A Compact Omnidirectional Parabolic Shape Tilted Dipole Array With Improved Axial Ratio Bandwidth

Current omnidirectional circularly polarized (CP) antennas meet the difficulty of limited bandwidth and bulky configuration. An exact analytical far-field solution for a conventional tilted dipole array (TDA) has been proposed, where the origin of axial ratio (AR) bandwidth limitation has been investigated. Then, it has been shown that the AR bandwidth can be efficiently improved when a conventional straight dipole is folded into a multi-segment one, where the deviation of two perpendicular far-field components,
$E_{\theta }$
and
$E_{\phi }$
, can be reduced at both the lower and higher frequencies. The dipole was further modified to be of a parabolic shape, a continuous and optimized variation for the multisegment one, which brings about an even wider AR bandwidth. For a compatible wide impedance bandwidth, several impedance matching techniques like radiator tapering, parasitic loading, and feeding network design have been applied. Measured results show the proposed TDA has a 3 dB AR bandwidth (also the antenna usable bandwidth) of 65%, within which the desired omnidirectional radiation is well maintained. Compared with the performances of some recent omnidirectional CP antennas, our design has a much wider usable bandwidth, and is also superior in the compact size, simple structure, and ease of fabrication.