We are developing a microwave hyperthermia system for the treatment of chestwall recurrence of breast cancer. To improve power control of heating applicators, we intend to measure tumor temperature noninvasively during treatment, using radiometry. We are designing single-arm Archimedean spirals for use as receive antennas with a radiometer collecting thermal radiation from different tissue volumes at 1.9-2.3 and 3.7-4.2 GHz. We modeled the antennas numerically. First, we studied the antennas in terms of impedance matching to feedlines. Second, we investigated radiation mechanisms of the spirals radiating into lossy tissue. For small spacing between turns, the surface currents on the spiral were in phase on several neighboring windings, producing strong radiation from a circular, wavelength related region. At these locations, surface currents were also in phase on opposite sides of spiral, contributing to a more centrally peaked radiation pattern with deeper energy penetration than is obtained with a widely dispersed pattern. Finally, we studied the effect of distance from the spiral feedpoint to the radiating region on antenna efficiency. We found this distance should be minimized to reduce power loss from the less useful inner turns of the spirals. The optimization of these design parameters may produce significant improvement of antenna efficiency and improve depth-sensing capability of microwave radiometry.