This article presents a comprehensive overview of the state-of-the-art, design trends, and challenges in the field of highly reconfigurable and co-designed RF/microwave filtering devices. Starting with BPFs with tune-all capabilities in terms of center frequency, BW, TZs, out-of-band rejection profile, and even type of filtering function, their co-integration with other RF functionalities in highly adaptive co-designed circuits are also addressed: e.g., low-noise amplification, phase shifting, attenuation, isolation, and nonmagnetic nonreciprocity by means of STM and transistor-based configurations. Subsequently, the topic of highly adaptive multiband BPF and BSF components for multi-RF-signal-acquisition and multi-RF-interference-suppression tasks is covered. Furthermore, other design trends and challenges in this exciting field are briefly discussed.

This article presents higher-order independently tunable dual-band bandpass filters (BPFs) with constant absolute bandwidth (CABW) based on odd-modes-only quarterwavelength (λ/4) resonators. The main contribution is breaking the intrinsic tuning interdependence between the fundamental (f1) and third-harmonic (f3) modes, enabling compact λ/4 structures to realize higher-order BPFs. This is attributed to the first-time creation of an internal artificial voltage null point of f1 (A-VNP1) by series varactor-loading. By series-loading a varactor at the current null point of f3 (CNP3) and parallel-loading another at the A-VNP1, a dual-mode dual-null-point loading scheme is established, achieving independent tuning of f1 and f3. Based on this mechanism, third- and fourth-order prototypes are designed and measured.

An on-chip reconfigurable dual-band millimeterwave (mmWave) bandpass filter (BPF) is presented in a GaAs integrated-varactor platform. Independently tunable passbands with asymmetric reconfigurability are realized using a parallelcoupled topology comprising a varactor-loaded half-wavelength resonator and a dual-mode capacitor-loaded resonator, which effectively decouples the two bands, a functionality seldom demonstrated in reported on-chip mmWave filters. The lower band tunes from 25 to 26.5 GHz with 10.2% FBW and 3.48–4.16 dB insertion loss, whereas the upper band tunes from 35.25 to 37.25 GHz with a reconfigurable 4.4–9.2 GHz bandwidth and 3.7–5.15 dB insertion loss. Good linearity is achieved, with a maximum IIP3 of 30 dBm.

This paper presents the design, fabrication, and performance evaluation of a flexible loop antenna integrated with LC sensors for wireless dielectric sensing in oral care applications. The antenna, implemented on a flexible substrate, enables conformal placement and reliable inductive coupling with passive LC sensors, facilitating non-invasive detection of dielectric property changes in biofluids. The flexible antenna was also tested at various bending angles. Two sensor types—big and small spiral laminated LC structures (BSL and SSL) were characterized under various test media, including deionized water, artificial saliva, and multiple concentrations of rosemary essential oil emulsions. Experimental results demonstrate stable resonance behavior under mechanical deformation and effective frequency shifts corresponding to liquid permittivity changes.