Synthesis and Optimization of Time Modulated Antenna Array

Abstract

The thesis designed time modulated antenna arrays using optimization techniques. Conventional antenna arrays use complex feed network with expensive phase shifters to meet the demand of radiation pattern with specific requirements. Time modulation in antenna arrays produces asymmetric radiation pattern with low or ultra-low side lobe level without the use of phase shifters. Time modulated antenna arrays (TMAAs) provides an additional control parameter “time” to achieve the desired amplitude and phase excitation. TMAAs equipped with RF switches are turned on/off using predetermined switching sequences. The switching sequence of RF switches can easily and accurately be calculated and tuned to produce pattern with desired radiation characteristics. Due to ON/OFF switching harmonics or sidebands are generated in TMAAs causing power wastage. The dissertation studied different optimization techniques to design time modulated antenna arrays. It discussed various radiation parameters followed by the mathematical analysis of the TMAA. The thesis simulated TMAAs of half wave dipoles to obtain desired radiation performances. It designed TMAAs using artificial bee colony (ABC) and particle swarm optimization (PSO) and obtained broad side radiation pattern with fixed sidelobe level (SLL) and first null beam width (FNBW) constraining side band level (SBL). It simulated scanned beam pattern with fixed SLL and FNBW using progressive phase shift between array elements. The dissertation designed a TMAA where each element of the array was controlled by radio frequency (RF) switches and excited with a common complex time exponential signal with unit amplitude at the switching frequency resulting in relative amplitude weight and phase difference between the elements at the central frequency without phase shifters. The design applied ABC to compute the switching intervals and produced cosecant squared and scanned beam patterns at the fundamental frequency with reduced side band radiations. Owing to the beneficial properties of printed antennas, the thesis discussed TMAA designs consisted of printed dipoles for achieving patterns with different radiation characteristics. It simulated a printed dipole with microstrip balun in Computer simulation technology (CST). iv After investigating the parametric optimization of ground plane of printed dipole, the thesis designed a single band TMAA and a dual band TMAA of two printed dipoles with different ground planes to demonstrate beam steering without using phase shifters. It designed single band and wide dual band printed power dividers to feed the TMAAs. The elements of TMAAs, coupled with PIN diode RF switches, used a common complex exponential excitation signal and modified the timing sequences of RF switches to create phase and amplitude variations. The timings of RF switches were controlled by a micro-controller based circuit. The phase difference among the antenna elements steered the beam in different directions. In dual band TMAA, SLL and SBL at both the resonating frequencies were reduced to increase the dynamic efficiency of the array using differential evolution (DE). The thesis also designed an 8-element TMAA of printed dipoles to obtain a radiation pattern with a specified SLL and SBL for a fixed half power beam width (HPBW). The design used 1:8 printed power divider to feed the array. Enhanced charged system search (ECSS) optimization was used to compute the timing sequence of the array and desired radiation pattern was obtained with increase in directivity. The optimization programs were written in matlab. The printed TMAA designs were simulated in CST and measured using network analyzer and spectrum analyzer and radiation pattern was characterized in anechoic chamber.