This doctoral thesis deals with designs and developments of multi(wide)-band multifunctional antennas based on folded dipoles.
In the beginning, the concept of Orthogonal Folded Dipoles (OFD) are put forward. Orthogonal folded dipoles are formed by two identical two-port folded dipoles orthogonally joining with each other at the center, and can be fed through different combinations of feed nodes to offer dual- and wide-band modes, respectively. The impedances of the 2 modes are studied both by analytical models and by commercial electromagnetic simulation tools. The properties of the linearly polarized radiation patterns in the two modes vary quite little, except for ±45◦ rotation of two principal planes. In this way, orthogonal folded dipoles can possibly provide pattern reconfigurability in a context of switchable types and orientations of polarizations.
Next, the concept of Log-Periodic Folded Dipole Array (LPFDA) is proposed. It stems from the traditional log-periodic dipole array, whereas folded dipoles instead of straight dipoles are applied as the elements of the array. Two configurations, i.e. partly-scaled LPFDA and fully-scaled LPFDA, are studied through simulations and optimizations. The comparison shows that the latter outperforms the former in terms of higher directivity, reduced front-to-back ratio and lower crosspolar level.
The key parts of this thesis focus on exploiting Eleven antennas, based on the LPFDAs, in a variety of applications. First, the 4-port L-band lab model for use in satellite terminals demonstrates that the radiation patterns for monopulse tracking can be achieved through different excitation combinations of the multiport Eleven antenna. Second, a decade bandwidth, an unchanged phase center and nearly constant directivities over the whole band can be regarded as the major figure-of-merit of the Eleven antenna, which makes it suitable as feed for prime-fed reflector antennas. Through endeavors of using Genetic Algorithms, the wideband Eleven antennas have been gradually optimized in term of matching as well as efficiencies. Besides, the rotationally symmetric circular Eleven feed is a very promising solution for improving the BOR1 efficiency and therefore the aperture efficiency by a fraction of dB. Last but not least, the two multiport L-band lab models measured in a reverberation chamber demonstrate that the Eleven antenna with MIMO diversity ports can possibly overcome narrowband multipath fading in a real radio link and improve the link quality in terms of a significant diversity gain and high maximum available capacity.