Appl Phys A (2011) 103: 537–540 DOI 10.1007/s00339-010-6183-1 Dual mode composite right-left-handed unit cells Amr M.E. Safwat Received: 29 December 2009 / Accepted: 3 December 2010 / Published online: 29 December 2010 © Springer-Verlag 2010 Abstract This paper presents a new category of composite right-left-handed (CL-CRLH) unit cells. The new cells rely on complementary stubs, e.g., open and short circuited stubs in planar transmission lines or dual modes in coupled lines. They have geometrical circuit model, do not rely on com- mercial off-the-shelf components, and do not require vias. Theoretical expectations have been confirmed by EM simu- lations and measurements. 1 Introduction CRLH transmission lines have paved the way for the real- ization of compact microwave devices, and for the imple- mentation of novel applications [1–4]. Some designs of the unit cell were persuaded by the first implementation of meta- material [5], where split ring resonator (SRR) were used, while others relied on patterned or commercial off-the-shelf (COTS) lumped-components. This paper revisits some conventional microwave struc- tures, short/open stubs and coupled-lines. It shows the appli- cation of these structures as CRLH unit cells. The proposed cells have geometrical circuit model, and are not limited to specific values as in COTS components. 2 Complementary stubs CRLH unit cell Series-short and shunt-open circuited stubs, shown in Fig. 1(a) and 1(b), respectively, are basic components in RF A.M.E. Safwat ( ) Electronics and Communication Eng. Dept., Faculty of Engineering, Ain Shams University, 1 El-Sarayat St., Abbassia, 11517, Cairo, Egypt e-mail: asafwat@ieee.org circuits. At λ/4, their equivalent circuit models are shunt resonant and series resonant circuits, respectively, given in Fig. 1(c) and 1(d), and at λ/2 their equivalent circuit models are series resonant and shunt resonant circuits, respectively, shown in Fig. 1(e) and 1(f). Combining these two simple structures, we get a CRLH unit cell. If the dispersion rela- tions of the short- and open-circuited stubs are identical, the transition frequencies for both stubs will be the same, and consequently a balanced CRLH TL will be achieved at λ/2. Figure 2(a) shows the microstrip implementation of the proposed unit cell, where the slot in the ground behaves as a series short-circuited stub [6]. The length of the parallel stub was tailored to achieve the balance condition at the sec- ond transition frequency, f = 3 GHz. The optimum length was found to be d p = 20.5 mm. The corresponding disper- sion relation is shown in Fig. 2(b). The second left-handed band appears at 6 GHz, however, the balance condition at this frequency could not be achieved. This is mainly due to the difference in dispersion relations of the series and shunt stubs. The performance of seven cascaded cells was simulated using the EM, HFSS [7], and circuit simulations. A compar- ison between the circuit model and EM simulations when d p equals 20.5 mm is shown in Fig. 3(a). Transmission bands alternating between right- and left-handed appear at the out- put. However, both EM and circuit simulations results show imbalance between left- and right-handed bands. The length of the parallel stubs was optimized in the EM simulations to reduce the imbalance at the lower frequency, the optimum value was found to be 24 mm. The results, which are also depicted in Fig. 3(a), show that there is a slight increase in S 21 for the first left-handed band; however, the imbalance could not be eliminated. The optimized structure was fab- ricated on FR4 (h = 1.6 mm, ε r = 4.4, tan δ = 0.02). Fig- ure 3(b) shows the measurement and the EM simulation re-