Realization of dual-frequency and wide-band VSWR performances using normal-model helical and inverted-F antennas

The effects of parasitic elements on the voltage standing wave ratios (VSWR´s) of two antennas are investigated. First, a parasitic monopole is used for a normal-mode helical antenna. The VSWR investigation shows that dual-frequency operation is obtained by the parasitic element effects. The dual-frequency operation is revealed as a function of monopole position above a ground plane (monopole height). As the monopole height decreases, the separation of a higher resonance frequency f_H from a lower resonance frequency f_L increases. For a monopole length of L_MP≈0.4λ_HX , where λ_HX is the resonance wavelength of the helix, the frequency bandwidth for a VSWR=2 criterion is 12.5% in the lower frequency f_L region and 5.2% in the higher frequency f _H region, with a frequency separation ratio of f_H/f_L=2.14. Secondly, L-figured parasitic elements are used for an inverted-F antenna (IFA). The parasitic elements improve the VSWR performance. The frequency bandwidth for a VSWR=2 criterion is approximately two times as wide as that of the single IFA. It is also found that bending the horizontal sections of the IFA and parasitic elements contributes to reducing the antenna size while not significantly deteriorating the VSWR bandwidth. The radiation patterns are also presented and discussed