Abstract: This dissertation discusses the approach and benefits of implementing an automatic antenna tuning unit (ATU) for the further development of software defined and cognitive radio. The focus of the topic of this dissertation is to simplify radio frequency (RF) front-end design for multi-band, multi-mode radios by using reconfigurable antennas as a frequency-agile selective component and develop appropriate analog and mixed-signal control circuitry as well as algorithms to allow these antennas to be reconfigured on demand by the radio's baseband processor. Electrically small antennas (ESAs) are utilized as tunable filters as well as radiating elements to simplify the RF front-end design, which has the potential to lower the cost of next generation commercial and military radios while simultaneously enhancing performance. The automatic tunability of the frequency selective antenna is achieved by a closed-loop ATU. In implementing the ATU, impedance synthesizers, i.e., tunable matching networks, are developed to match a more or less arbitrary load to a convenient impedance value, usually 50 Ω. To generate feedback data that can be used to optimize the impedance synthesizer, the incident and reflected powers at the input to the impedance synthesizer are measured using a power sensor block comprising directional coupler, power detector diodes, and analog-to-digital converters (ADCs). In order to validate the design of the ATU, a hardware implementation of the ATU prototype has been demonstrated to verify narrowband automatic tuning ability of the ATU under constantly changing environment conditions.