Wideband Excitation Technology of <formula formulatype="inline"><tex Notation="TeX">${\rm TE}_{20}$</tex></formula> Mode Substrate Integrated Waveguide (SIW) and Its Applications

Abstract

The higher order mode substrate integrated waveguide (SIW) has advantages in the applications, which are: 1) simplifying the structure with reduced fabrication cost and 2) enhancing the stability of performance by relaxed fabrication tolerance. In this paper, two wideband ${\rm TE}_{20}$ mode excitation structures are presented and investigated for the SIW. A slot along the mid line in the longitudinal direction of the SIW is employed to convert the electromagnetic field pattern between slotline and the ${\rm TE}_{20}$ mode SIW. The second structure is based on the slot aperture coupling and can realize wideband direct transition between the ${\rm TE}_{20}$ mode SIW and microstrip line. Both transitions have a simple and compact structure, as well as broadband characteristics. The wideband transition performance is demonstrated in this paper. As application examples of the transitions, two broadband substrate integrated baluns are presented and fabricated based on the ${\rm TE}_{20}$ mode excitation structures and ${\rm TE}_{20}$ mode characteristics. The 180 $^{\circ}$ out-of-phase and equal magnitude of the balun outputs in a wide frequency range can be achieved inherently. The balun based on the slotline excitation has a fractional bandwidth of 50.2%, from 7.3 to 12.2 GHz, with measured return loss, amplitude imbalance, and phase imbalance better than 10 and 0.45 dB and 3.8 $^{\circ}$ . The balun based on the aperture coupling excitation shows a fractional bandwidth of 50.3%, from 7 to 11.7 GHz, with measured return loss, insertion loss, amplitude imbalance, and phase imbalance better than 10, 1, and 0.27 dB and 2.4 $^{\circ}$ , respectively. Both baluns not only show good performance, but also demonstrate the existence of the ${\rm TE}_{20}$ mode in the SIW.