I. Introduction
Wireless communication technologies and applications have shown tremendous growth lately and continuously strive for better performance, lower power consumption, and smaller device size. Antennas are usually the largest component in radio-frequency (RF) front-end circuits and their miniaturization on a printed circuit board (PCB) has attracted significant research interest lately [1]–[4]. An effective antenna is a quarter-wave monopole collapsing on the PCB. Without conductor on the back to form a resonator, it maintains a low-Q feature of a suspended antenna and is broadband in nature. In many portable wireless applications where the complete communication system is built on a small circuit board, it usually allows very small area for an antenna. It is possible to reduce the antenna area and decrease impedance bandwidth by using a smaller (capacitive) antenna together with a tuning inductor, such as the inverted F antennas. In any case, the characteristics of an antenna are mostly determined by its electrical length. High dielectric-constant materials could be useful in reducing antenna physical length. Recently, the use of ultraslow wave transmission line structures to reduce the size of passive components has received noticeable attention. The use of periodic inductive or capacitive loadings on a transmission line either right- or left-handed could tailor the - diagram and results in an ultralarge phase constant or an ultrasmall transmission-line wave length. Various miniaturized microwave passive components have been reported for both antenna and circuit applications, based on the slow-wave periodic transmission line [5]–[8]. A common feature of these passives components is the use of ground plane to provide a current return path, and in many cases, the ground plane is patterned to provide addition loadings.