Exploiting Reflectionless Filter Design Techniques in Absorptive Antennas: Emerging Trends and Challenges (Invited)

Antennas are indispensable components in all wireless communication and sensing systems, serving as the fundamental radio-frequency (RF) interface that enables the transmission and reception of electromagnetic (EM) waves between free space and guided media [1]. The overall performance of any RF system is inherently linked to the EM behavior of its antennas—particularly in terms of radiation efficiency, impedance matching, operational bandwidth, and radiation pattern characteristics. Traditional antenna designs have mainly focused on maximizing in-band radiation/reception efficiency, ensuring optimal impedance matching, and achieving the desired spatial response. However, emerging and increasingly complex RF scenarios demand more advanced functionalities beyond conventional performance metrics. In particular, there is a growing need for antennas capable of not only radiating efficiently within their operating band but also suppressing unwanted out-of-band or non-radiated RF power within the antenna itself, rather than reflecting it back to the source. This requirement has motivated the development of a new class of antennas—commonly referred to as reflectionless or absorptive antennas—which draw inspiration from RF reflectionless filtering concepts [2]- [4]. These antenna architectures offer an effective alternative to traditional reflective counterparts by inherently mitigating out-of-band RF-signal reflections [5]- [12]. Thus, they provide natural protection to preceding RF active stages (e.g., amplifiers or mixers of frequency converters) without requiring additional isolators or attenuators, and they enhance robustness in multi-transceiver environments by minimizing cross-channel interference and oscillation effects.This overview paper summarizes the latest research progress in reflectionless/absorptive antenna technologies. It reviews key design principles and recent trends, ranging from fundamental low-reflection radiators to more sophisticated architectures, such as multi-functional and array-based configurations (e.g., filtering antennas with single/multi-band and/or balanced characteristics). Experimental results from representative proof-of-concept prototypes developed in various technologies (e.g., planar, multi-layer, and substrate-integrated waveguide) are discussed. The remaining challenges and future research directions in this emerging field are also highlighted.