Textile antennas efficiency

When dealing with wearable textile antennas, several adverse effects, which influence the performance characteristics, often occur. It is very important for the design engineer to be able to efficiently model these effects in order to predict variations in antenna performance. The main adverse effects that are treated in the present literature on textile antennas are briefly summarized as follows. 

For textile antennas without a ground plane, the presence of the human body produces substantial effects on the antenna performance because it acts as an absorber for the radiated field and represents an additional loading. This mainly alters the antenna input impedance and efficiency. Radiation patterns also change with respect to the free-space situation the human body acts as a lossy reflector, making the radiation pattern more directional in the off-body direction. An example of such a behavior, on a wearable antenna without ground plane, is shown in Fig. 26.5, where the on-body and free-space horizontal pattern of a wearable UWB monopole antenna on a polyi-mide substrate (realized by the EM group of Ghent University), are compared. 

For wearable systems, including textile antennas, power efficiency represents a crucial matter, especially for the so-called autonomous systems, where the necessary power for the system operation is ideally obtained entirely by means of energy harvesting [16] from the surrounding environment, and no additional power supply is needed. Even in the case of wearable systems equipped with wearable battery units, it is very important to keep power consumption as low as possible. To this aim, several techniques were recently envisaged, by means of innovative textile antennas, such as active antennas as well as the use of multiantenna-processing techniques, such as diversity with multiple wearable antennas. 

F. Boeykens, H. Rogier, L. Vallozzi, An efficient technique based on polynomial chaos to model the uncertainty in the resonance frequency of textile antennas due to bending, IEEE Trans. Antenn. Propag. 62 (3) (2014) 1253—1260. 

In summary, we remark that embroidered textile antennas indeed demonstrated no compromise on antenna performance and efficiency. This is in addition to their... 

Introduction of microelectronic microwave components close to the antenna demands for appropriate interconnect technologies on textiles. This aspect has been dealt with in various publications, both with woven and non-woven materials However, only little data is available on the microwave properties of different woven and non-woven materials This deficiency must be overcome. Interconnection to the microwave monolithic integrated circuit is an important area of research and development. One possible approach is the implementation of ribbon type interconnects, which can efficiently be used for power supply and low ficquency outyut signals. 

Efficiency to maximize the radiated power, the antenna designer wiU aim at a large radiation efficiency. As previously discussed, the total efficiency is the product of two terms. The first one, the impedance mismatch factor M, can he maximized by minimizing the reflection coefficient. The second term, the conductive-dielectric efficiency Ced can be maximized by using textile materials with low ohmic and dielectric losses, ie, electrotextiles with high conductivity and textile dielectric substrates with low tan 6. 

A microstrip patch is a representative candidate for a wearable integration, because it can be thin, lightweight, low maintenance, robust, and easily integrated into a garment and coupled with RF circuits (Wang et al., 2012). Moreover, the conductive textile used for antenna purposes has to have a low and stable electrical resistivity (<10/sq.) to minimize losses (Locher et al., 2006). Several properties of the materials can influence the behavior of the antenna properties. For instance, the permittivity and the thickness of the substrate change the bandwidth and the efficiency of a planar microstrip antenna (Liu et al., 2011). In general, fabrics present a complex structure, in term of density of fibers and hence air volume and size of the pores, which allow a very low dielectric constant with a reduction of the surface wave losses and an increase of the impedance bandwidth. 

Giddens, H., et al., 2012. Influence of body proximity on the efficiency of a wearable textile patch antenna. In 2012 6th European Conference on Antennas and Propagation (EUCAP), pp. 1353-1357. 

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