Electric-field distribution near rectangular microstrip radiators for hyperthermia heating: theory versus experiment in water

A rectangular microstrip antenna radiator is investigated for its near-zone radiation characteristics in water. Calculations of a cavity model theory are compared with electric-field measurements of a miniature nonperturbing diode-dipole E-field probe whose 3-mm tip was positioned by an automatic three-axis scanning system. These comparisons have implications for the use of microstrip antennas in a multielement microwave hyperthermia applicator. Half-wavelength rectangular microstrip patches were designed to radiate in water at 915 MHz. Both low ( epsilon _r=10) and high ( epsilon _r=85) dielectric constant substrates were tested. Normal and tangential components of the near-zone radiated electric field were discriminated by appropriate orientation of the E-field probe. Low normal to transverse electric-field ratios at 3.0 cm depth indicate that the radiators may he useful for hyperthermia heating with an intervening water bolus. Electric-field pattern addition from a three-element linear array of these elements in water indicates that phase band amplitude adjustment can achieve some limited control over the distribution of radiated power.