This chapter has shown that efficiency measurements performed using textile antennas on live human beings can be performed in an accurate and controlled manner, with the repeatability being as close as 2%. The magnitude of on-body losses experienced by a given textile antenna with a small ground plane is seen in this case to be a function of the material properties of that antenna - a higher conductivity, thinner (copper based) textile material which is more efficient in free space is seen to perform worse when placed on body as compared to a lower conductivity and lossier free space textile antenna with the same overall design topology. This is proved to be due to the fact that the lower conductivity material based antenna has given rise to lower electric fields in the body as more power has been lost in the antenna itself for the same input power. This is a remarkable result from this study, constituting new knowledge which can have a profound impact on the material choice for these small sized antennas, in the sense that a higher conductivity material would appear not always to be the best option when operating in close proximity to a human being.
The magnitude of efficiency losses on body has been experimentally shown to be mitigated somewhat by a variation in the distance from the body - a small 20 mm distance from the body (for antenna SHSL) in this case was sufficient to show that a reduction in radiation efficiency can be eliminated by up to 22%. For the single band (SHSL) antenna, at higher frequencies the 20 mm (off) body result approached the radiation efficiency value in free space.
From the experiment that looked to assess the effect of loading the antenna via bending, one can conclude that this aspect is not good practice - the antenna was severely loaded for a considerable period of time (more so than for example if placed on a body part that relaxed the antenna from time to time), and thus the de-tuning performance was seen to be severe in this scenario. From the results seen here this condition cannot be recommended.
For the dual band antennas, the investigation has determined that the higher conductivity based material provides a more resilient operation on-body in terms of its efficiency performance. The results also show this aspect is valid both in free space and on-body operational roles which in this case is entirely expected. This facet is different from the single band study, thus it can be surmised that in addition to the conductivity of the textile material, the ground plane size of the antenna is also a crucial design parameter.
Two facets have presented themselves in the dual band study. Firstly, the ShieldIt material selection had a tendency to frequency de-tune in the lower band of operation. In this case it manifested itself as a frequency shift of 90 MHz. Therefore, any future designs using the shieldIt material that aim to operate on-body should aim to compensate for this de-tuning in the lower band by designing the initial centre frequency with the body in mind to account for the frequency shift. Potentially, another option remains here to increase the distance between the antenna and human subject where we expect reduced coupling. For the copper based antenna, obtaining an enhanced impedance match in the initial design would solve the problem in the lower band.
Secondly, although both measurement and simulated results showed the higher conductivity material used in this study to perform better (in terms of efficiency at the dual band frequencies), the fabricational construction of the copper tape material should be improved for a more robust design. The adhesive backing used for the copper tape construction has a tendency to degrade over time meaning that the edges of the Sierpinski gasket began to peel and deform - this is believed to have affected the practical high frequency efficiency performance of the antenna. A more robust method might be to use a copper based textile and stitch this to the fleece substrate instead. On the whole however, the investigation has confirmed the textile antenna is a strong candidate for use in the on-body role. With regards to the measurement practices and facility, the methods utilised have yielded an accurate and repeatable series of results; this validates the RC‟s use for this type of measurement. Repeatability in the dual band measurement results is also seen to be in the order of 2% at the maximum and the uncertainty in the order of 0.2 dB, which proves that any (inevitable) subtle human movements have not affected the accuracy.
It has also been proved that the statistical measurement environment is consistent irrespective of different on-body locations or different human subjects. This is a useful fact, proving the suitability of the RC facility.
3.14 References
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