Measurement errors introduced by the use of co-axial cabling in the assessment of wearable antenna performance in off-body channels

This paper presents the results of an investigation into the effect of using co-axial cables in ultra-wideband off-body radio channel characterisation and performance evaluation for wearable antennas. Experiments were carefully designed to faithfully compare the use of a co-axial feed cable for a wearable antenna versus an optic fibre feed, and thus report on any errors introduced into the measurements due to the use of such reflective cabling. Detailed results are presented for a range of body-centric antenna positions for stationary measurements and general observations for mobile tests are also introduced and discussed. Presented results show that the use of co-axial cables has a marked effect on the radio channel characterisation, affecting received power, mean delay and delay spread results, seen to greater extent in low reflection environments and for non line of sight and highly shadowed configurations. Co-axial cable-fed antenna tests yielded greater received power than with an optic-fibre feed for a user-stationary scenario in low reflection environments. As either the line of sight component or the measurement environment´s reflectivity was increased, the difference between the two systems diminished. It was also found that the use of a co-axial cable altered the statistical fading channel model for mobile tests.