The protection of the environment and reduction of the environmental footprint of air transport is very demanding Among others, growing air traffic also significantly contributes to the emissions contributing to climate change and local air quality issues around airports. Very Ambitious goals of restricting CO2 emissions have been set which require many operational improvements and adoption of new technologies. One new technology is to replace the protruding communication antennas by integrated one with the aircraft outer skin can be used to contribute to the defined requirements.
The antenna represents the interface between the transmitted and/or received microwaves traveling on free space and the signal processing hardware and software. Optimizing the antenna characteristics can lead to great improvements to the overall system performance, like lower noise figures, suppressed multipath and interference signals and higher signal levels, i.e. better accuracy, better aerodynamics, lighter weight, etc.
The structurally integrated antennas cause less additional drag, noise and turbulence in comparison with classical protruding antennas. These antennas also reduce maintenance costs and possible operational delays by avoiding collisions of protruding parts with airport cargo vehicles. More and more conformal antennas are used in communication and navigation technologies in Aircraft, Spacecrafts, and other vehicular bodies. The advantage of this choice is that they do not suffer from air drag or breakage and can be formed into any desirable shape. As they are structurally integrated there is no need of conventional radomes as non-conformal antennas require when they are incorporated on nonplanar structures.
Also, another important feature of these antennas is their radiation characteristics. They usually provide broader beams than their planar counterparts. This quality is an advantage when, for instance, omnidirectional patterns are required, which normally are achieved applying an array. The use of conformal antennas for the same propose provides simpler manufacturing since it may be only one single antenna with simpler performance, avoiding the ripple problems characteristic of arrays configurations.
Regarding the antenna performance, by varying the shape of the conformal antennas the gain can be controlled, being possible to reach nearly omnidirectional coverage while with planar antennas the almost full coverage is limited to approximately ± 60°. This very broad beam radiation patterns with non-conformal antennas can be only achieved with array configurations.
Another possible purpose for using these antennas is making them less disturbing, i.e. less visible to the human eye since there are integrated on the structure. This attribute might be useful for urban or military environments. In addition, their radar cross section (RCS) can be much lower than for their planar counterparts. The military has a great interest in Stealth platforms. Well-designed stealth platforms such as aircraft are with very low radar cross-section (RCS). However, antennas on those stealth platforms are one of the main scattering sources. So it is of great interest to reduce RCS of antennas mounted on those platforms.
Unmanned Aerial Vehicles (UAVs) are currently restricted from flying in the National Airspace System as a result of the FAA requirement that they stay within radio line of sight of their operators on the ground station. One solution is to use satellite communications but the necessary antenna systems tend to be heavy and increase drag. NASA is hoping to develop flexible antenna with foundations made from aerogel, thereby providing satellite communications while minimizing the weight and drag of the antenna.
Conformal Antenna Technology
Conformal antennas can be almost any geometry, although the main structures investigated so far are cylindrical, spherical and conical. In this project, the conformal antennas are divided into singly and doubly curved antennas, depending on how many curvatures the geometry has.
Antennas on singly curved surfaces are the simplest conformal antennas. The main purpose of such antennas is to improve the azimuth coverage, obtaining even in some cases omnidirectional coverage. Especially the circular cylindrical antenna is commonly used in conformal antenna applications since it is one of the most straightforward non-planar geometry. The conical antenna is also in a singly curved surface with particular interest for applications in the noses of aircraft or missiles.
Another important factor in the antenna performance is the substrate thickness. Already small differences can affect considerably the bandwidth while the radiation patterns are nearly not affected. The thickness of the substrate is critical for the conformal antenna
manufacture since the flexibility of the material is worse for thick substrates and very thin materials may be too fragile to bend.
Microstrip antenna technology is most suitable for conformal applications because of their ability to conform to non-planar structures. Their low profile and low weight property and easy of fabrication makes them a good candidate for antennas mounted on platform especially aircrafts. However the patch and ground of the microstrip antenna form a cavity and resonant modes are easily excited which contribute a lot to RCS.
Researchers are now developing antennas based on liquid metal alloy that can be moved around to meet specific needs and is embedded in the aircraft structure, without compromising the structural properties. The liquid state allows for the antennas to be reconfigured to provide tunable frequency and directional operation and go so far as being multi-operational. These liquid metal antennas reduce the structural alteration to the craft.
Microstrip antenna technology
Microstrip antenna patches are placed above what may be characterized as a conducting plane with a dielectric substrate separating the patch from the conducting plane. The properties of such an array depend strongly on whether it is small compared to the radius of curvature of mounting body, in which case it behaves nearly like a planner array or whether it is comparable or large to the radius.
Conformal antenna may be put in the belly of aircraft then there is no need of radome. Besides this, shaping and resolution can be improved by using conformal antenna as more elements can be accommodated. In conformal antenna it is found that resonance frequency is same for various curvatures. However as curvature increases the pattern broadens.
However the patch and ground of the microstrip antenna form a cavity and resonant modes are easily excited which contribute a lot to RCS. Many different methods for RCS reduction for microstrip patch antennas have proposed by researchers in the past decades. Etching slots on patch and ground are use to suppress these resonant modes to reduce RCS. Solid ground is replaced by meshed ground which exhibits frequency selective property in, thus out of band energy can pass through the ground to achieve RCS reduction. Fractal patch antenna is also designed to achieve miniaturization and reduce RCS
Shaping is one of the main strategies used to reduce RCS of stealth platforms. Shaping is useful when the structure is electrically large. It can be used in the antenna design where the working frequency for radiation is below the frequency where RCS reduction is considered.
Tunable Liquid Metal Antennas
Researchers have held tremendous interest in liquid metal electronics for many years, but a significant and unfortunate drawback slowing the advance of such devices is that they tend to require external pumps that can’t be easily integrated into electronic systems.
Team of North Carolina State University (NCSU) researchers have created a reconfigurable liquid metal antenna controlled by voltage only by using electrochemical reactions to shorten and elongate a filament of liquid metal and change the antenna’s operating frequency. Applying a small positive voltage causes the metal to flow into a capillary, while applying a small negative voltage makes the metal withdraw from the capillary.
The shape and length of the conducting paths that form an antenna determine its critical properties such as operating frequency and radiation pattern. “Using a liquid metal—such as eutectic gallium and indium—that can change its shape allows us to modify antenna properties more dramatically than is possible with a fixed conductor,” explained Jacob Adams, coauthor and an assistant professor in the Department of Electrical and Computer Engineering at NCSU.
The positive voltage “electrochemically deposits an oxide on the surface of the metal that lowers the surface tension, while a negative potential removes the oxide to increase the surface tension,” Adams said. These differences in surface tension dictate which direction the metal will flow.
This advance makes it possible to “remove or regenerate enough of the ‘oxide skin’ with an applied voltage to make the liquid metal flow into or out of the capillary. We call this ‘electrochemically controlled capillarity,’ which is much like an electrochemical pump for the liquid metal,” Adams noted.
Myriads of potential applications await within the realm of mobile devices. “Mobile device sizes are continuing to shrink and the burgeoning Internet of Things will likely create an enormous demand for small wireless systems,” Adams said. “And as the number of services that a device must be capable of supporting grows, so too will the number of frequency bands over which the antenna and RF front-end must operate. This combination will create a real antenna design challenge for mobile systems because antenna size and operating bandwidth tend to be conflicting tradeoffs.”
This is why tunable antennas are highly desirable: they can be miniaturized and adapted to correct for near-field loading problems such as the iPhone 4’s well-publicized “death grip” issue of dropped calls when by holding it by the bottom. Liquid metal systems “yield a larger range of tuning than conventional reconfigurable antennas and the same approach can be applied to other components such as tunable filters,” Adams said.
In the long term, Adams and colleagues hope to gain greater control of the shape of the liquid metal — not only in one-dimensional capillaries but perhaps even two-dimensional surfaces to obtain nearly any desired antenna shape. “This would enable enormous flexibility in the electromagnetic properties of the antenna and allow a single adaptive antenna to perform many functions,” he added.
References and resources also include:
Radar cross section reduction for microstrip antenna using shaping technique, Longjian Zhou ; Feng Yang, http://ieeexplore.ieee.org/document/7762470/