Browsing by Author "Karthik, R."

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Dual Band Circularly Polarized Slot Antenna Loaded with Asymmetric Cross Strips
(2018) Puneeth, Kumar, T.R.; Karthik, R.; Krishnamoorthy, K.
In this paper, a dual band circularly polarized (CP) slot antenna loaded with asymmetric cross strips is proposed. It consists of a corner perturbed slot and two asymmetric strips oriented diagonally with the slot. The slot antenna is excited by 50\Omega microstrip line. The proposed antenna resonates at 2.7 GHz and 3.2 GHz. Two orthogonal degenerate modes produced by the perturbed slot antenna gives CP in lower band and the combination of slot and strip mode gives CP in upper band. The proposed antenna is fabricated and experimentally verified with axial ratio (AR) bandwidth of 13.27% and 13.31% in lower and upper band respectively. The peak gain of 4.68 dB at lower band and 4.81 dB at upper band is achieved. The simulated and measured results are in well agreement and it is good candidate for WiMax and WLAN applications. � 2018 IEEE.
Euclidean thermodynamics and Lyapunov exponents of Einstein–Power–Yang–Mills AdS black holes
(Springer Nature, 2025) Karthik, R.; Dillirajan, D.; Ajith, K.M.; Hegde, K.; Punacha, S.; Naveena Kumara, A.N.
We study the thermodynamics of Einstein–Power–Yang–Mills AdS black holes via the Euclidean path integral method, incorporating appropriate boundary and counterterms. By analyzing unstable timelike and null circular geodesics, we demonstrate that their Lyapunov exponents reflect the thermodynamic phase structure obtained from the Euclidean action. Specifically, the small-large black hole phase transition, analogous to a van der Waals fluid, is signaled by a discontinuity in the Lyapunov exponent. Treating this discontinuity as an order parameter, we observe a universal critical exponent of 1/2, consistent with mean-field theory. These results extend previous insights from black hole spacetimes with Abelian charges to scenarios involving nonlinear, non-Abelian gauge fields, highlighting the interplay between black hole thermodynamics and chaotic dynamics. © The Author(s) 2025.
Perturbations of black holes surrounded by anisotropic matter field
(American Physical Society, 2025) Sagar J, C.; Karthik, R.; Hegde, K.; Ajith, K.M.; Punacha, S.; Naveena Kumara, A.N.
Our research aims to probe the anisotropic matter field around black holes using black hole perturbation theory. Black holes in the Universe are usually surrounded by matter or fields, and it is important to study the perturbation and the characteristic modes of a black hole that coexists with such a matter field. In this study, we focus on a family of black hole solutions to Einstein's equations that extend the Reissner-Nordström spacetime to include an anisotropic matter field. In addition to mass and charge, this type of black hole possesses additional hair due to the negative radial pressure of the anisotropic matter. We investigate the perturbations of the massless scalar and electromagnetic fields and calculate the quasinormal modes (QNMs). We also study the critical orbits around the black hole and their properties to investigate the connection between the eikonal QNMs, black hole shadow radius, and Lyapunov exponent. Additionally, we analyze the graybody factors and scattering coefficients using the perturbation results. Our findings indicate that the presence of anisotropic matter fields leads to a splitting in the QNM frequencies compared to the Schwarzschild case. This splitting feature is also reflected in the shadow radius, Lyapunov exponent, and graybody factors. © 2025 American Physical Society.