arXiv subjects

Morteza Shahpari

Publications and source records attributed to Morteza Shahpari.

6 recordsLinked to original sources

arXiv2018

Fundamental limitations for antenna radiation efficiency

Small volume, finite conductivity and high frequencies are major imperatives in the design of communications infrastructure. The radiation efficiency $η_r$ impacts on the optimal gain, quality factor, and bandwidth. The current efficiency limit applies to structures confined to a radian sphere $ka$ ($k$ is the wave number, $a$ is the radius). Here we present new absolute limits to $η_r$ for arbitrary antenna shapes based on $k^2S$ where $S$ is the conductor surface area. For a dipole with an electrical length of $10^{-5}$ our result is four orders of magnitude closer to the analytical solution when compared with previous bounds on the efficiency. The improved bound on $η_r$ is more accurate, more general, and easier to calculate than other limits. The efficiency of an antenna cannot be larger than the case where the surface of the antenna is 'peeled' off and assembled into a planar sheet with area $S$, and a uniform current is excited along the surface of this sheet.

physics.class-ph↗

arXiv2015

The impact of reduced conductivity on the performance of wire antennas

Low cost methods of antenna production primarily aim to reduce the cost of metalization. This might lead to a reduction in conductivity. A systematic study on the impact of conductivity is presented. The efficiency, gain and bandwidth of cylindrical wire meander line, dipole, and Yagi-Uda antennas were compared for materials with conductivities in the range $\mathrm{10^3}$ to $\mathrm{10^9}$ S/m. In this range, the absorption efficiency of both the dipole and meander line changed little, however the conductivity significantly impacts on radiation efficiency and the absorption cross section of the antennas. The extinction cross section of the dipole and meander line antennas (antennas that Thevenin equivalent circuit is applicable) also vary with radiation efficiency. From the point of radiation efficiency, the dipole antenna performance is most robust under decreasing conductivity. Antennas studied in this study were fabricated with brass and graphite. Radiation efficiency of the antennas were measured by improved Wheeler cap (IWC) method. Measurement results showed a reasonable agreement with simulations. We also measured the extinction cross section of the six fabricated prototypes.

physics.gen-ph↗

arXiv2015

Point Contacts in Modeling Conducting 2D Planar Structures

Use of an optimization algorithm to improve performance of antennas and electromagnetic structures usually ends up in planar unusual shapes. Using rectangular conducting elements the proposed structures sometimes have connections with only one single point in common between two neighboring areas. The single point connections (point crossing) can affect the electromagnetic performance of the structure. In this letter, we illustrate the influence of point crossing on dipole and loop antennas using MoM, FDTD, and FEM solvers. Current distribution, radiation pattern, and impedance properties for different junctions are different. These solvers do not agree in the modeling of the point crossing junctions which is a warning about uncertainty in using such junctions. However, solvers agree that a negligible change in the junction would significantly change the antenna performance. We propose that one should consider both bridging and chamfering of the conflicting cells to find optimized structures. This reduces the simulation time by 40% using FDTD modeling, however no significant reduction is obtained using the MoM and FEM methods.

physics.optics↗

arXiv2014

Polarizablity of 2D and 3D conducting objects using method of moments

Fundamental antenna limits of the gain-bandwidth product are derived from polarizability calculations. This electrostatic technique has significant value in many antenna evaluations. Polarizability is not available in closed form for most antenna shapes and no commercial electromagnetic packages have this facility. Numerical computation of the polarizability for arbitrary conducting bodies was undertaken using an unstructured triangular mesh over the surface of 2D and 3D objects. Numerical results compare favourably with analytical solutions and can be implemented efficiently for large structures of arbitrary shape.

physics.optics↗

arXiv2013

An investigation into the Gustafsson limit for small planar antennas using optimisation

The fundamental limit for small antennas provides a guide to the effectiveness of designs. Gustafsson et al, Yaghjian et al, and Mohammadpour-Aghdam et al independently deduced a variation of the Chu-Harrington limit for planar antennas in different forms. Using a multi-parameter optimisation technique based on the ant colony algorithm, planar, meander dipole antenna designs were selected on the basis of lowest resonant frequency and maximum radiation efficiency. The optimal antenna designs across the spectrum from 570 to 1750 MHz occupying an area of $56mm \times 25mm$ were compared with these limits calculated using the polarizability tensor. The results were compared with Sievenpiper's comparison of published planar antenna properties. The optimised antennas have greater than 90% polarizability compared to the containing conductive box in the range $0.3<ka<1.1$, so verifying the optimisation algorithm. The generalized absorption efficiency of the small meander line antennas is less than 50%, and results are the same for both PEC and copper designs.

physics.optics↗

arXiv2012

Hat Feed Antenna Capable of Sum and Difference Pattern Synthesis

Potentially Separate waves could propagate independently with arbitrary amplitude and phase, if one longitudinal septum is inserted in the H-Plane of the circular waveguide. The presented paper has implemented this idea in order to excite hat feed antenna simultaneously with two in-phase and out of phase signals and leads to sum and difference patterns.

physics.optics↗