Theoretical limits of SAR distributions of a four-element square array of dipole-type antennas

Four-element dipole microwave antenna arrays with square insertion patterns are commonly used clinically for interstitial hyperthermia. One major disadvantage with this type of antenna array is the presence of a large dead length at the tips because the current gradually decreases from maximum at the junctions to zero at the tips. This dead length is usually 1.5-2 cm along the central axis of a 2 x 2 cm array of regular dipole antennas. Many attempts to improve the performance of dipole antenna arrays have been made by designing antennas with increased current at the tips. While some dipole antennas of new design show negligible dead lengths at close proximity in the single-antenna configuration, phantom experiments have demonstrated that these antennas exhibit at least 1.1 cm dead space along the central axis of the four-antenna array. Therefore, there seems to be a limit to which the array dead length can be reduced by the improvements in the dipole-type antenna design. The goal of this work is to find the theoretical minimum of the array dead length. This was done by assuming a uniform current distribution along the entire antenna. The specific absorption rate (SAR) patterns were calculated for an array with an insertion depth of 7 cm (resonant length for 915 MHz) and a variable spacing between antennas (1-3 cm). It was found that there is a dead length of 6 mm along the central axis of the 2 x 2-cm array with the uniform current distribution, which can be considered as the theoretical limit of the dead length for this array.(ABSTRACT TRUNCATED AT 250 WORDS)