SIW-Based Input-Reflectionless Filter/Filtenna Using Complementary-Diplexer Architectures

In this article, a class of input-reflectionless filter/ filtenna on substrate-integrated-waveguide (SIW) technology that exploits complementary-diplexer design principles is proposed. By extracting and controlling the couplings between the adjacent SIW-based cavities to satisfy a predesigned coupling matrix, a bandpass-type filtering functionality with out-of-band RF-input-reflectionless capabilities can be obtained. To this aim, a pair of frequency-complementary channels is employed, namely a bandpass channel for transmission and its complementary resistively loaded bandstop counterpart for stopband RF-power absorption. Based on this design approach, a third-order input-reflectionless SIW bandpass filter and a second-order input-absorptive SIW diplexer are realized. Subsequently, the last cavity resonator of the bandpass channel is replaced by a SIW-based radiator to form a filtenna with the same input-reflection frequency response. In this manner, based on the aforementioned input-reflectionless filter and diplexer, single- and dual-band SIW-based filtennas with reflectionless characteristics are conceived. Measured results of manufactured proof-of-concept prototypes are provided to experimentally validate the proposed design methodology of input-reflectionless SIW-based filter/filtenna. For these built devices of filter, diplexer, and single- and dual-band filtennas, wide-band reflectionless frequency ranges being $47.5\times $ , $18.06\times $ , $27.7\times $ , and $11.6\times $ larger than their associated operational frequency bands are, respectively, demonstrated.

I. Introduction

With the rapid development of wireless communication technology, filtennas, which co-integrate filtering and radiation functionalities so as to reduce the loss and size of the whole RF system, have been extensively studied. Nevertheless, most of previously reported filtennas mainly focus on the in-band characteristics so that the out-of-band/nonradiated RF-signal power is often reflected back to the preceding stages [1], [2], [3]. However, these unwanted/nonradiated RF-power reflections or echoes may cause adverse effects especially in RF active stages, such as the inducement of RF amplifiers to operate in nonlinear regime, additional undesired/spurious intermodulation products in mixers of frequency-conversion stages, standing-wave generation in transmission lines with negative impact in the conversion gain of mixers, interferences, or self-oscillation. This may lead to an unexpected operation and performance deterioration of the full RF front-end chain. In this context, isolators or attenuators are usually employed to reduce the negative impact of out-of-band reflected RF signals. However, they increase the entire size of the overall RF system, exacerbate its loss, and/or augment dc-power consumption when using interblock active isolators. Thus, the development of reflectionless filtennas able to dissipate inside themselves the nonradiated/out-of-band RF-signal energy has recently emerged as a suitable choice to overcome the referred problem.