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

This paper introduces a novel wideband transversal
filtering inverted-F antenna structure without using any extra
feeding network. The proposed antenna structure can produce
true efficiency zeros (EZ) 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 3^rd-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 at the WiFi 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.