A compact tri-band and tetra-band short helical antenna electromagnetically coupled feed by a loop antenna

A very compact dual frequency antenna composed of helical elements and an exciting loop which are electromagnetically coupled has been reported for the application to sensor networks. In this paper, a tri-band and tetra-band small antenna are presented by inserting the helixes to the dual frequency antenna without increasing the antenna size. Performance of a prototype of the tri-band antenna with axial length of 26mm was measured and the return loss less than -6dB have been achieved with fractional bandwidth of 1% at 440MHz, 920MHz and 2.4GHz. The radiation efficiencies for the above frequencies are 35%, 65% and 96%, respectively excluding the dielectric loss for the element support. A tetra-band antenna has been also designed by adding a helix for the 5GHz band and the input and radiation performances are ensured by numerical calculations.