Interference due to unwanted electromagnetic waves has become a significant concern along with the development of high-speed digital systems, microwave devices and wireless communication equipment. These electronic products should satisfy the electromagnetic compatibility (EMC) requirements to obtain access to some of the worlds markets. The upper bound of EMC regulation frequency is increasing up to the microwave frequency range in these days.
Electromagnetic anechoic chamber equipped with electromagnetic wave absorber has been widely used as an accurate and convenient environment for EMC test in place of an open area test site (OATS). Therefore, it is necessary to study broad bandwidth wave absorber to keep up with the upper limit of increasing EMC regulation frequency. Absorbers with excellent performance were developed from the mid-1980s when the techniques of calculating and measuring their reflectivity were introduced. However, these researches concentrated on urethane pyramid or wedge absorbers, and urethane hybrid absorbers, which have electrically thick dimension to meet lower frequency limit. Electrically thin ferrite absorbers were developed as an alternative to urethane pyramids and wedges, but have poor characteristics above 1 GHz. Nowadays, ferrite/ferrite hybrid absorbers were invented to extend the absorption bandwidth.
In this dissertation, various kinds of ferrite/ferrite hybrid structures are presented and analyzed using finite difference time domain (FDTD) method. Ferrite is a frequency dispersive material with high permeability. Electromagnetic wave absorption mechanism of this absorber is impedance matching and magnetic loss. Therefore slight variation in the dimension of ferrite absorber can make large difference in absorption characteristics. So, the accurate calculation method for ferrite geometry is needed.
FDTD method is a useful computational technique for a wide variety of problems in electromagnetic radiation and scattering. In the dissertation, it is modified to calculate the ferrite/ferrite hybrid absorber as follows.
First, locally tensor conformal FDTD (LTCFDTD) method for modeling arbitrarily shaped dielectric surfaces is introduced. This new algorithm is based on an effective medium theory that makes effective dielectric tensor. The LTCFDTD method can be applied to frequency dispersive material with easy. The curved and slanted ferrite boundary is calculated with the LTCFDTD method. The FDTD analysis of TE scattering by rotated two-dimensional (2-D) dielectric cylinders shows excellence of the LTCFDTD.
Second, new modified perfectly matched layer (NMPML) is presented to absorb large evanescent modes caused by re-radiation process between the ferrite array structures. The Berenger's PML can absorb propagation modes more than 100 dB but cannot absorb evanescent modes at all. It only acts as free space with thickness of PML for evanescent modes. The modified PML (MPML) can absorb evanescent modes but not sufficient. For verification of the NMPML, the reflection coefficients of parallel plate waveguide with other absorbing boundary conditions are compared.
With these two modifications, ferrite pyramid absorber, ferrite cone absorber, ferrite rounded cone absorber and ferrite arbitrary cone absorber are designed and optimized by calculating their absorption characteristics. The optimized absorbers are suitable for broad bandwidth electromagnetic absorber. The comparison of numerical results for the pyramidal ferrite absorber with measured data in the frequency range from 30 MHz to 3 GHz shows good accuracy of the presented method.
