A Compact and Low-Profile Tunable Loop Antenna Integrated With Inductors

We present a frequency-tunable, compact loop antenna which consists of a transmission line on a ground plane, two shorting posts, and two inductors which are serially connected between the posts and the edge of the transmission line. By properly choosing the inductance of the inductors, the operating frequency of the antenna can be controlled without seriously sacrificing fractional bandwidth. To demonstrate the operating mechanism, the equivalent circuit of this antenna is included. The characteristics of the antenna with various inductors integrated are also investigated. Fabricated antennas show that the operating frequency can be shifted from 2.07 GHz to 1.2 GHz using off-the-shelf inductors. Using two 33-nH inductors achieves an antenna with an electrical size as small as 0.118lambda times 0.013lambda times 0.047lambda. The validity of this antenna is demonstrated by experimental results.