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.
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.
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,
- Low maintenance,
- No set-up requirements,
- No damage from obstacles (robust),
- Unnoticeable (military).
The antenna polarization is defined by the selected application and in this particular case the antenna has to be 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 studied. The principle of the antenna structure and can be seen in the figure below. The dot in the patch is the antenna feed point.
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.
Measured 3D radiation patterns are shown in the next figures for bent and unbent antennas to demonstrate the 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. Measured return loss of the final manufactured antennas is shown in figure below. The results indicate good repeatability between two individual antennas (-10 dB bandwidth).
Finally, measured total efficiency under various bending conditions is shown in the next figure. Bending has moderate effect on the antenna total efficiency.
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.
The project was successfully finished on the 30th of October 2009 as planned in the original schedule.
The project main achievements are summarized below:
- Suitable materials for the antenna were selected and characterized,
- Antenna structure maintaining circular polarization under bending was developed and analyzed,
- The antenna was manufactured using purely textile materials,
- Several antenna samples were manufactured and measured. Measurement results showed good repeatability.
- The achieved total efficiency is comparable with rigid antennas having solid copper metallization,
- The textile antenna has been demonstrated to be operational in real life applications. The highlight of the project was a phone call with Iridium phone employing the developed antenna (web link to the published news): http://www.esa.int/esaCP/SEMQKBKIWZF_index_0.html
- Manufactured antenna fulfils and exceeds the project goals:
- Operability demonstrated using standard Iridium phone (primary goal),
- Operability demonstrated using standard GPS receiver (additional feature).
- Project was finished on schedule.