Dual-mode leaky-wave excitation in symmetric composite right/left-handed structure with center vias

In this paper a novel four port symmetric coupled composite right/left-handed leaky-wave (LW) transmission-line is introduced. Unlike the other coupled CRLH-TLs, this structure has the ability to support both right-handed as well as left-handed guided and leaky-wave propagations under both common-mode and differential-mode excitations. These unique properties are achievable through providing physical short circuit by means of ground vias at the center of each unit-cell along the symmetry plane of the structure. The advantage of this structure is that not only backward-wave is supported under both common- and differential-mode excitations but also when the TL operates in the fast-wave region, the radiated field under differential-mode excitation has an orthogonal polarization in comparison to that of the common-mode excitation resulting in a dual polarized four port coupled CRLH LW-TL antenna. The operation mechanism which is based on the direction of the current maximum will be discussed in this paper. For illustration purposes, a 25-cell four port coupled CRLH LW-TL is simulated, fabricated and measured and the obtained results verify the unique features of the structure mainly CRLH nature as well as its dual polarity under both common-mode and differential-mode excitation.