Jin-Young Choi

This paper presents the design and realization of a tunable wideband triple-mode bandpass filter (BPF) with two transmission zeros (TZs) using liquid crystal (LC). The proposed filter employs a capacitively coupled open-ended stub-loaded resonator (SLR) to realize the extinct resonant characteristics. In addition, a high-impedance line loaded with a radial stub is introduced to preserve the resonant characteristics and to enable the operation of the LC director under bias voltage, despite the use of open-ended SLRs at both ends simultaneously. Through simulations with respect to the stub length and the dielectric constant of the LC, the proposed filter demonstrates advantages in bandwidth design flexibility and stable frequency-agile tunable response without pole merging or pole isolation. Experimental results show that the center frequency can be tuned from 10.61 GHz to 9.48 GHz with a maximum bias voltage of 30 V, while maintaining a triple-mode response with two TZs. Regardless of the applied voltage, the normalized 3-dB fractional bandwidth remains between 29.11 % and 29.55 %, the peak attenuation level exceeds 24 dB, and the shape factor is less than 1.48, which is the lowest reported value among LC-based BPFs. With its wideband and highly selective response, substantial frequency tuning capability, saturation achieved at moderate bias levels (10–12 V), and consistently low passband insertion loss, the proposed filter demonstrates practical advantages and makes a notable contribution to the integration of reconfigurable RF front-end platforms such as cognitive radio and radar systems.

In this paper, a capacitively coupled-fed reconfigurable wideband bandpass filter (BPF) is proposed based on liquid crystal (LC) technology, which achieved three transmission poles across varying bias voltages (V_B). An open-ended stepped impedance resonator configuration enables multi-mode resonance, offering significantly wider bandwidth compared to uniform-impedance resonators. The fractional bandwidth (FBW) and transmission pole positions are determined by the impedance ratio of the two resonators, allowing the filter to meet specific design requirements. An analytical methodology employing multilayer transmission line formulations and resonant frequency ratios was used to predict the modal stability of transmission poles under dielectric constant variation, which was subsequently validated through simulation. Experimental results show that the center frequency can be adjusted from 10.76 to 9.47 GHz with a maximum V_B of 30 V, achieving a tuning range of 12.71%. The normalized 3 dB FBW exceeds 64.66%, and the return loss remains above 10 dB from 0 to 30 V, offering the widest FBW among the reported LC BPFs without pole merging or mode collapse. The frequency response of the fabricated filter shows good agreement with the simulation results.

This paper introduces an electrically reconfigurable narrowband bandpass filter that exploits liquid-crystal technology. The filter uses two electrically-coupled open-loop resonators to produce two transmission zeros, resulting in a more compact structure than a single open-loop resonator. The spacing between resonators contributes to easy manipulation of the fractional bandwidth (FBW) and return loss. Theoretical calculations of the electrical length using multilayer microstrip line equations are presented. Experimental results demonstrate that the center frequency can be tuned from 9.70 GHz to 10.86 GHz with a maximum bias voltage of 30 V, achieving a tuning range of 11.34%. With applied bias, the maximum FBW reaches 8.10% and the maximum return loss attains 15.97 dB in each biased state.

Liquid crystal (LC) is a valuable material for constructing tunable devices in the sub-Terahertz (sub-THz) spectrum, and to utilize it effectively, it is imperative to accurately measure the permittivity of the LC. Frequency Selective Surfaces (FSS) serve as an effective tool for assessing LC permittivity in the sub-THz frequencies. However, existing research lacks measurements of the loss tangent and exhibits certain limitations and sources of error. In this paper, we propose a multi-loop complementary FSS (CFSS) structure to extract the permittivity of LC at sub-THz. The designed multi-loop CFSS exhibits high sensitivity despite featuring an extremely thin LC cell gap of less than 10 μm, the smallest cell gap to be reported in literature for such applications. The structure can estimate LC permittivity across a broadband range by utilizing multi-resonant characteristics. Moreover, we propose a procedure using an averaged inhomogeneous model, thereby enabling accurate calculation of LC permittivity. The proposed method estimates the parallel and perpendicular permittivity and loss tangent of the LC using the measurement results and comparison with the finite element simulation results. The permittivity extraction procedure is experimentally verified in the D-band (110 - 170 GHz).

We evaluated hexadecyltrimethylammonium bromide (HTAB) for liquid crystals (LCs) in layered ITO cells with various cell gap conditions. HTAB is a surfactant that can self-align vertically on the surface of indium tin oxide (ITO) substrates and induce homeotropic alignment of the LC molecules. For implementing RF devices with HTAB and LCs, we should consider limitations caused by the design conditions which are different from conventional liquid crystal displays such as cell gap. We quantified the concentration of HTAB ([HTAB]) that is necessary to form and maintain a sufficiently dense vertical alignment of 5CB (4-Cyano-4′-pentylbiphenyl). The required [HTAB] for full-homeotropic alignment was increased to the cell gap until it was too large to support the transfer of the surface alignment to the LC molecules, due to the weak anchoring nature of HTAB. We also showed the phase-change characteristic of the LC mixture related to [HTAB] for the design of RF devices driven by light or heat. This study may help to guide the development of new approaches to designing efficient RF devices that use LCs.

We evaluated hexadecyltrimethylammonium bromide (HTAB) for use in radio frequency (RF) applications in which liquid crystals (LCs) are used in layered structures that have various cell gap conditions. HTAB is a surfactant that can self-align vertically on the surface of indium tin oxide (ITO) substrates, and can induce homeotropic alignment of the LCs. For implementing RF devices with HTAB and LCs, we should consider limitations caused by the design conditions which are different from conventional liquid crystal displays such as cell gap. We quantified the concentration of HTAB ([HTAB]) that is necessary to form and maintain a sufficiently dense vertical alignment of 5CB (4-Cyano-4'-pentylbiphenyl). The required [HTAB] for full-homeotropic alignment was proportional to the cell gap until it was too large to support the transfer of the surface alignment to the LC, due to the weak anchoring nature of HTAB. We also showed the phase-change characteristic of the LC mixture depended on [HTAB] for the design of RF devices driven by light or heat. This work may help to guide the development of new approaches to designing efficient RF devices that use LCs.

A liquid-crystal (LC)-based floating electrode-free (FE-free) coplanar waveguide (CPW) phase shifter with an additional LC layer is demonstrated for the first time. An LC layer is overlain on the electrodes of the original model; this change increases the amount of electric flux that the proposed structure can confine in the tunable region, and thereby greatly increases the figure-of-merit (FoM) while maintaining the benefits of the simple coplanar structure. We simulated the variations in the phase shifter’s FoM, characteristic impedance, and driving voltage while sweeping the additional LC layer thickness up to 300 μm with each electrode condition at 28 GHz. In the case of electrode thickness variation, the FoM increased as electrode thickness increased, regardless of the presence of the additional LC layer. However, in the case of the signal electrode width variation, we obtained an opposite FoM tendency depending on the presence of the additional LC layer. This work shows the possibility of an efficient LC-based FE-free CPW phase shifter design for a given LC layer and electrode conditions.