11 July 2001
�SIGNAL Magazine 2001
Spray-on Antennas Make Their Mark
Experimental technologies offer elegant, inexpensive solutions.
By Henry S. Kenyon
Researchers are studying applications and materials for creating radio antennas that are sprayed onto a surface. Made from commercially available materials, these devices consist of a conductive substance sprayed over a template with a radio aerial pattern on it. The antennas can be applied directly to walls, windows or fabric shelters, allowing military commanders and relief workers to set up communications networks quickly.
Transporting, establishing and maintaining radio systems for military and humanitarian operations is often a logistics balancing act because of weight and space considerations. The ability to use any convenient surface as a mount for an aerial provides planners with additional flexibility when deploying in areas that are devastated or lack infrastructure.
The Defense Advanced Research Projects Agency (DARPA), Arlington, Virginia, is considering a number of possible applications and techniques for using spray-on antennas. According to Dr. Paul J. Kolodzy, program manager, DARPA Advanced Technology Office, the goal is to develop easily transportable antenna apertures that can be deployed rapidly. For example, a command post staff could set up its communications infrastructure by spraying antennas onto tents, walls or windows. This concept could also be used where there is a temporary need for a large antenna. Instead of diverting resources to transport and assemble a large structure, communications specialists may find it more efficient to spray a pattern onto a large piece of plastic to create a dish, he says.
Kolodzy cautions that the technology is still in its early concept phase. DARPA is examining materials that can emit or receive radio waves and is studying the capabilities of those substances in a particular antenna design. Once these operational parameters have been determined, researchers will be able to decide where to direct work on potential applications, he explains.
The technology for the antennas is not exotic, Glynda Benham, president of MegaWave Corporation, Boylston, Massachusetts, explains. MegaWave is conducting a phase-two small business innovation research study for DARPA on possible uses for sprayable antennas.
According to Benham, the antenna material is available in the form of metal-based paints such as nickel or silver and carbon-graphite-based paints. To create an antenna, a template is placed on the desired surface, the paint is sprayed over it and a connector is attached.
These paints are currently used for electromagnetic interference shielding in cases for electronic equipment such as laptop computers. Marshall Cross, vice president for research and development at MegaWave, notes that an antenna could be built into a laptop casing on the production line simply by using a template to separate a section of the paint. This would be less expensive than installing a piece of metal for an antenna, he explains.
One potential difficulty will be creating a paint that maintains its integrity on fabric surfaces such as tents. The antennas must be durable enough to withstand repeated folding and washing for long-term applications, Benham says.
DARPA research also will determine what kind of power a sprayable antenna will be able to put out in a transmission mode. This is important because too much power can destroy the antenna material, Kolodzy says. However, the primary goal of the research is to develop passive receiving systems, he adds. Additional studies seek new ways to find optimal patterns for antenna templates and to create apertures easily.
MegaWave also has experimented with wideband antenna designs with bandwidth ranges from 200 megahertz to 3 gigahertz. While efficiency remains a concern, the goal of the current research is to see where the engineering and design boundaries lie. Kolodzy notes that if the broadband studies are successful, the results can be applied directly to narrowband applications.
The paints currently used for the antenna material are opaque. DARPA is interested in possible applications for transparent paints. Transparent antennas would be unobtrusive and could be installed on vehicle windshields, Kolodzy says. He notes that the military would like very large apertures for their antennas, and a windshield is often the largest uninterrupted surface on a vehicle that is available for mounting such a device.
However, MegaWave researchers determined that no commercially available aerosol spray products currently meet transmission requirements because high temperatures are required to make the paint transparent. While several companies make metal-oxide-based materials for use at room temperature, they do not possess the minimum conductivity capacity of 100 to 500 ohms per square inch necessary for an antenna.
The study did reveal how much conductivity is needed to make this class of antennas, Cross observes. "It's almost like negative research. We figured out that with today's state of the art, you couldn't do it," he says.
DARPA and MegaWave are also cooperating on developing "invisible" antennas built into transparent surfaces such as glass or plastic. Unlike sprayable antennas, these devices consist of films embedded into or placed over a windshield or a window to create a receiver. This technology also utilizes existing materials.
Cross notes that automobile windows are coated with a metal-oxide film. This material currently serves three functions: as a safety laminate to hold the glass together during an accident, as protection for the vehicle's interior and occupants from ultraviolet and infrared rays, and as a demister or defogger when a current passes through it. A fourth and new function would be as a wideband antenna, he adds.
In much the same way spray-on antennas can be created by separating a small template from a larger painted area, a transparent antenna can be made by cutting out and isolating an area of window film, Cross explains. This type of device could receive a variety of signals such as amplitude modulation, frequency modulation, global positioning system, cellular telephone and personal communications systems transmissions. Megawave currently is working with Southwall Technologies Incorporated, Palo Alto, California, to develop this technology for automotive use, Benham says.
MegaWave also developed a prototype aircraft window for the Federal Aviation Administration. While the program did not proceed beyond the concept stage, it did demonstrate possible aerospace applications for the technology, Cross observes.
The concept was to place a metal-oxide film on commercial aircraft windows to provide protection from sunlight and to shield communications and navigation systems from electromagnetic radiation emitted by personal electronic devices. The windows also could be used to detect and locate individual personal devices affecting aircraft avionics. "It's more bang for the buck. One piece of film does three things," Cross contends.
Kolodzy hopes that these antenna technologies will be employed in areas such as the automotive industry and disaster relief. He believes that continued research will reveal more uses and capabilities for these devices. "It's going to be fun. If this works out, you're only limited by your imagination," he says.
Besides developing unobtrusive and sprayable antenna technologies, DARPA is focusing on combining multiple aerials into a single unit. As part of its phase-two SBIR program with the agency, MegaWave developed a broadband antenna that would allow multiple radios to transmit and receive signals.
According to Cross, the goal of the research is to develop multifunction antennas for law enforcement vehicles. "The idea is that you wouldn't drive around in a vehicle with five or six antennas on it because that tips off who you are. We got a used Ford Taurus and developed the whip and invisible windshield antennas and demonstrated that you could put many very high frequency and ultrahigh frequency radios on a single antenna and communicate just as well as with many antennas," he says.
The modified device functions as a broadband whip antenna operating over multiple octaves. It is a resistantly loaded antenna--the resistances are distributed along the aerial's length in a manner that allows for wideband use without compromising much of its gain, Cross notes.
The company has recently redesigned the whip with genetic algorithm techniques to optimize it for certain frequency bands, Benham explains. Genetic algorithms generate and produce efficient functions, much like real genetic traits are passed along by the dominant genes. This creates an automated method for optimizing antenna configurations. "When you do this process with genetic algorithms, instead of getting a continuous loading process, you get a piecewise, continuous profile that is totally random. It's not something you would ever get if you sat for 100 years and optimized it by hand," she says. --HSK
Additional information on MegaWave Corporation's military antenna programs is available on the World Wide Web at http://www.megawave.com/.