS-20) located at 68.5˚E on the azimuth angle of 57.5˚ with respect to Durban.
A datacube parametrization-based model for bistatic scattering coefficient estimation, and pattern reconstruction is presented in this work for electromagnetic wave scattering from rough surfaces with low to high subsurface dielectric constants. A datacube of bistatic scattering coefficients is simulated using the Stabilized Extended Boundary Condition Method (SEBCM). The polarization-combined bistatic scattering patterns of the datacube are fit with elliptical (or circular) contours that are parameterized across magnitude level, center location, and major axis length in normalized wavenumber space. These parameters depend on the surface roughness, dielectric contrast, as well as the angle of wave incidence. The polarimetric bistatic scattering patterns can be reconstructed through fast interpolation over the contours and projection onto the polarization unit vectors. Good agreement is achieved between the reconstructed bistatic scattering patterns compared with the original ones in the input datacube. Though not physics-based, this datacube parametrization-based model allows quick estimation and construction of the polarimetric bistatic scattering coefficients and patterns. The model development approach can also be adopted to parametrize datacubes from simulations with other configurations or targets, e.g., surface with different correlation functions, multilayer surfaces, surface covered with vegetation, etc.
In this paper, the exerted electric and geomagnetic forces on the electrified hydrometeors in thunderclouds are compared. The parameters of geomagnetic field are acquired from International Geomagnetic Reference Field (IGRF) model. First, the calculations showed that the magnitude of the electricforce exerted on a charged hydrometeor dominates the magnitude of the geomagnetic force in troposphere. These results revealed the significance of electricforce in the formation of thunderclouds' charge structure. Moreover, as the electric field increases in thunderstorm conditions, (regarding the dependence of the induction mechanism of cloud electrification to the intensity of the electric field) the increased electric field strengthens the induction mechanism of cloud electrification and influences the electrical properties of thunderstorm. Second, using satellite-based/ground-based data and reports, an inverse relation has been revealed between the totalgeomagnetic field and the mean annual lightningactivity in most of the hot spots on the Earth. Moreover, a comparison between the global annual thunder days' map and the map of global total geomagnetic field showed an inverse relation between these two maps. Furthermore, regarding the horizontal and vertical correlation coefficient matrices of the geomagnetic field and the global mean annual lightning activity (in the global tropics and subtropics), approximately in latitudes and longitudes with high lightning density, the reverse relation between the average annual lightning activity and the total geomagnetic field is stronger.
A novel method for the estimation of a forest vertical structure using Polarimetric Synthetic Aperture Radar (PolSAR) data only without the need to interferometry data is proposed in the present paper. Electromagnetic (EM) simulation is used to develop the proposed method, where the SAR pulse is simulated as a plane wave incident in the direction of the side looking angle of the SAR. For this purpose, the forest canopy layer is modeled as clouds of randomly oriented thin straight dipoles which are randomly distributed within an inclined prism volume, whereas the forest soil surface is modeled as a random rough surface. This prism has a horizontal rectangular base and parallelogram sides parallel to the direction of the incident plane wave (side looking angle of the SAR). The proposed method aims to estimate the average height of the canopy layer above the soil surface, the canopy layer thickness and the roughness of the forest ground surface. The proposed method is based on the Radar Vegetation Index (RVI) and the normalized Radar Cross Section (RCS) calculated from the PolSAR data and their relevance to the parameters of the forest vertical structure. Some examples are presented to demonstrate the capability of the proposed method using some PolSAR images obtained through EM simulation of the scattering from forest regions and by applying the theorem of SAR target composition with the Multiple Component Scattering Model (MCSM). The phase differences between the components of scattering obtained from the solution of the SAR target decomposition problem are used in the estimation process. The accuracy of the proposed method is assessed by calculating the percentage error of the estimated vertical structure and ground roughness for each resolution cell of the simulated forest region. It is shown that the percentage errors of the estimated parameters are very low, which reflects the accuracy and efficiency of the proposed method.
Resonance interaction between the electromagnetic radiation from a dipole antenna and a cylindrical density depletion aligned with an external static magnetic field in a magnetoplasma is studied in the case where the antenna is located outside such a density irregularity. A distinctive feature of the presented analysis is using a realistic distribution of the antenna current instead of the assumed one. It is shown that such an antenna can excite plasmon resonances of the density depletion, along with the resonance at the plasma frequency of the outer region. In addition, previously unrevealed resonances of the total field, which are related to excitation of complex modes of the cylindrical density depletion, are discussed. The results obtained can be helpful in understanding the basic properties of resonance interaction of the antenna fields with cylindrical density irregularities in a magnetoplasma and planning the related experiments in the ionospheric and laboratory plasmas.
There is an increasing demand in real-time imagery applications such as rapid response to disaster rescue and security screening to name a few. The throughput of a radar imaging system is mainly controlled by two parameters; data acquisition time and signal processing time. To minimize the data acquisition time, various methods are being tried and tested by researchers worldwide. Among them is the computational imaging (CI) technique, which relies on using coded apertures to encode the radar back-scattered measurements onto a set of spatio-temporarily incoherent radiation patterns. Such a CI-based imaging approach eliminates the requirement for a raster scan and can substantially simplify the physical hardware architecture. Equally important is the processing time needed to retrieve the scene information from the coded back-scattered measurements. In CI, the simplification in the hardware layer comes at the cost of increased complexity in the signal processing layer due to the indirect mapping and compression of the scene information through the spatio-temporally incoherent transfer function of the coded apertures. To address this particular challenge, this paper presents a hardware-based solution for CI signal processing using a Field Programmable Gate Array (an Xilinx Virtex-7 (XC7VX485T) FPGA chip) architecture. In particular, the proposed method consists of calculating the CI sensing matrix using the FPGA chip and storing it on the FPGA platform for image reconstruction. For the adjoint operation, the calculated sensing matrix is applied on the measured back-scattered waves from the target object. We demonstrate that the FPGA based calculation can reach 21.9 times faster speed than conventional brute-force solutions.
As computing power and algorithmic advances have evolved rapidly in the recent past, it is now feasible to solve complex electromagnetic (EM) problems involving scattering, radar cross section, antenna design, microwave circuit design, artificial EM materials etc., using full-wave numerical methods. Several general-purpose commercial software packages are routinely used in industry in all these domains for EM analysis or design. However, the task of processing large sets of data output from these design studies and analyses is generally beyond the realm of commercial software packages, and the designer spends many hours writing problem-specific computer programs to extract the desired performance parameters. Some examples where auxiliary processing is needed for the extraction of EM parameters of interest include determination of coupling coefficients or the unloaded quality factor of a dielectric resonator, de-embedding feed lines from antenna currents, removal of discontinuity effects, and the extraction of equivalent circuit models. The same considerations as simulated data apply to the parametric analysis of measured data in the presence of noise. This paper presents a versatile data-driven spectral model derived from a state-space system representation of the computed or measured EM fields, from which all the parameters of interest can be extracted. An attractive feature of the state space method is its ability to identify a small number of the system transfer function poles uniquely associated with a specific scattering mechanism or modal response, thereby enabling its isolation from the total response for detailed study. For example, using SSM, specular reflection and creeping waves on a smooth convex surface can be analyzed and the diffraction at the edges can be isolated from the composite RCS of a large body. The desired field parameter is extracted or estimated from synthetic or measured data using a linear system of a relatively small model order that characterizes the specific modal response of interest. Illustrative examples will be presented to demonstrate the usefulness of the proposed approach for parametric extraction.
Direct Antenna Modulation (DAM) is explored recently in many wireless communication systems. In this paper, we explore the modulation process of electromagnetic signals in the antenna circuit design directly. The proposed antenna consists of two non-concentric elliptical patches for broadband applications to suit the spread spectrum applications. To perform a Differential Phase Shift Keying (DPSK) modulation, two identical antennas are fed by a two-branch microstrip line with a phase shift. Utilizing Computer Simulation Technology of Microwave Studio (CSTMWS) based on Finite Integral Technique (FIT), an optimization based-on numerical analysis is adopted for designing the transmission line configuration at the desired frequency bands. The other significant aspect that has been achieved in this research is reducing the patch size to be suitable for wearable devices. Therefore, a cylindrical substrate is utilized for bending the proposed antenna structure. The proposed antenna design shows a gain of 4.73 dBi and 2.5 dBi for the planar and folded antenna profile respectively. Two high-speed Positive Intrinsic Negative (PIN) diodes as switching elements of the RF signal are inserted between the identical antenna elements through a transmission line. Switch 1 (SW1) and switch 2 (SW2) are used to control the phase shift between the antenna elements by changing the switching state from (OFF-ON) and vice versa. The designed antenna is further investigated to realize the effects of radiation leakage from the antenna elements on the human body in the context of wearable applications. This study is conducted to the antenna performance when it is bent on a cylinder and compared to the flat case on four human body regions: arm, head, thigh, and chest. The proposed antenna based on PIN diodes is fabricated, measured, and tested. Using a 3D axis field strength meter, the proposed antenna system field strength is measured for different conditions at various locations of the human body. Finally, an excellent agreement is found between the obtained numerical results and measurements.
A novel dual-band conical-helix/monopole antenna is proposed to operate as an on-body central antenna for Wireless Body Area Network (WBAN). The proposed antenna communicates in three ways: (i) off-body communication through its end-fire radiation with the ceil-mounted WiMax antenna at 5.8 GHz, (ii) on-body communication through its broadside radiation with the on-skin biosensor antennasat 3.0 GHz, and (iii) in-body communication with the in-body (implanted) biosensor antennas at 3.0 GHz. The characteristics of the proposed antenna are investigated through electromagnetic simulation and experimental measurements where a prototype of this antenna is fabricated for this purpose. The antenna is matched with 50 Ω coaxial feeder over the dual frequency bands, mounted on a copper circular disc, and covered with a very thin dielectric radom for mechanical protection. Such an antenna covered by the radom is shaped like a hemispherical button that can be attached to patient clothes and, hence, it can be considered as a wearable antenna. The radiation patterns obtained by experimental measurements show good agreement with those obtained by the CST® simulator and are shown to be appropriate for communication with the ceil-mounted WiMAX antenna and the biosensor antennas at 5.8 GHz and 3.0 GHz, respectively. The distribution of the microwave power density near the body surface is evaluated by simulation and experimental measurements to ensure the realization of the electromagnetic exposure safety limits. The Specific Absorption Rate (SAR) distribution inside the human tissues of concern is evaluated showing a safe level of electromagnetic exposure. Quantitative assessment of the WBAN communication system performance is achieved when the proposed antenna is employed as an on-body central antenna for the WBAN. Thanks to the optimized design of the proposed antenna the Bit-Error-Rate (BER) is shown to be very low even when the input power fed to the antenna is only 1 mW.
Mathematical expressions for the components of the magnetic field produced by a conically-shaped current sheet and by a tightly-wound conical coil are presented. The conical current sheet forms the frustum of a cone. In the limit as the top radius of the frustum approaches the bottom radius, a cylindrical current sheet is formed. Mathematical expressions for the magnetic field produced by a cylindrical current sheet are then compared to known and published results.