Human proximity effects on circular polarized handset antennas in personal satellite communications

Abstract

Satellite-based systems are the next step in mobile communications. Recently, several low and medium earth orbit mobile communication satellite systems have been proposed and are currently being deployed. For all these systems, high-performance circularly polarized antennas for the 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 presents an analysis of representative circularly polarized helical handset antennas. Although other physically less obtrusive circular polarized antennas are available for the handset, the helix is used because of its wide bandwidth properties and capability to provide both circular and linear polarization. Thus, this element could be used for handsets that are planned to work with both the terrestrial-based communications systems of today and the satellite-based communication systems of tomorrow. The intent is to characterize the effects the close proximity of a human head model has on the computed performance of the circular polarized antenna. The method of moments (MoM) and finitedifference time-domain (FDTD) numerical techniques are used to study various helix structures on top of a small box representing a handset. In order to be able to effectively apply FDTD, a novel square helix structure is introduced. Results computed with these two techniques are compared to illustrate the accuracy of each implementation. The results indicate significant polarization purity degradation caused by the presence of a human head. For the particular geometry simulated, the presence of a head model degraded the average axial ratio within a verticle 50 cone from 2.9 to 9.1 dB. This significant increase in axial ratio can have profound effects on link budgets.

Theoretical Approach of the Interaction Between a Human Head Model and a Mobile Handset Helical Antenna Using Numerical Methods

“…The presence of the human head significantly distorts the circular polarization pattern of a helix mounted on a mobile handset, as it is mentioned in [6], leading in an increase in the axial ratio. When compared to a monopole antenna, a helix of an about 40% smaller length appears to have a narrower impedance bandwidth as reported in [4] while inducing higher values of Specific Absorption Rate (SAR) at the mobile communications frequencies.…”

“…In [4][5][6] the standard helix antenna mounted on a mobile handset is replaced by an equivalent square-helix structure in order to be accurately modeled within the FDTD scheme. The results obtained appear to be rather approximate, compared with those arising from the application of the MoM or conducted measurements.…”

Investigations of polarization purity and specific absorption rate for two dual-band antennas for satellite-mobile handsets

“…Reasonably good circular polarization coverage is observed over the elevation angle range of 640 for both bands of interest. The circular polarization purity for both bands decreases in the presence of the head by 2 to 5 dB, which is much better than for the single axial mode helix [1].…”

“…Further, specific absorption rate (SAR) assessments in comparison with terrestrial mobile handset antennas have been presented in [2] and [3]. In [1] it was found that the presence of the head degraded the average axial ratio within a 50 cone by between 2.9 and 9.1 dB, which can have a significant effect on system performance. The antenna was rather larger than would be realistic, but the intention was to characterize the polarization issue only.…”

“…Degradation of circular polarization purity (equivalent to degradation of axial ratio performance) of radiated fields from satellite-mobile handsets in the proximity of the human head has been addressed in the past [1]. Further, specific absorption rate (SAR) assessments in comparison with terrestrial mobile handset antennas have been presented in [2] and [3].…”