  Microstip antenna topology is commonly loaded with a narrow slot to
manipulate the resonance frequency or impedance bandwidth. However, the tuning
of the resonance frequency or impedance bandwidth results in the variation of
the current and field distributions. In this regard, this work adopts the
concept of characteristic modes to gain an initial understanding of the
perturbation mechanism of the rectangular patch when loaded with a slot. The
performance of microstrip antennas with finite ground plane is then studied
using full-wave simulation. It has been found that the distribution of the
induced current density is highly dependent on the orientation of the slot The
incorporation of a narrow slot suppresses the nearby orthogonal eigen mode and,
as a consequence, the radiation behavior is affected. Specifically, in the
presence of biological tissues in the near-field region, both antenna input
impedance properties and the realized gain are dependent on the slot
orientation. Different examples are included for understanding the impact of
slot loading on the energy absorption by biological tissues, by calculating the
the specific absorption rate (SAR). The proposed analysis facilitates the
design of miniaturized antenna geometries for biomedical applications via
systematic loading of narrow slots.
