Development of omnidirectional collinear arrays with beam stability for base station and mobile applications.

Since the 1800’s, inventors have considered electromagnetic radiation as a medium for communications between ship and shore and base to mobile. Generally, these were narrow band services, supported by simple series fed arrays. These arrays are limited in bandwidth by an inherent tapered phase profile which exhibits an undesirable vertical beam angle shift with excessive change in frequency. With a symmetrical tapered or linear phase profile, the radiation pattern remains on the horizon over at least 10% bandwidth.

This thesis aims to presents the development of an antenna which has a unique compact planar architecture, and has achieved the performance required for broad band communications channels in the 900 MHz band. With low cost; volume manufacture, PIM and PIP considered.

A major achievement in this work has been the development of a planar passive coupled dipole array with coplanar feed network presented on a single sided low cost flexible PCB. In this format, the array can be re produced accurately at a fraction of the cost of the conventional sleeve dipole arrays. Due to PCB processing limits for track spacing, the array has been designed with a main transmission line track spacing of 0.65mm resulting in a higher impedance of 86 Ohms. At this track spacing, the main transmission line can withstand a 2.1 kV ionisation test allowing for a PIP value of 1 Kw. To compliment this planar array, a specially designed planar dipole; matched to the new feed network scheme. With slots to augment the radiation pattern for improved directivity and reduced azimuth ripple, and notches to stagger tune the dipole to cover a broader than otherwise band and compensate for the reactance associated with close proximity to the slotline tracks. This dipole meets broad bandwidth and omnidirectional pattern specifications similar to cylindrical sleeve dipoles, while still maintaining inter-stage isolation.

To further compliment the array attributes, a robust coaxial cable connection to the thin and flexible substrate employs a novel passive coupler transition circuit. The proposed transition is electromagnetically coupled to source and ground terminals of the array, eliminating the need for soldering, resulting in low loss, low PIM “Passive intermodulation” typically <-148 dBc, This coupler also provides the impedance transformation from 70 Ohms to 50 Ohms.

An omnidirectional six element “coplanar waveguide(CPW)” fed collinear array was assembled from the planar feed network, dipole elements and passive coupler components, having a total length approximately 1.8 meters. This large array has the unique attributes of wide band, less than 1 degree beam tilt over a frequency range of 850 to 960 MHz (for a measured reflection coefficient magnitude greater than 14 dB), with low measured passive intermodulation below -140dBc “Decibel power ratio relative to transmit carrier”. The prototype collinear array can also withstand a wind loading of 240 km/h. The resulting antenna array meets the desired frequency band and has extremely stable radiation pattern, with a gain of 10 dBi.