Design and performance of small electromagnetically coupled monopole antenna for broadband operation

An electromagnetically coupled small broadband monopole antenna has been investigated. The antenna structure consists of a circular disk-loaded monopole and a probe with a spiral strip line monopole packed within a small electrical volume. Broad bandwidth can be achieved through electromagnetic coupling between these two monopoles that generate two resonances closely spaced in frequency. The antenna with a circular disk has an electrical size kr of 0.563 and has 430 MHz of impedance bandwidth for VSWR < 2 with a centre frequency at 2.185 GHz, which is similar or equal to 19.7% of a fractional bandwidth. Measured efficiency of the circular disk-loaded antenna is 98% at its centre band region. The gain of the antenna is > 1.55 dBi and the antenna has good omni-directional radiation characteristics.