Wideband Millimeter-Wave Co-Linearly Polarized In-Band Full-Duplex Open-Ended Waveguide Antenna

In this article, a novel self-decoupled method implemented by the propagation modes cancellation of a rectangular waveguide is proposed for wideband and high-isolation-level co-linearly polarized in-band full-duplex (IBFD) antennas. This antenna comprises an enlarged open-ended rectangular waveguide serving as a radiator, and two integrated feeding waveguides that connect to the TX and RX channels. When the antenna operates, the TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub>, TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub>, and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes of the open-ended waveguide are simultaneously excited and superposed within the waveguide, generating a weak-field region near the RX port. In this way, the mutual coupling between the TX and RX channels can be effectively suppressed. Since these modes are propagation modes, they persistently coexist once the operating frequency exceeds their cutoff frequencies, thereby enabling wideband performance. Besides, two tapering ridges are employed in the open-ended waveguide to maintain a stable high in-band isolation level. They can adjust the propagation constants of the three modes to synchronize them, thereby achieving better cancellation effects. This antenna is fabricated and measured. The measurement reveals that the antenna achieves a bandwidth of 51%, with an in-band isolation level exceeding 30 dB, thereby providing a good tradeoff between bandwidth and isolation level. Moreover, the open-ended waveguide can be extended into different horn shapes to meet different gain requirements, demonstrating good scalability.