Optimization of aperture coupled patch antenna element for mobile communication applications

We discuss the optimization and design of a single-layer, single-patch antenna element for base station array antennas. Such an element should have a minimum of 10% bandwidth and preferably a return loss of 20 dB or more. The antennas used in base stations for mobile communications usually consists of linear arrays of radiating elements in order to achieve the desired pattern: narrow beamwidth in the vertical plane and a broad beam in the horizontal plane. Other aspects of the pattern are also important: constant beam peak angle over the frequency band (i.e. no beam squint), suppression of sidelobes above the main beam and null-fill below the main beam to ensure coverage close to the base station site. For the array to meet these requirements over the frequency bandwidth, it is important to maintain good control over the phase and amplitude distribution to the elements. Furthermore, due to the high power used and the increasing demand for low intermodulation levels, the use of power dividers which provide isolation between the output ports, such as the Wilkinson divider or the 90/spl deg/ hybrid, is very limited. Therefore, any impedance mismatch at the elements will disturb the power distribution of the array and degrade the pattern. The above considerations means that the usual patch antenna designs presented, where a VSWR<2 over the band, are inadequate for use in a base station antenna array. In fact, even if an input VSWR of 1.5 (14 dB return loss) is acceptable for the array, the elements should preferably be matched to a 20 dB return loss to ensure pattern stability.