Filtering Antennas: Design Methods and Recent Developments

With wireless communication technologies continually and rapidly developing, RF front-end systems are experiencing a profound evolution toward the goal of highly integrated and miniaturized RF front ends with improved performance. Traditionally, passive components in the RF front end, such as filters, antennas, and duplexers, have been designed individually and cascaded through a 50-Ω interface. This inevitably leads to bulky physical dimensions and complex circuits, high insertion loss, and signal distortion, especially at the band edges. In multiband or multistandard communication systems, multiple operating bands of the base station antenna are normally achieved using several separate subarrays operating at different frequencies, which can cause serious interference among different bands for various services due to the limited space on the antenna platform. Highly integrated multifunctional RF front ends would provide a high-efficiency solution to those advanced and miniaturized wireless systems. As the key components in any RF front-end system, the integration of filters and antennas can significantly improve the frequency selectivity and bandwidth; stability of the in-band gain, out-of-band rejection, and system efficiency. They have therefore received much research interest and opened up a broad avenue for antenna R&D in recent years.