Tightly Coupled Array of Horizontal Dipoles Over a Ground Plane

The design and performance of a circular array of tightly coupled horizontally polarized dipoles placed close to a ground plane is discussed. The dipoles are resonated with the ground plane below, a conducting reflector behind, a capacitive load in front, and a dielectric slab ring above to provide a 3.45:1 impedance bandwidth and consistent radiation patterns near the horizon, both as an omnidirectional radiator and as an array of four sectoral antennas. The design process is simplified by using a unit cell with periodic boundary conditions, and the effects of each resonance are characterized to provide design insight. Then, a theoretical analysis of the radiated fields of the array is performed, explaining nulls found in the radiation pattern near the horizon. A dielectric slab is added to mitigate this problem, and the effects of its dimensions on the radiation pattern and impedance match are presented. The array is fabricated and measured, and excellent agreement between simulation and measurement is observed. The array is found to provide > t1 dBi of horizon gain per sector (8 dBi max) with a cross-polarization ratio better than 18 dB or > t9 dBi of gain as an omnidirectional antenna at the horizon (8 dBi max) with a cross-polarization ratio better than 9 dB.