Reflector antenna configurations for radio base stations in cellular communications

Abstract

INTRODUCTIONNew microwave communication services have raised the interest in base station antennas capable of radiating an omnidirectional pattern in the aziniuthal plane as it is tlie case for cellular conimunications. Those antennas usually comprise linear dipole or slot arrays and, at higher frequencies, printed antenna arrays. Although such arrays inay produce adequate radiation patterns, main disadvantages of their use rest on reduced bandwidths and increasing losses with frequency due to phase and power control circuits.Alternatively, at microwave or millimetric ranges, one niay employ reflector antenna configurations for base stations, comprising reflector surfaces shaped in order to radiate uniformly in tlie horizontal plane while taking into account free -space attenuation via cosecant squared -like power patterns in vertical planes, with radiated energy concenmted below the horizon line for reduced interference. 'I'he main characteristics of two such configurations are reviewed in what follows. REFLECTOR ANTENNA CONFlGURATlONSThe antenna configuration shown in Fig. I comprises a (circularly symmetrical) conically shaped reflector surface fed by an axial horn. Concentrical circular sections of tlie feed horn have their radii chosen (Ap<A/2 along tlie structure) so as to ensure propagation of dominant TEM mode only, producing an axially polarized field on the horn aperture which independ on the aziniuthal coordinate. The horizontal plane feed pattern is therefore omnidirectional with vertically polarized fields and a null on the symmetry axis. The primary field is then reflected along a ray structure defined, in the vettical plane, by the reflector generatrix, i.e., a Geometrical Optics (GO) synthesis procedure is inipleniented for tlie reflector surface in order to yield a cosecant squared radiation pattern in that observation plane. Also, the pointed shape of the retlcctor surface near tlie symmetry axis, together with the feed pattern null in that direction, contribute to low return losses.Since GO syntheses do not consider diffraction effects. one may expect some discrepancies between specified and obtained radiation patterns in the extremes of the coverage region. This is, nevertheless, very small for observation angles (from the horizon) greater than 45" nnd one may then consider a larger upper litnit for synthesized fields in order to obtain a better pattern control u p to 45". Also, greater semi-angles (6,) for tlie edge cone of rays from the feed phase center, and lower (rellector) edge illumination levels, contribute to alleviate diffraction effects by reducing ripple amplitudes niore observed in weaker field portions of the radiation pattern. Greater such angles also contribute to reduce sidelobes near the horizon due to spillover but, on the other hand, niay iniply lower front-to-back ratios. The latter effect may be irlleviated by employing a dual reflector design such as proposed in [I-21. Also not considered herein is a feed design where TEol and TMol modes are excited to provide transmitti...

Performance Enhancement of High-gain STDA Antennas with Reflector for 4G LTE and Sub-6 GHz 5G Applications: Design, Measurement, and Analysis

“…The parasitic fragment-type element placed between two antennas reduces the mutual coupling by reflecting electromagnetic waves from one antenna back to the other, achieving a maximum isolation of 29 dB. This technique did not significantly impact radiation patterns, making it a promising and easy-to-implement method for suppressing mutual coupling [25]. The use of parasitic fragment-type elements also offers greater flexibility and versatility than conventional isolation techniques, resulting in substantial improvements in isolation, diversity performance, radiation patterns, and radiation efficiency of antenna structures reported in [26,27].…”