BITS Faculty Publications
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Item Parametric insights into square loop-based artificial magnetic conductors(IEEE, 2025-03) Singhal, RahulThis study investigates the band-stop frequency in both Transverse Electric (TE) and Transverse Magnetic (TM) modes by examining a square loop Artificial Magnetic Conductor (AMC) with varying inner edge widths, while keeping the distance between unit cells and their dimensions constant. The analysis focuses on the reflection and transmission characteristics to determine the band-stop frequency, bandwidth, and phase attributes. These insights are critical for the antenna design community, enabling the precise adjustment of the inner edge width to meet specific application requirements. The findings facilitate the selection of appropriate AMC unit cell dimensions, ensuring optimal performance in various electromagnetic environments. This research provides a valuable reference for engineers and designers in the field of antenna technology, aiding in the development of more efficient and tailored electromagnetic solutions.Item Improved boresight gain in slotted trapezoidal ground UWB monopole with non-concentric circular loops(IEEE, 2025-03) Singhal, RahulThe design and performance analysis of a microstrip-fed circular ring-based planar monopole Ultra Wide-Band (UWB) antennas are investigated in this paper. Three non-concentric circular rings are inserted within the microstrip-fed circular ring monopole antenna backed by reduced ground planes of distinct shapes such as rectangular ground (RG), elliptical ground (EG) and trapezium ground (TG) respectively. The non-concentric rings are scaled optimally, for a comprehensive analysis of their impact on impedance bandwidth and antenna gain in boresight direction. By integrating such structures, the study addresses significant challenges associated with traditional UWB monopole antennas, particularly the degradation of radiation performance at higher frequencies. It is found that non-concentric ring based UWB monopole antenna offers UWB impedance band while demonstrating positive gain in boresight direction up to 8 GHz. Thus, the proposed antenna structures exhibit enhanced UWB impedance bandwidth and improved boresight gain, thereby overcoming the limitations of conventional designs.Item Ground size optimization for high boresight gain in offset-fed wide slot antennas(IEEE, 2025-03) Singhal, RahulThis paper presents a microstrip-fed wide rectangular slot antenna that achieves improved boresight gain while functioning in the 6−8.5 GHz frequency band. The radiation and impedance characteristics of the slot antenna for distinct dimensions of ground plane are examined and analyzed. It is observed that optimizing the ground plane size enhances the gain of a wide rectangular slot antenna. This occurs when direct waves from the slot radiator interfere appropriately with the waves that are diffracted from the ground plane edges. The optimized rectangular slot antenna utilizing an FR-4 substrate, with a slot size of 0.31λ0×0.22λ0, has overall dimensions of 1.2λ0×0.77λ0×0.04λ0, where λ0 denotes the free-space wavelength at the midpoint frequency of the operating band. Measurements of the fabricated antenna prototype reveal an impedance bandwidth ranging from 5.96 to 8.54 GHz and a bidirectional radiation pattern. The antenna exhibits an average boresight gain of 5.33 dBi within this range, with a gain variation of 1.35 dBi. The key advantage of the proposed method is the precise selection of the ground plane size, which allows for nearly uniform gain improvement across the entire impedance bandwidth, while maintaining a bidirectional radiation pattern.Item A Terahertz Metasurface Based Refractive Index Sensor(IEEE, 2022) Singhal, RahulThe proposed dual-layer highly coupled patch metasurface terahertz sensor achieves a maximum sensitivity of 0.65 THz/RIU, figure-of-merit of 1.828 RIU-l in the sample refractive-index from 1.6-1.7, with quality factor of 6.3 for the bio-chemical applications.Item Performance Analysis of Traditional Band Pass/Band Stop Frequency Selective Surfaces for Distinct Frequency Domains of Electromagnetic Spectrum(IEEE, 2022) Singhal, RahulConventional square loop (SL) and complementary square loop (CSL) based frequency selective surfaces (FSSs) for distinct frequency range are designed, analyzed and compared in this paper. The SL and CSL type FSSs are designed to demonstrate band stop and band pass characteristics for S-band, Ku-Band, Ka-Band, G-band, Terahertz (THz) and Infrared (IR) frequency range respectively. The influence of dielectric substrate and metallic layer on FSS transmission characteristics are also studied for a purpose to identify the appropriate dielectric substrate and metallic layer to design the FSSs to obtain desired operating frequency, bandwidth of stop band and pass band and signal isolation/transmission. It is found that the signal transmission characteristics of FSSs at THz and IR frequencies deteriorates. Also, low lossy substrate materials are desired to improve the signal transmission and isolation levels at THz and IR frequency domains.Item Planar X-band reflecting surfaces based on rhombic loop pair(IEEE, 2022) Singhal, RahulThis paper presents design and analysis of two band stop FSSs offering wider band rejection along with polarization independence and linear reflection phase characteristics separately. The polarization dependent vertical dual rhombic loop (VDRL) unit cells arranged with periodicity ratio 1:2 as well as horizontal dual rhombic loop (HDRL) with periodicity ratio 2:1 exhibit two separate stop bands for TE and TM incident EM waves. Both FSSs are evolved from orthogonally oriented polarization dependent arrays of rhombic loop pairs those are first printed on both sides of dielectric substrate back-to-back (BTB) to form a BTBDRL FSS and later merged together on same side of dielectric substrate to form merged dual rhombic loop (MDRL) FSS respectively. The transmission responses of both VDRL and HDRL FSSs for orthogonally polarized incident EM waves coincide when they are cascaded back-to-back on both sides of dielectric material. The non-linearities in their reflection phase responses are compensated by merging both the arrays on the same side of dielectric substrate. The array of BTBDRL exhibits 4.15 GHz wide stop band with two transmission nulls at 7.46 GHz and 9.52 GHz respectively while the MDRL FSS provides 4.8 GHz wide stop band centered at 8.2 GHz.Item High-Electron-Mobility Transistor-Inspired Freestanding AlGaN/GaN/AlN Optical Waveguide for High-Pressure Sensing Applications(Wiley, 2023-01) Singhal, RahulHerein, a high-pressure sensor based on freestanding high-electron-mobility transistor (HEMT)-inspired optical waveguide comprising AlGaN/GaN/AlN layers on silicon carbide (SiC) substrate is proposed and studied for harsh environment applications using finite element method (FEM). The working principle of the sensor is based on the change in birefringence due to applied pressure. Further, the transmission spectra for different wavelengths are evaluated. For the best possible outcomes, the dip wavelengths under different values of pressure are tracked and studied for different gallium nitride (GaN) core thicknesses. After optimizing GaN core thickness and AlGaN ratio, the sensor displays a good linear response within the pressure range of 90–150 MPa. The two dip wavelength shifts demonstrate maximum spectral sensitivities of −209.6 and 180 pm MPa−1, respectively, for the pressure range of 90–150 MPa within the communication wavelength regime of 1500–1600 nm. By considering the features such as cost-effectiveness, portability, simple design, and easy detection process, the proposed pressure sensor is optimum for harsh environment measurements and studies.Item Probe-Fed Polygonal Patch UWB Antennas(Intecopen, 2023-03) Singhal, RahulThe chapter deals with the design of probe-fed planar antennas to operate at wider bands and techniques to improve peak or boresight gain using reflectors. The phenomenon of frequency excitation in dual-band, that is, C-band and X-band using the technique of partial removal of the ground plane, is well demonstrated here. The impedance bandwidth achieved by the sample antenna is 285 MHz and 380 MHz, respectively. The reduced ground plane technique is further exploited along with modifications in the shape of the ground plane to cover the entire ultra-wideband (UWB) range in a probe-fed hexagonal monopole antenna. Due to the existence of higher modes and especially when fed with a probe, UWB antennas are only capable of providing mediocre gain at higher frequencies. An approach to increase the probe-fed hexagonal UWB antenna’s peak gain involves the utilization of an appropriate reflector. The antenna is given an artificial magnetic conductor (AMC)-based reflector, which increases the peak gain as well as boresight gain across a band ≤ UWB. Peak and boresight gains of 3.74 dB and 5.5 dB, respectively, are observed with AMC. The equivalent circuit model and simulated impedance results of the sample antennas are validated with the measurement results.Item Performance analysis of frequency selective reflectors based on rectangular loop pair for wideband applications(Elsevier, 2023-07) Singhal, RahulThis paper presents performance analysis of single layered bandstop frequency selective surfaces (FSSs) for C-band and ultra-wideband (UWB) frequency ranges respectively. These FSSs are based on polarization dependent (PD) single rectangular loop pair (RLP). RLP arrays in square and rectangular grids are developed and simulated to evaluate signal rejection and reflection for incident transverse electric (TE) and transverse magnetic (TM) waves. Further, orthogonal patterns are printed back-to-back (BTB) on dielectric substrate to study wave propagation and effect on wave polarization. Non-linearities in the reflection phase of BTB arrays are then mitigated by printing orthogonal FSS arrays on the same side of the dielectric substrate. The equivalent circuit models (ECMs) of these FSSs are also presented to predict their transmission characteristics. Later, two FSSs with wideband and ultra-wideband rejection capability are developed and experiments are carried out. The array of merged RLP arranged in square grid exhibits stopband from 4.5 GHz to 8.4 GHz with polarization independence (PI) and it is angularly stable while merged array of RLP arranged in rectangular grid exhibit ultra-wide stopband from 2.3 GHz to 11.5 GHz but it is not angularly stable. The measured transmission responses of all FSSs proposed here are close to simulated results.Item Electromagnetic Scattering Analysis of Conventional Frequency Selective Surfaces for Terahertz Applications(IEEE, 2023) Singhal, RahulConventional designs of arrays of metallo-dielectric frequency selective surface (FSS) for terahertz (THz) frequency range such as square conducting pads (Pixels), metallic mesh, mesh-loop (ML), square loop (SL) and complementary square loop (CSL) are analyzed and compared through simulations. Pixel FSS demonstrates 0.58 THz wide stop-band and 0.14 THz wide pass-band centered at 2.96 THz and 3.35 THz respectively. By complementing, the pixel array becomes a mesh array that appears as a band-pass FSS from 1.89 THz to 3.45 THz. ML array is formed by removing a square ring from the mesh array and provides a 0.32 THz wide pass-band and 0.3 THz stop-band centered at 0.82 THz and 1.15 THz. After eliminating the outer mesh of ML array, SL array yields a 0.32 THz wide band-stop FSS centered at 1.22 THz. Later, the CSL array is formed when SL array is complemented, and the array demonstrates a 0.6 THz wide pass-band at 1.14 THz. The pixel array provides a high signal rejection capability and the widest −10dB stop-band in the frequency range of 2.58-3.16 THz. This analysis and comparison is significant for reduction of EM coupling or interference in THz based integrated circuits.