arXiv subjects

Georgios Kyriakou

Publications and source records attributed to Georgios Kyriakou.

6 recordsLinked to original sources

arXiv2026

Non-uniform Antenna Loading Effect on Embedded Element Patterns and Application to Fault Detection

An iterative technique is presented to calculate the Embedded Element Pattern (EEP) transformation from a set of patterns computed for a uniform antenna port loading (scaled identity matrix) to a set of those computed for a non-uniform one (arbitrary diagonal matrix). Inverting this transformation to derive the non-uniform impedances of the arbitrary load leads to a new, efficient algorithm which disposes of the redundancy of the trivial use of all \( N \) EEPs at the faulty array condition, requiring only one such EEP for numerically stable impedance fault calculation. As the EEPs are envisioned to be obtained primarily through measurement, our method is also tested with the inclusion of various noise components and its convergence is evaluated, leading to suggested measurement apparatus SNR and fading levels, as well as the optimal choice of reference antenna to minimize the estimation error.

astro-ph.IM↗

arXiv2026

Precise Measurement of the Absolute Sky Brightness at 60-350 MHz

Accurate knowledge of the low-frequency radio sky is essential for modelling foregrounds in experiments targeting cosmic dawn and the epoch of reionization. Measurements below 1 GHz are also needed to understand the Galactic cosmic ray electron spectrum, constrain nanojansky radio source populations and dark matter models, investigate the origins of the diffuse radio background, and improve calibration of long-wavelength radio telescopes. Here we present a precision measurement of the radio sky brightness over 60-350 MHz using a new receiver architecture that self-calibrates its noise contribution and bandpass in situ while connected to an antenna. Our measurement used a log-periodic SKALA4.1 antenna on a 40 m diameter SKA-Low station ground mesh in the Southern Hemisphere to observe approximately half of the celestial sphere. We show that current all-sky maps and the 2016 Global Sky Model (GSM2016) require substantial corrections over this frequency range. GSM2016 must be scaled upward by a factor of 1.2 over 60-200 MHz, increasing to 1.5 at 350 MHz. A smaller offset correction of approximately 100 K is also required below 100 MHz. The revised scaling significantly increases previously inferred excess radio background, motivating review of faint source populations and dark-matter decay models. Sky models scaled to our measurements can set the absolute flux-density scale for SKA-Low and other low-frequency radio telescopes. They can also improve foreground characterisation for cosmic dawn and epoch-of-reionization experiments.

astro-ph.CO↗

arXiv2026

Dichroic Dual-Angle Refractor: Multi-Cell Huygens' Metasurface-Based Circuit Approximation

A dichroic dual-angle refractor based on a 2D Huygens' metasurface model is treated by means of resonant-circuit approximations of multiple cells. The whole parameter space of refraction angles is tested for the analytic approximations involved, whereas the realised refraction guides the choice of optimal such angles. Our results indicate the possibility of sythesising a rectangular array of a number of cells which eventually depends on the low-frequency band diffraction limit. Aberrations from the ideal performance are also examined and attributed to certain approximation caveats.

physics.optics↗

arXiv2026

Design of a Dichroic Transmissive Huygens' Metasurface Unit-Cell Presenting Refraction Angle Duality

A purely transmissive Huygens' metasurface model under plane-wave illumination is used to derive circuit parameters describing a constituent unit cell, such that diverse refraction angles are attained at two distinct frequency bands. Various levels of accuracy of the circuit description approaching the analytical are possible by constraining certain numbers of parameters. This theoretical study is then tested by calculating the exact formulas of the two representations for the various strategies proposed. By using simulations of a candidate unit-cell, we then examine whether such circuit parameters correspond to rudimentary versions of the geometry of a so-called parallel 'dogbone' structure. A device of this type is intended as dual-band (dichroic), dual-angle beam refractor diverting an incoming beam at different directions in two different bands without reflections.

physics.optics↗

arXiv2024

Spectral Smoothness of Ground Plane Backed Log-Periodic Dipole Antennas for Radioastronomical Applications

The spectral smoothness properties of the low-frequency array of the Square Kilometer Array (SKA), namely SKA-Low, are an important issue for its scientific objectives to be attainable. A large array of 256 log-periodic dipole antennas, installed on top of a 42~m circular ground plane, will work as an SKA-Low station in the frequency range 50-350 MHz. In this article, the ground plane induced effects are examined in terms of antenna beam spectral characteristics, while different antenna placements are considered. Results are produced both at isolated antenna and at array level in the band 50-100 MHz, by employing an approximate method for the speeding-up of array simulations. We attempt to distinguish the ground plane effect from that of mutual coupling among antennas, which appears to be more severe at specific frequencies, using 2 figures of merit. The Discrete Fourier Transform (DFT) components of gain pattern ratios identify the fundamental spatial components of the ripple, while the Envelope Correlation Coefficient quantifies the penalty to considering an infinite ground plane.

astro-ph.IM↗

arXiv2023

Characteristic Modes Analysis of Mutually Coupled Log-Periodic Dipole Antennas

Characteristic Modes Analysis (CMA) is a widely used method with recent progress in multi-antenna systems. We employ this method to characterize the mutual coupling phenomenon between two SKALA4.1 antennas, the low-frequency array elements of the future radiotelescope Square Kilometer Array (SKA-Low). The CMA accuracy is first validated at the lowest frequency range of interest with respect to a standard Method of Moments (MoM) solution by decomposing the single antenna into its characteristic modes. We then examine critical frequencies of a two-antenna system in modal decomposition, and characterize those responsible for the radiated electric field spurious spectral features owing to the mutual coupling. We connect these modes to first-order coupling of single antenna CMA modes, by using the eigenvalue data of both single- and two- antenna simulations.

astro-ph.IM↗