Electric and magnetic fields have been known to be a manifestation of the same physical phenomenon for almost two hundred years, but some aspects of electromagnetism are not in balance in nature around us. We are faced with restrictions due to lack of real magnetic charges, currents, and conductors. However, it has turned out that virtual magnetic currents and conductors and magnetic materials can be engineered and used in a variety of applications, antennas being one amongst them. This doctoral thesis focuses on magnetic materials and artificially created magnetic responses in antenna applications. The thesis is divided into three parts concerning antennas in which the magnetic part of Maxwell's equations plays a key role. The common goal in each of these parts is to improve the properties of the antennas in question or to conjure new types of antennas. The first part of the thesis deals with antenna miniaturisation using magnetic materials. A new method to determine the optimal material for a given small resonant antenna is proposed. The method is based on identifying the radiating field (equivalent current) of the antenna. A mobile-terminal antenna is miniaturised with dielectric and magneto-dielectric (both dielectric and magnetic responses) materials. Guidelines for an objective comparison of filling materials in antenna miniaturisation are presented. In the second part the antenna in question is based on a mushroom type high-impedance surface (HIS). The modes of an HIS are studied by using an analytical model for oblique incidence angles of the incident plane wave. Two of the resonance modes responsible for the HIS behaviour in the infinitely-sized surface are identified and used as a basis for designing a finite antenna with two orthogonal radiation patterns. A helical antenna geometry based on wires and loops is studied in the third part. By using a combination of short electric dipoles and loops acting as magnetic elementary dipoles, a Huygens' source antenna for circular polarisation (CP) from two helices is designed. Huygens' source antenna has a unidirectional radiation pattern and perfect polarisation purity: In theory it can radiate perfect CP in every direction, making it, for example, an attractive receiving antenna for space-to-Earth communications. The geometry of the antenna has also a peculiar effect when an incoming wave is scattered from the object, as the scattered field is directed away from the object when the incoming field arrives from certain directions.
|Translated title of the contribution||Magneettiset materiaalit ja vasteet antennisovelluksissa|
|Publication status||Published - 2012|
|MoE publication type||G5 Doctoral dissertation (article)|
- magnetic material
- mobile antennas
- high-impedance surface
- receiving antennas