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This article proposes a methodology for designing co-polarized in-band full-duplex (IBFD) antenna arrays with high isolation. An equivalent circuit model of IBFD antenna arrays is established, and transmission zeros are introduced by configuring controllable coupling paths between the arrays. Based on the proposed equivalent circuit model, the high-isolation co-polarized IBFD antenna arrays are designed by utilizing a wavefront phase compensation technique and a controllable coupling structure. Two co-polarized IBFD cavity slot antenna arrays based on the TM510 mode cavity are designed for validation, where a transmission zero is obtained in the passband to improve the isolation between the transmitter (Tx) and the receiver (Rx) ports. Compared with the reference antenna array, the isolation of the designed IBFD antenna array is improved by about 22 dB. The theoretical analysis and design process of the IBFD antenna array is given in detail. A second-order co-polarized IBFD antenna array is fabricated and measured, and the measured results are in good agreement with the simulated results.

A dual-band coradiator antenna with a beam-scanning capability for the sub-6 GHz and millimeter-wave (mm-wave) bands is proposed in this article. First, four subarrays based on cascaded triangular cavities are arranged using a mirror operation to form the mm-wave antenna array. Then, four mm-wave subarrays are connected at the position where the electric field is at a minimum on their edges. Finally, the entire mm-wave array structure resonates in the microwave band, which can be equivalent to a shorted quarter-wavelength patch antenna in the sub-6 GHz band. Hence, the same radiator can work as both a sub-6 GHz antenna and an mm-wave antenna array with broadside patterns in both cases. Attributed to the novel co-radiator structure, the proposed design is simple and compact. An interdigital coupled line with a high-pass filtering characteristic is inserted into each mm-wave feeding line to enhance the isolation. Besides, two parasitic patches are loaded to increase the microwave bandwidth. The experimental results show that the peak gain at 2.56 GHz is 3.2 dBi and at 28 GHz is 14.3 dBi for the sub-6 GHz and mm-wave bands, respectively. The isolation among different feeding ports is greater than 15 dB and the mm-wave array also provides ±38° beam-scanning performance.

In this article, a decoupling network is proposed for the design of in-band full-duplex antennas, whereby high isolation between the transmitter (Tx) and receiver (Rx) ports is attained by adjusting the coupling within the antenna to generate transmission zeros in the passband. To guide antenna design, analyses are carried out on both first-order and third-order decoupling networks. A first-order full-duplex antenna is designed based on a slot antenna array and a metal block antenna, which are capable of obtaining a transmission zero in the passband. A third-order full-duplex antenna is designed based on a TM120/210 mode cavity and a cross-shaped radiant metal block, which obtains three transmission zeros in the passband. The internal coupling relationships of the antennas are analyzed in detail, and two full-duplex antennas are fabricated and measured. The measured results demonstrate that the isolation between the Tx antenna and Rx antenna is greater than 30 dB in the passband.

The analyses of the numerical stability and dispersion of the 3-D hybrid implicit–explicit finite-difference time domain (3-D HIE-FDTD), including the lumped elements, are systematically researched. Specifically, three lumped elements of the resistor, inductor, and capacitor are discussed, and the explicit, semi-implicit, and implicit schemes are discussed. First, according to the techniques of von Neumann and Jury criterion, the numerical stability of the extended 3-D HIE-FDTD method is studied. Research results demonstrate that: 1) as to the lumped resistor and inductor under the semi-implicit and implicit schemes, and capacitor under the implicit scheme, the numerical stability condition of the extended 3-D HIE-FDTD method is consistent with the 3-D HIE-FDTD method unloaded lumped elements and 2) for the lumped resistor and inductor under the explicit scheme, the numerical stability condition becomes more stringent than that of the 3-D HIE-FDTD method unloaded case, which is dependent on the value of the resistance or inductance. Second, based on the Norton equivalent circuit, the numerical dispersion characteristics of the extended 3-D HIE-FDTD method are analyzed, and the numerical impedance for the lumped elements under different difference schemes is deduced. Finally, to testify the validity of the theoretical results, the numerical results of microstrip circuits are provided.