Abstract
Ferromagnetic core loop antennas with high permeability core materials are typically used in size constrained applications requiring the detection and reception of weak Very Low Frequency (VLF) and Extremely Low Frequency (ELF) signals. However, these antennas have two limitations that constrain their performance in real world applications. First, the high permeability materials are susceptible to saturation in the presence of strong magnetic fields such as those generated by power lines, in research laboratories, or through natural phenomena. Second, the demagnetizing field inside the antenna core limits its effective permeability and prevents receiving the full benefit of the core material's soft ferromagnetic properties.
Here, a system is presented that actively controls the magnetic flux in the core of the antenna by applying a control signal to an independent set of windings on the same core. This control signal is phase locked to the offending signal and the two signals destructively combine in the core. Also, methods of optimizing core geometry are explored in an attempt to improve antenna performance. In total, eleven experimental core shapes were simulated using finite element analysis electromagnetic simulation software.
The results show that it is possible to actively cancel the fundamental component of a strong interferer in the core of a high sensitivity ferromagnetic core loop antenna and prevent core saturation. This approach eliminates signal distortion caused by magnetic saturation that passive signal cancellation or signal processing in the receiver do not. Thus, high sensitivity ferromagnetic core antennas can be used in applications where they were previously thought unsuitable. Furthermore, simulation and testing of antenna core prototypes have shown that modifications to the geometry of the antenna core's ends can improve effective permeability and result in better antenna performance.
