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A compact substrate‐integrated waveguide (SIW) filtering multiple‐input multiple‐output magnetic dipole antenna with high selectivity and high isolation is proposed. Each filtering antenna element contains a SIW cavity, a half‐mode SIW cavity, and a magnetic dipole antenna with backed ground. Meanwhile, the SIW cavity and the half‐mode SIW cavity are vertically connected through magnetic coupling, and second‐order filtering frequency responses can be obtained. Owing to the cross‐coupling between the feeding line and antenna, two radiation nulls can be created to realize high selectivity. Based on the radiation mechanism of the magnetic dipole antenna and polarization diversity, high isolation can be obtained. The measured isolation and the envelope correlation coefficient are greater than 24 dB and less than 0.004, respectively.

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Compact and low-loss U-band shielded asymmetrical coplanar stripline (SACPS) to unilateral and antipodal finline transitions are proposed in this article for millimeter-wave circuits and systems applications. The principles of mode conversion and impedance matching are applied to the proposed transitions. In the SACPS to unilateral finline transition, the SACPS is firstly transformed into the asymmetrical unilateral finline to convert the quasi-TEM mode to the quasi-TE 10 mode and then a tapered unilateral finline section is applied to realize impedance matching. For the U-band back-to-back SACPS to unilateral finline transition, the measured return loss is larger than 15 dB and the insertion loss is less than 1.5 dB with a minimum of 0.5 dB. In the SACPS to antipodal finline transition, the SACPS is firstly transformed into the defected ground microstrip (DGM) without changing the characteristic impedance. Then, the DGM is transformed into the asymmetrical antipodal finline to make mode conversion.At last, a tapered antipodal finline section is used for impedance matching.For the U-band back-to-back SACPS to antipodal finline transition, the measured return loss is larger than 12 dB and the insertion loss is less than 1.5 dB with a minimum of 0.3 dB.