Effect of upper and lower shielding on the elimination of the slot-line mode in coplanar waveguides

An alternative method to eliminate the slotline mode in coplanar waveguides (CPWs) is to use upper and/or lower grounded conducting shields. These shields will not significantly affect the dominant (even) CPW mode if they are at a sufficient distance from the slots, because the fields generated by this mode die quickly due to the antiphase magnetic currents in the two slots. However, these shields should reduce the (odd) slotline mode because the slot magnetic currents of this mode are in phase, and so are the magnetic current images generated by the shields. The resultant large magnetic impedance obstructs the magnetic current of the slotline mode. To investigate this alternative method (without air-bridges), numerical results are obtained for the S-parameters of a CPW filter for normal CPW, CPW with botton shield, and CPW with top and bottom shields. The bottom shielding significantly reduces the slotline mode, which is reduced slightly further when a top shield is added. However, the shielded structures suffer a significant power leakage due to the parallel plate TEM (transverse electromagnetic) mode in the dielectric. This TEM mode can be eliminated by via holes.<>