Calorimetric evaluation of antennas used for microwave interstitial hyperthermia

Several quasi-adiabatic calorimeters have been constructed for determining the heating efficiency of microwave antennas. The absorber is 0.9% saline which is stirred continuously, making equilibration times short (several seconds). Depth of insertion data have been obtained for standard BSD and CTC antennas at 915 MHz. Efficiency varies only slightly with insertion depth in the range 7-16 cm and can be as high as 65%. At shallower depths efficiency drops off, while reflected power and leakage fields increase. Data obtained in site visits conducted by the Hyperthermia Physics Center show that antenna efficiency of the BSD and CTC models can vary by as much as 35% within sets of functional 'identical' antennas.