Directional handset antennas and human head at Ka-band: importance of skin layer

To achieve wideband data transmission associated with multimedia, communication systems operating at microwave frequencies such as Ka-band (26.5-40 GHz) are recently drawing much attention in terrestrial and satellite communications. Naturally, the electromagnetic interactions with human body at these high frequencies would be of great interest for the design of personal communication terminal devices. Popular numerical techniques such as Finite-Difference Time-Domain (FDTD) method [l]-[3] have severe limitations in evaluating EM interactions at those high frequencies due to excessive computer storage requirements and computation time. For example, the size of a typical adult head (18 cm) is -75 (dielectric wavelength) at 30 GHz. On the other hand, Eigenfunction Expansion Method (EEM) can be utilized to efficiently assess the EM interactions associated with thin biological layers (e.g. skin) at high frequencies. It has been found that the unaVcraged peak specific absorption rate (SAR) at the skin layer can be very high with non-directional (monopole or dipole) antennas. This is because most of the radiated energy of the antenna is absorbed in the skin layer. For example, the unavcraged peak SAR is -450 W/kg with 1W of the delivered power at 30 GHz, and the 1-g averaged peak SAR is ~ 10 W/kg. However, the peak SAR-and the absorbed power-can be greatly reduced by employing the directional antennas; e.g. the unaveraged peak SAR is -3 W/kg and the 1-g averaged peak SAR is -0.1 W/kg. It can be shown that the directional antennas are more efficient at Ka-band as compared with those at much lower frequencies (e.g. 900 MHz or 1.9 GHz). In this paper, we will discuss the power absorption properties and radiation patterns of directional and non-directional antennas at Ka-band.