HUMAN PROXIMITY EFFECTS ON CIRCULAR POLARIZED HANDSET ANTENNAS IN PERSONAL SATELLITE-COMMUNICATIONS

Satellite-based systems are the next step in mobile communications. Re cently, several low and medium earth orbit mobile communication satell ite systems have been proposed and are currently being deployed. For a ll these systems, high-performance circularly polarized antennas for t he mobile terminals are of importance. Although considerable material is available on circularly polarized antennas, there is an absence of information on how the human's close proximity to the antenna affects the circular polarization purity of the radiated field. This paper pre sents an analysis of representative circularly polarized helical hands et antennas. Although other physically less obtrusive circular polariz ed antennas are available for the handset, the helix is used because o f its wide bandwidth properties and capability to provide both circula r and linear polarization, Thus, this element could be used for handse ts that are planned to work with both the terrestrial-based communicat ions systems of today and the satellite-based communication systems of tomorrow, The intent is to characterize the effects the close proximi ty of a human head model has on the computed performance of the circul ar polarized antenna, The method of moments (MoM) and finite-differenc e time-domain (FDTD) numerical techniques are used to study various he lix structures on top of a small box representing a handset. In order to be able to effectively apply FDTD, a novel square helix structure i s introduced. Results computed with these two techniques are compared to illustrate the accuracy of each implementation, The results indicat e significant polarization purity degradation caused by the presence o f a human head. For the particular geometry simulated, the presence of a head model degraded the average axial ratio within a verticle 50 de grees cone from 2.9 to 9.1 dB, This significant increase in axial rati o can have profound effects on link budgets.