Debidas Kundu

Reciprocity between a transmitter and receiver is a foundational requirement in wireless communications. A few recent works have suggested that reciprocity is broken under reflection by reconfigurable intelligent surfaces (RIS) when the reflection phase becomes incident angle dependent. In this work, we rigorously show that these claims are based on the use of idealized reflection coefficients that ignore mutual coupling between heterogeneous unit cells, surface-truncation effects, and structural scattering contributions from the RIS. Full-wave electromagnetic simulations of transmit/receive antennas and a finite-size RIS implemented via a particular unit cell design are performed to quantitatively demonstrate that reciprocity holds even in the presence of incident-angle dependent reflection phases. To show this, we calculate two-port antenna scattering parameters and evaluate the electromagnetic reciprocity integral to support our claims.

A metasurface reflector unit cell is proposed for achieving near-complete-range of complex reflectance (i.e., magnitude and phase) for versatile beamforming capabilities. The unit cell is based on tunable coplanar coupled resonators: a split ring resonator (SRR) with a lumped capacitor and a lumped resistor, and a dipole ring resonator (DRR) with another lumped capacitor. The DRR inserted inside the SRR creates a coupled resonance configuration which results in an enhanced complex reflectance range at the desired frequency. To provide a physical insight and explain the operation principle of the structure, the response of the unit cell is modeled as a coupled Lorentz oscillator via the effective surface susceptibilities, where a unique plasma frequency, damping coefficient, and resonant frequency can be attributed to each resonator. The proposed unit cell is demonstrated in an array configuration for linear-polarized beamforming, where full-wave simulations are used to demonstrate beam-steering, gain control, side-lobe level control and dual- and triple-beam generation, as illustrative examples. Finally experimental demonstration is performed to validate the full-wave results and obtain in-depth electrical characterization of the reflectors.

A novel metasurface reflector unit cell based on two coupled resonators is proposed and demonstrated for real-time reconfigurable beamforming in the X-band (8-12 GHz). The unit cell is composed of a split-ring resonator (SRR) with tunable capacitance and a dipole-ring resonator (DRR) with tunable resistance, whose collective variations allow for control over the complex reflectance at a desired frequency. To gain physical insight into its working mechanism, the proposed unit cell is modeled as a coupled Lorentz oscillator using the surface susceptibility description of the unit cell. Thereafter, a metasurface reflector based on the proposed unit cell is demonstrated in full-wave simulations to achieve various beamforming examples, such as beam-steering, side-lobe level control, beam-steering with amplitude control, and multi-beam patterns, from a single normally-incident plane wave excitation. Three metasurface reflectors are fabricated to experimentally demonstrate the proposed concept; the first is based on an SRR with a varactor diode, the second is based on a DRR with a PIN diode, and the third is based on the proposed coupled SRR-DRR configuration with both varactor and PIN diodes, for simultaneous magnitude and phase control. The metasurface reflector based on the coupled resonator unit cell is experimentally demonstrated to achieve versatile beam transformations including beam-steering with amplitude control and multi-beam patterns.

This work presents a dual-polarized broadband switchable rasorber/absorber for applications requiring radar cross-section reduction. The rasorber/absorber is designed in a low-profile configuration using frequency selective surfaces (FSSs) on the both sides of two dielectric layers separated by an air gap. The top layer consists of a lossy square loop FSS loaded with lumped resistors and a bandpass FSS realized by four miniaturized slotted square patches inside the loop. The bottom layer has a square-slot-shaped lossless bandpass FSS. A partial set of initial geometry parameters is synthesized from the equivalent circuit model using particle swarm optimization to ensure rapid optimization of the geometry with good performance. Both the lossy and the lossless FSSs are loaded with p-i-n diodes to enable switching between rasorber and absorber modes. The design exhibits a lower absorption bandwidth from 4.2 to 7.2 GHz [fractional bandwidth (FBW) of 52.63%], an upper absorption bandwidth from 9.69 to 14.1 GHz (FBW of 37.07%), and a transmission band at 8.5 GHz with an insertion loss of 0.38 dB in the rasorber mode. In the absorber mode, the design exhibits a continuous absorption band from 4.6 to 11.1 GHz (FBW of 83%). The simulated results are validated through experimental measurements with good agreement.

A low-radar cross section (RCS) circularly polarized (CP) reflectarray antenna with a linearly polarized (LP) feed is designed. The out-of-band RCS of the reflectarray for both the TE and TM polarized incident waves is reduced by replacing its ground with an absorptive frequency selective reflector (AFSR). A systematic methodology based on synthesis of the equivalent circuit model (ECM) is presented to expedite the design of the AFSR. The unit phasing element of the reflectarray is designed in such a way that it exhibits minimum coupling between the two orthogonal reflected LP-field components. Moreover, during the reflectarray construction, a novel phase synthesis is used which takes into account possible lowest of the existing mutual coupling while providing quadrature phase difference between the orthogonal LP components with minimum phase error. The proposed design is fabricated using 14×14 phasing elements to experimentally validate the numerically simulated results. The performance of the reflectarray is compared with a conventional reflectarray without the AFSR. It is observed that when the reflectarray is integrated with the AFSR, its gain is reduced only by 1.16 dB. However, its out-of-band RCS is reduced significantly. The 8-dB RCS reduction bandwidth can be observed from 3.55-7.2 GHz (67.9%) and from 11.85-14.21 GHz (18.11%). Moreover, it provides peak co-pol CP gain of 21.62 dBi, aperture efficiency of 41%, and 1-dB gain-bandwidth from 8.68-8.95 GHz, respectively.

A broadband linearly polarized reflectarray antenna is designed by modifying a typically narrowband square patch unit element with cross-loop slot (CLS). Due to the introduction of the CLS, the unit patch element becomes multi-resonant and miniaturized. Besides, the outer length and width of the CLS are one-to-one related to the length of the patch. As a result, more than 480° phase range with quite good linearity is achieved. In addition, the phase ranges are almost parallel to each other from 22.5–26.5 GHz, which signifies its broadband property. Using such $36\times 36$ CLS patches in a square aperture of 180 mm $\times180$ mm size, an offset-fed reflectarray is constructed for an off-broadside radiation pattern. A prototype has been fabricated to experimentally validate the numerical results obtained using CST full-wave simulations. The measurement results exhibit a broad 1-dB gain bandwidth of 17.6% (22.44–26.78 GHz) and a peak gain of 32.2 dBi with an aperture efficiency of 60.6%. Moreover, both the cross-pol and side-lobe levels (SLLs) remain more than 40 dB and 21 dB, respectively, below the peak co-pol level throughout the 1-dB gain bandwidth. The proposed reflectarray can be used as a CubeSat antenna for K-band inter-satellite links (ISL).