Three working modes, omni-, sector and adaptive modes, for a compact array antenna are introduced. The compact array antenna is an electronically steerable parasitic array radiator (Espar) antenna, which has only a single-port output, and carries out signal combination in space by electromagnetic mutual coupling among array elements. These features of the antenna significantly reduce its cost, size, complexity, and power consumption, and make it applicable to mobile user terminals. Signal processing algorithms are developed for the antenna. An omnipattern is given by an equal-voltage single-source power maximization algorithm. Six sector patterns are formed by a single-source power maximization algorithm. Adaptive patterns are obtained by a trained adaptive control algorithm and a blind adaptive control algorithm, respectively. The experiments verified the omnipattern, these six sector patterns and the adaptive patterns. It is hope that understanding of the antenna’s working modes will help researcher for a better design and control of array antennas for mobile user terminals.
Espar Antenna Design
This section describes the structure and the design of the Espar antenna.
A 2.484GHz Espar antenna is illustrated in Figure 1 and Figure 2. The seven-element monopole elements are arranged on a finite circular ground structure. The centre element is the feed element. The remaining M (= 6) elements are parasitic and make a 0.25λ ring around the centre element, where λ=12.07 cm is the free space wavelength corresponding to the operating frequency of 2.484 GHz. The bottom of each parasitic element is loaded with a variable reactance. A bias voltage Vm on it adjusts the value of the reactance. Variable beamforming is carried out by controlling the six bias voltages (control voltages) Vm, (m=1, 2,…, M), and thus the values of the reactances. A skirting is used on the ground plane to reduce the main lobe elevation. The radius of the circular ground plane is 0.5λ, and the skirting height is 0.25λ (Ojiro, 2001), which provides maximum gain of the antenna’s radiation in its horizontal direction.
A fabricated 7-element Espar antenna