Effect of practical layered dielectric loads on SAR patterns from dual concentric conductor microstrip antennas

Radiation patterns of 2 and 4cm square Dual Concentric Conductor (DCC) micr ostrip antennas were studied theoretically with Finite Difference Time Doma in (FDTD) analysis and compared with experimental measurements of power dep osition (SAR) in layered lossy dielectric loads. Single and array configura tions were investigated with 915 MHz excitation applied across either one, two or four sides, or four corners of the square apertures. FDTD simulation s were carried out for realistic models of a muscle tissue load coupled to the DCC antennas with a 5 mm thick bolus of either distilled water or low l oss Silicone Oil. This study characterizes the effect on SAR of adding thre e additional thin dielectric layers which are necessary for clinical use of the applicator. These layers consist of a 0.1mm thick dielectric coating o n the array surface to provide electrical isolation of DCC apertures, and 0 .15mm thick plastic layers above and below the bolus to contain the liquid. Experimental measurements of SAR in a plane 1 cm deep in muscle phantom ag ree well with theoretical FDTD simulations in the multi-layered tissue mode ls. These studies reveal significant changes in SAR for applicator configur ations involving low dielectric constant (epsilon(r)) layers on either side of a high epsilon(r) water bolus layer. Prominent changes include a broade ning and centring of the SAR under each aperture as well as increased SAR p enetration in muscle. No significant differences are noted between the simp le and complete load configurations for the low epsilon(r) Silicone Oil bol us. Both theoretical and measured data demonstrate relatively uniform SAR d istributions with > 50% of maximum SAR extending to the perimeter of single and multi-aperture array configurations of DCC applicators when using a th in 5 mm water or Silicone Oil bolus.