Debdeep Sarkar

This article presents a novel finite-difference time-domain (FDTD) framework for the analysis and design of multiport nonreciprocal (NR) circuits based on spatiotemporally modulated (STM) resonators. Traditional frequency-domain methods, such as harmonic balance (HB) simulations, face significant challenges in modeling time-modulated elements due to harmonic truncation. In contrast, the proposed time-domain (TD) approach inherently supports arbitrary waveforms and adaptive components, enabling accurate and efficient analysis of space–time-modulated and tunable RF systems. Specifically, the proposed FDTD-based analysis framework introduces a generalized update scheme for interconnected transmission line (TL) junctions and lumped-element resonators using modified Telegrapher’s equations and custom boundary conditions (BCs), allowing to support complex circuit topologies, including NR-bandpass filters (NR-BPFs), NR-filtering power dividers (NR-FPDs), and NR-filtering couplers, and extract their key performance metrics such as TD waveforms, S-parameters, and constellation diagrams. Real-time simulations demonstrate the method’s ability to model direction-dependent behavior, frequency conversion, and broadband signal propagation with high accuracy. The method is exhaustively compared and validated with simulations and measurements through the manufacturing and testing of three experimental prototypes at the UHF band. These include: 1) a tunable 3rd-order in-line NR-BPF; 2) a tunable 4th-order single-band NR-BPF; and 3) a 3rd-order NR-FPD.

This paper presents a unified analytical, numerical, and experimental framework for wave propagation in lossy time-varying transmission lines (TVTLs) subject to arbitrary space-time modulation. Starting from generalized Telegrapher's equations, new formulations are derived that explicitly incorporate conductor and dielectric losses as well as temporal derivatives of L(z, t) and C(z, t). These formulations are numerically solved using MATLAB's ode113, validating fundamental effects such as frequency translation and non-reciprocity while exposing solver instabilities under strong modulation. To overcome these limitations, a finite-difference time-domain (FDTD) framework is developed with one-way excitation and distortionless-line PML boundaries, enabling stable simulation of lossy TVTLs under arbitrary modulation profiles. Comprehensive parametric studies reveal that losses impose uniform attenuation without suppressing frequency conversion, maximum non-reciprocity occurs when spatial and temporal modulation frequencies are matched, and optimal modulation indices maximize performance. Finally, a two-pole TVTL-based non-reciprocal bandpass filter (NR-BPF) prototype is fabricated and experimentally characterized. Measured results confirm harmonic generation, spectral redistribution, and modulation-dependent non-reciprocity, in excellent agreement with the analytical formulations and FDTD simulations. Together, these results establish the first complete modelingto-hardware validation platform for lossy TVTLs, providing both physical insight and design strategies for non-reciprocal microwave devices and spectrum-engineering applications.

In this letter, we propose the design of a single-feed low-profile digitally coded metasurface antenna for generation of quad-beam radiation. The design procedure involves initial synthesis of the desired reflection phase profile using far-field plane-wave illumination, followed by application of a correction factor which considers presence of near-field excitation source generating spherical waves. The profile of the proposed quad-beam metasurface antenna is significantly minimized using a coax-fed printed dipole as exciter, instead of the high-gain Helical or Horn antennas placed at a distance for feeding conventional reflectarrays. Furthermore, use of 1-bit coding scheme considerably reduces the design complexity of the metasurface. Full-wave EM simulations based on CST-MWS and experimental measurements on fabricated prototype confirm that the dipole-excited metasurface with aperture size $11\lambda _{0} \times 11\lambda _{0}$ (where $\lambda _{0}$ is the free-space wavelength at 10 GHz) generates four symmetrical beams in the $\theta =45^{o}$ plane with peak gain of 11.7 dBi. The proposed low-profile quad-beam metasurface antenna has potential to be used in future 6G communications as well as X-band Radar applications.

There exist inevitable deviations in the cosmic microwave background spectrum from that of a blackbody. The additive distortions from photons emitted over the epoch of recombination are one such deviation. We present a novel scalable broadband antenna design motivated by detecting these spectral distortions in the 2.5–4 GHz band. This antenna is capable of maintaining the spectral quality of the measured sky spectrum to 1 part in 10³ over the full band and outperforms conventional broadband antennas by at least an order of magnitude. While this is not sufficient to make a direct detection of the cosmological recombination radiation, it meets the baseline design criteria for an experiment that can make an absolute measurement of the sky spectrum at the millikelvin level, thereby making it capable of improving foreground models to global signal detection experiments (including those from cosmic dawn and the epoch of reionization) and addressing the problem of the excess radio background at 3.3 GHz reported by the ARCADE2 experiment.