Gain Enhancement of Ferromagnetic Composite Tunable Antenna
Authors:   , Alhassoon Khaled A. , Daryoush Afshin S. 
Published: July 2021

Low cost, small size, and tunable printed circuit antennas are in high demand for both civilian and military applications. The antenna tunability is a required feature in the most recent communication systems since these systems operate at various wireless communication frequencies, while are dramatically scale down in size. By employing the tunable antennas, a single antenna geometry can operate at different frequencies. Various methods and techniques, such as optically and electrically tuned varactor diodes, were reported in the recent years for this purpose of multi-frequency operation, but these solutions have high losses that degrade the performance of the tunable antenna. In addition, the tuning and switching functions are reported using ferrite structures, which are also prone to loss at microwave frequencies. Alternatively, ferromagnetic nanoparticles and ferroelectric have been demonstrated to have high tuning sensitivity covering broad bandwidths at microwave frequencies. Because of [epsilon]r>>1 in ferroelectric materials and [mu]r>>1 in ferromagnetic materials, a compact size antenna can be realized for a variety of wireless applications. This thesis presents design and realization on the FR4 substrate of ferromagnetic based composite annular ring antennas employing nanoparticles. FeCo nanoparticles form a low coercive material and have demonstrated high permeability of the order of 8 and high saturation magnetic moment. A 100 MHz frequency tuning had been obtained at a WiFi frequency band of 2.4GHz by utilizing an external permanent magnet of 1kG. However, a high loss tangent of about 0.25 results in lower efficiency than the baseline annular ring antenna realized without the ferromagnetic material. To improve the gain of the FeCo composite based antenna, the parasitic element stacking technique is proposed to maintain a higher gain tunable annular ring antenna. The directive gain and efficiency of the annular ring patch antennas using a composite substrate material of ferromagnetic type are not high, and passive stacked radiating elements are thus designed and implemented to resonate at the WiFi frequency band of 2.4GHz. The design concepts are in a manner similar to the Yagi-Uda antennas with enhanced antenna gain using passive radiators. The parasitic superstrate elements couple the broadside radiated power in the normal direction of the antenna radiation pattern. After investigating different superstrate materials and thickness, an optimum design was achieved with a 5dB gain and 20% efficiency with only one superstrate of the annular ring mounted on a low loss honeycomb substrate. Further improvement was accomplished after placing the second superstrate, where the ultimate efficiency of 50% was reached. The reasoning behind this antenna gain improvement is that the current of the driven active element is coupled to the first superstrate and this phenomenon is repeated through mutual coupling existing between the first and second passive radiators mounted on them. In other words, the mutually coupled current normally tapers down in the number of passive radiators and results in a higher antenna gain. Finally, as part of future work, alternative design implementation challenges for ferroelectric composite substrates are enumerated in terms of the performance of composite antennas employing TM12, low frequency control and efficiency of microstrip feed structures.