|
Flexible antennas are becoming evermore attractive, since the recent developments in wearable computing have opened several possibilities to integrate wireless functions to clothing. The scope of this study is to demonstrate that flexible antennas are feasible in personal satellite communications. The project work contains complete design flow, from substrate material selection and characterization to actual design implementation and verification. The resulting demonstrator antenna is capable to operate at Iridium band under nominal human body environment as part of clothing. This is the ultimate goal and the actual measure of success from the project point of view. The overall objective of this activity is to study and analyze the viability of the textile (flexible) substrate materials for antenna elements and arrays. More specifically, the objective is to demonstrate compliance with satellite antenna user requirements using textile (flexible) substrate materials for antenna elements and/or arrays. This shall be achieved by design, manufacture and testing of a breadboard textile antenna taking into account operational conditions. Selected demonstration application is the Iridium satellite phone system. The Iridium satellites operate at low altitudes (Low Earth Orbit, LEO) which make the communication possible between satellite and end-user without huge antennas. There is a coarse picture below about the use case, where textile antennas (blue patch) are located on both shoulders. Designed textile antenna shall be compliant with the Iridium system. 
click for larger image
Project output in hardware point of view is the antenna prototypes. No additional electronics e.g. antenna switches or transceivers are developed under this project.
The selected application defines the antenna polarization. It requires that the antenna is circularly polarized. The operational frequency band is the Iridium band (L-band). Since the antenna is intended to be integrated as part of clothing it is considered to be self-conformal. It means that flexible wearable antennas can adapt its form according to the body where the antenna is attached. In addition, the self-conformal antennas are capable of re-adjusting its form as a function of time. There is a figure of the antenna EM-simulation model and the preliminary antenna prototype built on flexible substrate. However, this is only a glimpse of the complete selection of different flexible substrate materials under study. The preliminary antenna structure and patch shape can be seen from the figure. The dot in the patch is the antenna feed point. It is emphasized that this is a preliminary design which requires further iterations. 
click for larger image
Final implementation of the textile antenna is presented. Flexibility is demonstrated by bending the antenna as shown in the figure below. The structure of the antenna allows it to be bent similar way in each direction: vertical, horizontal and diagonal. Furthermore, the antenna meets the electrical specifications under bending conditions. Radiating element is shielded against environmental conditions using protective clothing. 
click for larger image
Measured 3D radiation patterns are shown for bent and unbent antennas to demonstrate the designed antenna performance. Axial ratio values in the 3D radiation patterns are in dB. It is emphasized that the designed antenna maintains circular polarization even under bending conditions, which is commonly recognized to be hard to achieve with soft, wearable antennas. 
click for larger image
Project started on 1st May 2008. The total project length is 18 months. The work to be done during this time is divided into ten separate Work Packages (WP): WP0: Management & Project control
WP1: Survey of current state of the art textile antennas
WP2: Application selection and establishing the requirements
WP3: Preliminary antenna design
WP4: Study & selection of textile materials
WP5: Characterization of textile materials
WP6: Detailed textile antenna design
WP7: Manufacturing of textile antennas
WP8: Measurement of textile antennas
WP9: Conclusions & development plan The policy to monitor project status is to arrange progress meetings in 2 month intervals. Moreover, design review meetings will be organized according to the technical milestone schedule. Project will end 30th of October 2009.
There are three main issues. First issue is related to the antenna substrate material selection. It is needed to know the electrical behaviour of the material in order to select suitable substrate. It is common that this material data is not available and therefore electrical characterization of different textile materials is required. This is not a completely trivial task and different measurement techniques have to be used in order to evaluate the validity of the extracted parameters, e.g. dielectric constant. Second issue concerns the antenna performance under stress. Since antenna is assembled in clothing it experiences different kind of bending conditions. The real issue is to maintain the critical antenna parameters at acceptable levels in all conditions regarded as normal operation environment. Such parameters are: - Axial ratio (robust against bending),
- Operating frequency (mitigation of detuning due to bending),
- Efficiency (prevent human tissue to degrade efficiency).
Third issue deals with the actual manufacturing. The main concern is how to make the antenna robust enough against manufacturing tolerances.
The main benefits of the textile antennas are summarized: - Light weight,
- Inexpensive,
- Low maintenance,
- No set-up requirements,
- No damage from obstacles (robust),
- Unnoticeable (military).
The project has been on-going now for a bit over fourteen months and the work has proceeded as planned. Achievements so far include the completion of the survey of the state of the art textile antennas. Furthermore, the demonstration application has been selected and the requirements for the antenna are defined. Preliminary antenna design has been made in order to determine the antenna patch geometry. Textile substrate has been selected and electrically characterized.The antenna structure is iterated to its final implementation during detailed antenna design. As a result, compliant antenna for the selected application is achieved. Several antenna samples of the final design have been manufactured for measurements. Currently the measurement of the textile antennas is on-going. This means that the WP8 is running. The final work package (WP9) will start after the measurements are successfully completed.
Last Update: 13 Aug 2009
| 
