Slipping Stealthily Across the Seas
SwRI antenna helps U.S. warships escape enemy detection
In times past, a warship depended on a combination of firepower, maneuverability and defensive armor to survive battle. While these characteristics remain important, stealth has become essential for warships. Once detected, a ship can be engaged and defeated in a matter of seconds by today's high-tech weapons systems. The aim of the modern-day warship is to remain undetected by the enemy as long as possible.
Engineers must address several aspects of ship design to optimize stealth. Smaller surfaces prevent visual detection. Dispersing engine exhaust heat helps evade heat-seeking missiles. Decreasing machinery noise reduces sonar detection. Reducing radar cross-section (RCS) makes a ship a smaller, more elusive radar target.
RCS is a measure of how well an object reflects radar energy back to the enemy's radar transmitter. Reducing RCS involves shaping objects so that incoming radar signals are not reflected back to the radar transmitter, but are rather dispersed in every other direction so that the signature of the object can't be determined. The highest radar reflections occur when the radar transmitter is aligned straight on or "normal" to the target, reflecting a mirror image of itself to the transmitter.
Stealthy designs favor curved and faceted surfaces, angles other than 90 degrees between surfaces, and curved or saw-tooth-shaped access doors and hatches. Radar-absorbing material (RAM) on all surfaces absorbs some of the radar signal energy.
While existing antenna designs meet current antenna requirements, they are visible to enemy radar. Future shipboard applications demand a new class of stealthy, covert antennas.
Southwest Research Institute (SwRI) has supported U.S. Navy radio direction finding (RDF) since 1951, becoming the recognized worldwide leader in shipboard RDF antenna design, fabrication and support. To remain the preferred supplier of topside antenna systems, SwRI is designing a new series of stealthy antennas.
Using SwRI internal research funding, a multidisciplinary team has developed and validated the AS-141, a conformal, high-frequency (HF), direction-finding antenna. The AS-141 (patent pending) is a prototype of the next generation of covert shipboard HF band Communications Intercept (COMINT) RDF antenna. This antenna is ideal for the next generation of stealthy warships.
The AS-141 antenna is a combination of something old and something new. Old antenna design technology used low-profile ferrite core loops to reduce the visually observable profile of the antenna. Ferrite is a ceramic material manufactured to exhibit magnetic properties. Ferrite materials conduct electromagnetic radio wave energy much more effectively than air. Desktop AM radios use ferrite material to concentrate the electromagnetic energy of the radio signals, thus improving radio reception. In effect, the ferrite core antenna provides the same level of service as a much larger antenna, while keeping a much lower profile.
The new antenna design techniques embodied in the AS-141 include shaping the faces of the materials to eliminate highly reflective 90-degree angles between antenna surfaces and using RAM on surfaces visible to incoming radar signals.
The AS-141 also employs a single, large, printed wiring board, providing a uniform ground plane and mounting surface for the antenna and its components and electrical connections. A single wiring board is less expensive to assemble, provides better unit-to-unit performance match, and uses fewer components, making it more reliable. The design includes a loop of wire wound around the central junction of the ferrite bars, which allows engineers to inject a test signal, permitting automatic, computer-controlled built-in antenna assembly testing after installation.
Building a stealthy antenna
Efficient HF band (nominally 2 to 30 MHz) antennas are traditionally very large; existing land-based HF COMINT antennas are up to 120 feet tall. These large antennas are unsuitable for shipboard use. Current shipboard COMINT antennas include electrically short monopoles (5 to 23 feet), air-cored loops (2- to 4-foot diameter), and ferrite-cored loops (1.5- to 3-inch diameter).
SwRI engineers chose ferrite-cored loop antenna elements for the AS-141 because they are small, exhibit low observability and can be shaped to reduce RCS. Sensitivity, efficiency and antenna reception pattern tests demonstrated that this 2-inch diameter, ferrite-loaded antenna has performance comparable to a traditional 24-inch, air-cored loop antenna but is much stealthier.
RCS testing conducted in an SwRI anechoic chamber validated the design calculations. The anechoic chamber is shielded from outside interference by electrically conductive steel walls, flooring and ceiling, and large pyramid-shaped cones of RAM line its interior. This design permits accurate measurements of the electromagnetic energy reflected from the antenna. The antenna was tested both with and without RAM applied. While using RAM produces a lower RCS, even without RAM the AS-141 antenna has a significantly reduced radar signature compared to existing designs.
The U.S. Navy plans to install AS-141 antennas on the LPD-17, USS San Antonio class ships, currently under construction. The LPD-17 class is an amphibious transport dock ship. Commercial contractors also are investigating this technology for other ship concept designs, such as the U.S. Navy DD-21 next-generation warship. Covert building-mounted applications, as well as aircraft installations, are also potential uses of the AS-141 antenna.
In related stealth antenna developments, SwRI engineers have developed and patented conductive composite COMINT antenna designs for the VHF and UHF radio frequency ranges. These stealthy "plastic" antennas could be used in conjunction with the AS-141 for full frequency range HF/VHF/UHF COMINT coverage.
New ship designs not only call for new antenna materials and designs, but also for a general reduction of the numbers of antennas and co-location of transmitting and receiving antennas. To this end, SwRI is participating in a shipboard antenna design program called IVUL (Integrated VHF/UHF L-band antenna), which seeks to reduce ship RCS by reducing the total number of antennas required.
With its extensive background in shipboard antenna systems design, combined with its cutting-edge work in stealth technology, SwRI will continue to provide the U.S. military with new, more advanced antenna designs.
Comments about this article? Contact Saulnier at (210) 522-3758 or firstname.lastname@example.org.
Published in the Spring 2002 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.