Increasing the bandwidth of a normal mode helical antenna consisting of two strips

Abstract

A normal mode helical antenna (NMHA) is usually used for handsets in personal communication systems. As the small NMHA has low radiation resistance, it requires a matching circuit. The matching circuit has loss and the radiation efficiency decreases. An NMHA consisting of two wires (the 2-wire NMHA) has been suggested (Noguchi et al., 1997). The radiation impedance of the 2-wire NMHA can be matched by itself to the characteristic impedance of the feed line. The impedance matching is carried out by use of current modes that include balance and unbalance modes. However, it is difficult to increase the bandwidth of the 2-wire NMHA because reactance components of the antenna can not be eliminated by reactance in the balance mode of two wires. A small meander-line antenna of two strips has been suggested to increase the bandwidth of the antenna without a matching circuit (Noguchi et al., 1997). The reactance component of the antenna can be eliminated by reactance in the balance mode over a wide frequency range. In this paper, a small NMHA consisting of two strips (the 2-strip NMHA) is presented. The method of increasing the bandwidth is applied to this antenna.