An analysis of the bandwidth oscillations caused by finite ground planes using characteristic mode theory

Antennas;finite ground planes;characteristic mode theory;bandwidth;electrically small antennas

Bohannon, Nicole ; Bernhard ; Jennifer T.

关键词: Antennas;    finite ground planes;    characteristic mode theory;    bandwidth;    electrically small antennas;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/44094/Nicole_Bohannon.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

It has long been known that the impedance bandwidth for planar inverted-F antennas (PIFAs) changes as the rectangular ground plane changes length. Although previous research has characterized these changes, it has failed to adequately explain why the bandwidth and pattern changes occur. This thesis explains why these changes in bandwidth and radiation occur by creating a method for separating the effects of the ground plane from the effects of the antenna element. By replacing the element with an infinite ground plane, the structure can be analyzed including the effect of the feed and height, without including the antenna element.This structure is then analyzed using characteristic mode theory to correlate the modal behavior of the ground plane with the bandwidth minima and maxima. Overall, bandwidth minima occur for ground plane sizes where only one mode has the highest modal significance across the band, and bandwidth maxima occur when two modes shift the mode with the highest modal significance near the center frequency of the antenna. Because the developed process is not specific to PIFAs, it is then applied directly to two different planar electrically small antennas (ESAs). The narrow bandwidth that plagues ESAs makes it particularly attractive to understand where bandwidth maxima occur to create optimal designs. At first glance the process seems to fail to predict maxima for some ground plane lengths because the ground plane size where the two modes switch is slightly larger than predicted. However, the characteristic mode simulations must be done using a perfect electric conductor (PEC), whereas the bandwidth simulations are done using copper. By investigating the effect of using copper versus PEC, the shift in center frequency is quantified. Using PEC significantly lowers the center frequency of the antenna, causing the characteristic mode model to show the transition at a larger ground plane size.