Soldiers performing dismounted operations in the field use radios that have antennas with a distinct visible signature and can become easy targets. These antennas also tend to snag on other equipment or vegetation, creating a hazard and a distraction to any ongoing operation. Therefore, it has become necessary to develop an antenna that can conform to soldiers and be virtually indistinguishable from a soldier’s body armor.

The Patch Antenna (red box) is shown attached to the shoulder of an Army-issued jacket using a hook-and-loop connector. The antenna can be connected to a radio with a cable running down the back.
Traditional antenna materials such as metals tend to break under repeated cycles of loading and unloading, which makes them undesirable for this application. Existing wearable antenna technologies have limited bend radii, are bulky, and cannot easily be incorporated into clothing.

This innovation uses conducting polymer materials to create patch antennas that can easily conform to a soldier’s body and can match the performance of existing antennas. Antennas based on these materials have reached high enough electrical conductivities such that they have been developed for radio frequency identification (RFID) applications. These materials can be developed as flexible, conformable, and even transparent alternatives to traditional metal-based approaches.

Conducting polymers are electrically conducting materials that have high electrical conductivities (~105 S/m) and are extremely lightweight and flexible. Wires synthesized from these materials have a wide range of applications that can include smart textiles, neural probes, polymer-based actuators, sensors, and antennas. The fabrication technique involves an additive printing technique that can print numerous patterns, but with limited inherent conductivity (less than 600 S/m). There is also interest in generating highly electrically conductive polymer fibers that can be directly incorporated into clothing to function as an antenna, but a printing technique has to be developed for this process.

Polypyrrole films cannot be synthesized as long wires using traditional electrospinning or wet spinning techniques. The electrochemical deposition process involved in the fabrication of polypyrrole traditionally cannot create fibers; however, the thin films generated from this process have high conductivities. A new approach has been developed to manufacture wires of polypyrrole up to 10 m long having varying cross-sections. The pyrrole monomer was vacuum-distilled before use. Polypyrrole was electrodeposited on a glassy carbon cylindrical substrate at - 40 °C at a constant current density of 1.0 A/m2 for 8 hours. The deposition solution used was 0.05 M pyrrole in 0.05 M tetraethyl ammonium hexaflourophosphate in propylene carbonate. The cylindrical glassy carbon substrate is 85 mm tall and 75 mm in diameter. A thin layer of polypyrrole forms on the surface of the electrode approximately 10-20 μm thick.

The electrode is then mounted on a custom-built instrument that slices the wires. The instrument consists of 4 axes (1 rotary and 3 linear axes) with the electrode mounted on the rotary axis. One of the linear axes consists of a sharp blade that slides along with a rotating crucible. The blade slices the film by running over the crucible in a helical pattern. The blade is simultaneously oscillated along its length such that a fresh cutting edge is continuously presented at the point of contact with the crucible. This instrument has enabled the production of PPy microwires with widths as small as a few micrometers and lengths ranging from tens of millimeters to a few meters. It may also be used to slice microwires from films made of other types of conducting polymers.

These fibers can range from a few microns to tens of millimeters in width and thickness. The large conductivities and large strengths make these materials ideal candidates for direct incorporation into fabrics.

The design focuses on an easily removable patch design. This enables the user to easily attach and detach the antenna from their radios while at the same time avoiding the problems associated with repeated cycles of washing. It also provides a low-profile, lightweight, highly flexible design that can easily conform to any active surface. The base design consists of three layers: a top camouflage layer made of any type of flexible fabric, an intermediate feed/ radiator layer, and a bottom isolation/ attachment layer. The feed/radiator layer is uniplanar and can consist of a meander line monopole or a wideband radiator such as a bow-tie slot radiator that can be optimized for the particular application under consideration. The bottom layer isolates the middle layer from the body and contains attachments such as a hook-and-loop mechanism that can be used to attach the antenna to a uniform (see figure).

The overall device has dimensions of 200 mm × 200 mm and 1 mm in thickness, along with a very low bend radius. The connector cable in the current embodiment of the prototype is directly attached to the patch itself but it is not necessary for this to be the case. A connector can be directly connected to the patch and the device can be directly connected to the relevant radio.

The patch design was tested to access its viability as an antenna. The measurements were taken with a network analyzer and the radiation patterns were taken in a tapered anechoic chamber free from any outside interference. The chamber was calibrated to a known isotropic radiator. The test antenna was secured in a stable position with the electric field oriented vertically. The gains of the antennas were calculated at different frequencies. At 200 MHz the gain is about 0 dBi, at 250 MHz the gain is -1 dBi, and at 500 MHz the gain is 8.2 dBi. The lower gain is due to the low conductivity of the conducting polymer as compared to copper. A noticeable amount of directionality is visible in each tested frequency. Using automated sewing techniques, better and more precise geometries can be created that can make these antennas have a better return loss.

This work was done by Priam Pillai, Eli Paster, Lauren Montemayor, Chris Benson, and Ian W. Hunter of MIT. MIT-0003

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Development of Soldier Conformable Antennae Using Conducting Polymers

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This article first appeared in the April, 2011 issue of Defense Tech Briefs Magazine.

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