ABSTRACT
In this chapter, a band hopping / tunable band / frequency agile printed antenna is discussed. In this antenna, a band can be electronically tuned in a frequency range, which is referred as frequency agility or band hopping in printed antennas. In the structure of antenna, in a circular microstrip antenna (CMSA), an air gap between the ground plane and the substrate is inserted to enhance the operating frequency range of the antenna, and active devices are loaded on the patch to achieve tunable band/band hopping/agility of antenna. The resonant frequency of a 10 mm radius patch antenna is upward shifted from 4.9 GHz to 6.60 GHz using a 1 mm airgap between the ground plane and the substrate, and by loading a MOS, the antenna can be tuned down to 1.127 GHz operating frequency, which leads to compactness and band hopping of the antenna. Hence, the antenna can be tuned between 1.127 GHz and 6.595 GHz frequency of operation. A frequency agility of 76.20% was found for a MOS loaded CMSA, and a double MOS loaded antenna possessed 82.94% frequency agility, also referred as band hopping. Also, a frequency agile BST varactor loaded stacked CMSA is discussed, one of the two bands of the antenna is tunable by loading a BST varactor. These antennas are useful for a wide range of mobile communications such as GPS, UMTS, WiMAX, remote sensing, and other modern communication systems.
