The demand for wide-band small antennas is steadily increasing for both civilian and military applications due to the explosive growth of wireless communications systems.
Linearly polarized electrically small antennas can be generally classified as TM10 and TE10 mode antennas. For a TM10 mode antenna, the input impedance of the antenna is considerably reactive with a small real part. In contrast, the input admittance of a TE10 mode antenna is characterized by a high susceptance and a small conductance, i.e. the input impedance is almost a short. It is therefore critical to match the antenna to a receiver (or transmitter) to optimize the transfer of power in the frequency range of interest. With conventional passive matching networks, the
antennas can be only matched over narrow frequency bands. However, Non-Foster matching networks composed of negative capacitors and/or inductors can in principle match the antenna over wide frequency bands because Non-Foster matching networks can overcome the gain-bandwidth restrictions derived by Bode-Fano.
In this dissertation, the design, implementation, and measurement of two Non-Foster matching networks for a TM10 mode antenna and a Non-Foster loading network for a TE10 mode antenna are the topics to be discussed, which improve performance of both types of electrically small antennas over broad frequency ranges. These devices take advantage of the unique property of Non-Foster impedances, counter-clock wise rotation on the Smith chart as the frequency increases.
First, a systematic methodology is introduced to design a Non-Foster matching network for an electrically small antenna. Key steps in the proposed methodology are presented to demonstrate how to realize a fabricated Non-Foster capacitor for a 3′′ electrically small monopole receiver antenna. Based on experimental results, it is verified that Non-Foster matching networks will improve both the antenna gain and the signal to noise ratio.
Second, a Non-Foster matching network with a series connection of negative capacitor-inductor for the same 3′′ small monopole antenna is fabricated and tested. This Non-Foster matching network improves the performance of the antenna to higher frequencies. Through measured and simulated data, it is shown that the antenna with
a negative capacitor-inductor has an advantage over both the antenna with and without a negative capacitor. To the best of our knowledge, it is the first time that a series combination of a negative capacitor-inductor has been demonstrated with measured
data when connected to an actual antenna.
Lastly, this dissertation discusses a method to improve the performance of a small loop antenna by using a Non-Foster inductor loading. The Non-Foster loading network (located away from the input port of the antenna) is employed to improve the impedance mismatch and also maintain an omni-directional antenna pattern at higher frequencies than the case of a loop antenna with a Non-Foster matching network and a short loading (stands for a small loop antenna). Although the experimental radiation patterns are somewhat different from the simulated data, it is found that the
measured antenna gain and SNR with the Non-Foster inductor loading are improved when compared to case with a short loading. To the best of our knowledge, it is the first time that a fabricated Non-Foster impedance loading network was applied to an
actual antenna.