Electromagnetically coupled small broadband monopole antenna

The paper presents a small broadband monopole antenna. The proposed structure consists of a circular disk-loaded monopole and a probe with a circular spiral strip line monopole. Broad bandwidth can be achieved through electromagnetic coupling between these two monopoles that generate two resonances closely spaced in frequency. The antenna occupies a cylindrical volume with a radius of /spl rho/ /spl cong/ 0.04/spl lambda//sub o/ (5.5 mm) and a height of h /spl cong/ 0.08/spl lambda//sub o/ (11 mm) and has 430 MHz of impedance bandwidth for VSWR<2 with a center frequency at 2.185 GHz, which is fractional bandwidth of approximately 19.7%. The gain of the antenna is more than 1.7 dBi within the entire bandwidth with good omnidirectional radiation characteristics.

I. Introduction

Recently, there has been increasing demand for various kinds of multimedia services as well as for the downsizing of hand-held transceiver units in wireless communication systems. This trend has attracted considerable interest in the development of small and broadband antennas. Electrically small antennas, being studied for these reasons, are much smaller than a radian sphere and they have a narrow bandwidth because the quality factor, , of the antenna is very high. Therefore, the reducing the value of is an important factor in the design of electrically small antennas. It has been reported that of the antenna can be reduced, thereby increasing its bandwidth through the effective use of the smallest sphere or cylinder that can enclose the antenna [1]–[4]. Designs of several types of small antennas have been investigated to overcome the bandwidth problem [5] based on this theory. However, bandwidth enhancement without sacrificing other antenna characteristics is extremely difficult due to its fundamental limitations.