Design of a Highly Miniaturized, Inherently Matched, Spherical Folded
Dipole Antenna and Evaluation of its Quality Factor
Abstract
A highly miniaturized three-dimensional spherical folded dipole antenna
has been reported, for which inherent impedance matching is achieved
with respect to a practical source impedance by employing a simple
series-LC loading combination, thereby engineering its input impedance.
In order to maximize its bandwidth, the miniaturized antenna employs a
spherical helix structure as the folded arm that occupies the full
volume of the corresponding Chu sphere. A bifilar (two folded arms) and
a quadrifilar (four folded arms) helix loaded folded dipole antenna are
designed, and full-wave simulations show that both the resulting
antennas demonstrate excellent impedance matching when miniaturized by
85% in comparison to a resonant dipole operating at the same frequency.
Despite the high degree of miniaturization, the resulting radiation
efficiencies for the bifilar and quadrifilar antennas are found to be
87.1% and 90.6%, respectively. Furthermore, various quality-factor
definitions are explored for the quadrifilar antenna, and it is observed
that the resulting quality factor is around 1.83 (1.22) times that
predicted by the Chu (Thal) lower bound.