PERFORMANCE OPTIMISATION OF SMALL ANTENNA ARRAYS
AbstractThis thesis addresses radiation pattern synthesis problems for small linear pe-riodic phased arrays (with array elements less then 10). Due to the small arraysize conventional pattern synthesis techniques fail to produce the required re-sults. In the case of practical small arrays, mutual coupling and element patternasymmetric effect degrade the array radiation performance. The main perfor-mance metrics considered in this thesis include sidelobe level (SLL), gain, halfpower beamwidth (HPBW) and mainbeam scan direction. The conventionalpattern synthesis approaches result in sub optimal gain, SLL and HPBW dueto the limited number of elements and the mutual coupling involved. In caseof difference pattern synthesis these factors resulted in lower difference patternslope, degraded SLL and difference peak asymmetry. The sum and differencepatterns are used in monopulse arrays and a simplified feed that could produceboth patterns with acceptable radiation properties is of interest and has beenexamined (chapter 5).A conventional technique is applied to small arrays to synthesise a sectorbeam and there is limited control over the radiation pattern. It is shown thatthe mutual coupling has significant effect on the array radiation pattern andmitigation is necessary for optimum performance (chapter 6). Furthermore,wideband phased arrays may have a natural limitation of the HPBW in lowgain applications and minimisation of the variation becomes important. Alsothe SLL variations for wideband antenna arrays in the presence of mutualcoupling considerably degrade the radiation pattern. The mutual coupling de-grades significantly the radiation pattern performance in case of small scanningwideband arrays (chapter 7).It is the primary goal of this thesis to develop an optimisation scheme thatis applied in the above scenarios (chapters 3 & 4). The only degree of freedom assumed is the array excitation. Optimised amplitude and phase for each element in the array are determined by the proposed scheme, concurrently. The deterministic optimisation techniques reported in the literature for the pattern synthesis may involve complicated problem modelling. The heuristic opti-misation techniques generally are computationally expensive. The proposedIntelligent z-space Boundary Condition-Particle Swarm Optimiser (IzBC-PSO)is based on a heuristic algorithm. This scheme can be applied to a wider rangeof problems without significant modifications and requires fewer computationscompared to the competing techniques.In order to verify the performance of IzBC-PSO antenna array measure-ments were performed in the receiving mode only using the online and offlinedigital beamforming setups described in chapter 8. The measurement resultsshow that the proposed scheme may be successfully applied with both onlineand offline digital beamformers for a practical small array (chapter 8).