In this paper, effects of the parasite elements on an antenna impedance of a UHF RFID tag put on a high impedance surface (HIS) are experimentally studied in detail. It is shown that small parasite elements on a mushroom HIS structure can help to recover a mismatch of the impedance and this impedance recovery is brought by an in-phase frequency shift of the HIS due to a mutual coupling between the HIS and the parasite elements. The technique is applied to a commercial 953 MHz band RFID tag inlet antenna on a 53-cell HIS with the total dimension of 125751.5 mm3 and it is demonstrated that the impedance mismatch is successfully recovered and the tag operates with a reading range of 3 m even on a 2003002 mm3 aluminum plate.

The key concept of Physical Optics (PO), originally developed for a perfectly electric conductor (PEC), consists in that the high frequency fields on the scatterer surface are approximated by those which would exist on the infinite flat surface tangent to the scatterer. The scattered fields at arbitrary observation points are then calculated by integrating these fields on the scatterer. This general concept can be extended to arbitrary impedance surfaces. The asymptotic evaluation of this surface integration in terms of diffraction coefficients gives us the fields in analytical forms. In this paper, uniform PO diffraction coefficients for the impedance surfaces are presented and their high accuracy is verified numerically. These coefficients are providing us with the tool for the mechanism extraction of various high frequency methods such as aperture field integration method and Kirchhoff's method.