High-Selectivity Compact Wideband Transversal Filtering Inverted-F Antennas for Out-of-Band Decoupling in Terminal Applications

This article introduces a novel wideband transversal filtering inverted-F antenna (IFA) structure without using any extra feeding network. The proposed antenna structure can produce true efficiency zeros (EZs) at both sides of the operating band across the beamwidth, not restricted to the boresight. The filtering antenna comprises three coupled radiators. The driven inverted-F radiating element is directly excited from the port, while the other two parasitic radiators are excited through electromagnetic coupling with the driven radiator. By controlling the resonant frequencies and the coupling relationships, a transversal filtering antenna (TFA) with a third-order filtering response is realized with all quarter-wavelength radiators. The antenna exhibits an EZ on each side of its passband, significantly enhancing its selectivity and thereby enabling the suppression of interference from out-of-band signals. The design method of the proposed filtering antenna is presented. Some design guidelines are provided. Without increasing the antenna footprint, the techniques introduced in this work are not only helpful for improving selectivity but also very useful for bandwidth enhancement of conventional unbalanced antennas. To validate this technology, an antenna for the 5G new-radio (NR) frequency band is designed, fabricated, and measured. The experimental results align very well with the simulations. To evaluate the out-of-band interference suppression, a dual-antenna system was constructed and tested, with one antenna operating at the NR band (3.3–5 GHz, 42%) and the other antenna operating at the Wi-Fi band (5.15–7.125 GHz, 32%). Test results demonstrate excellent out-of-band decoupling up to 20 dB within the near stopband, showing promises for space-limited terminal applications.