Spies like us - communication and surveillance equipment

Materials World magazine
,
1 Mar 2010
Measured Gain Pattern versus RTCA standard for airborne GPS antennas

Robert Pearson and Javier Vazquez of Cobham Technical Services, Leatherhead, UK, describe how materials development has improved communication and surveillance equipment.

In the world of James Bond, novel technology can seemingly fulfill any requirement. In the real world of engineering, there are a number of drivers underpinning the need to develop advanced materials that can be used to fabricate miniaturised antennas and to increase bandwidth for a range of communications, surveillance and navigations applications. Although these requirements are still driven by the likes of Bond, mainstream markets are also focusing on size, weight, power and bandwidth.

Whether the application is reducing the drag caused by an antenna on an aircraft, providing a blueforce tracking system for soldiers, or enabling a disaster recovery team to carry a broadband microsatcom terminal on to the next flight to Haiti – size, weight and power are all critical.

Antennas, unlike other electronic equipment, cannot simply be miniaturised as semiconductor processing techniques advance, because the size is linked to the wavelength of the electromagnetic wave. Broadband satellite communications systems use the range seven to 44GHz (7-40mm). For mobile phones, global positioning systems and Inmarsat satellite communications the wavelengths are 15-30cm. However, at lower frequencies, such as VHF/UHF, still widely used for many communications applications (both terrestrial and satellite), the wavelengths are one to 10 metres. Here antennas tend to be electrically small compared to the wavelength and radiate inefficiently, so need to input more power to overcome limitations in the range of communication systems.

The idea of using artificial materials to reduce the size and weight of antennas was conceived during the 1950-60s when several researchers developed the concept of artificial dielectrics based on a typically periodic distribution of metallic or dielectric particles. Each of these particles could be polarised under an external electric field in the same way that atoms and molecules are polarised in a natural dielectric. In the late 1980s, advances in manufacturing technologies, new high frequency materials and accurate computer modelling were applied to make more attractive artificial materials. These materials can be applied to both omni-directional, which radiate and receive energy in all directions, and directional, which ‘beam’ the energy over a narrow angular region of space. Directional antennas tend to be large compared to the electrical wavelength, whereas an omnidirectional antenna is typically comparable to the size of the wavelength.

A modern example of an artificial material-based lens was produced in the early 90’s, which generates a directional beam and has intrinsic losses comparable to a natural dielectric. However, this one-metre lens’ mass, at seven kilogrammes, is less than 10% of a conventional plastic equivalent and its diameter is approximately 40 wavelengths at Ku-band – the 11-14GHz band which is also used for satellite TV.

The materials concepts developed in the late 1980s and early 1990s went beyond that of artificial dielectric. Electromagnetic band gap (EBG) materials used the properties of periodic distribution of particles to obtain frequency bands where propagation of electromagnetic waves in any direction is inhibited.

As EBG materials do not support waves at certain regions of the electromagnetic spectrum, they have been used to eliminate trapped waves (referred to as surface waves), which limit the performance of antennas. These materials can channel and guide the electromagnetic energy, creating ‘contactless’ transmission lines whose dimensions can be adjusted to change the propagation velocity and introduce a variable phase shift. This alters the pointing direction of a directional antenna beam. At this time, the term metamaterial started to be used to describe artificial materials, which exhibit properties not normally found in nature.

Since the late 1990s there has been significant interest in artificial materials, which have simultaneous negative permeability and permittivity values. Potential applications for these materials include perfect imaging lenses (to focus electromagnetic waves), cloaking of structures which can make them effectively invisible to incident radio waves and could reduce the impact of wind farms.

In 2007, Cobham Technical Services in Leatherhead, UK, established a collaborative research programme into the use of advanced materials, electromagnetic applications, particularly those relevant to antennas. The Advanced Materials for Ubiquitous Leading-edge Electromagnetic Technologies programme (AMULET) has been supported by a grant from the UK Government’s Technology Strategy Board and has been undertaken in collaboration with UK organisations Queen Mary University of London, the National Physical Laboratory (NPL) and Vector Fields (now part of Cobham).

Position is vital

A GPS is becoming a standard feature in handheld devices and has been incorporated into almost all forms of transport. This success is pulling the requirements of GPSs antennas in two directions – to reduce their size to fit into smaller packages and to operate over larger bandwidths to improve positional accuracy and make them compatible with satellite infrastructure, such as the European Galileo system.

The latter is particularly relevant for airborne applications, where new regulations call for high precision GPSs.

Antennas based on advanced materials will enable automatic landing and free-flight air traffic control services to reduce carbon emissions, as well as meeting the packaging requirements for space constrained platforms, such as helicopters and unmanned airborne vehicles.

Yet a reduction in the size of the antenna not only leads to a decrease in its bandwidth, but also degrades its pattern fidelity. The GPS signal is circularly polarised, such that the wave propagation advances by circles like the thread of a corkscrew, making it possible to receive GPS signals independent of the device’s orientation.

To design an antenna capable of meeting all these requirements, a magnetic artificial material, incorporating biased ferrites, has been developed. making it possible to obtain a naturally circularly polarised radiator, without use of the external circuitry.

This radiator has been developed as an airborne GPS antenna which has a high dimensions/bandwidth ratio of 25mm2 with a 40MHz bandwidth, three times more than a conventional antenna with similar size, due to the bandwidth enhancement obtained by realising mr>1 in the material. The antenna’s radiation pattern is circularly polarised and compliant with the new Radio Technical Commission for Aeronautics DO-301 template for airborne GPSs, with an efficiency of 80%.

High dielectrics

High dielectric constant materials (er>10) are commonly used to miniaturise antennas. Natural dielectric materials with this high dielectric constant typically suffer high losses, high density, poor permittivity tolerances and limited environmental performance, as they are sensitive to temperature variations. This imposes a limitation for use of these materials in space applications, where even a few grammes can significantly affect the cost of launching a satellite.

As a consequence, the artificial dielectric concept has been revisited to obtain materials with a large dielectric constant of 10-100, but crucially with low loss. A concept of artificial dielectric, based on thin substrates, has been developed, proving its feasibility in microwave applications up to three gigahertz. Patch antennas filled with artificial dielectric with er of 13-30 have been demonstrated for a generic L band application, showing efficiency and losses below those of ceramic materials at a fraction of the weight.

Broadband negative materials

Passive artificial materials with negative electromagnetic parameters (e,m) are bound to be dispersive – their parameters change with frequency. This is necessary to comply with Foster’s theorem, which defines the limits on the frequency variation of the electromagnetic parameters, to ensure a positive definition of the energy stored in the material. As a result, passive negative materials are essentially narrowband, limiting their use in real-life applications.

To obtain broadband negative materials (er<1), active components store ‘negative’ energy in the artificial material. The mechanical equivalent of these materials would be a material which expands rather than contracts when pressure is applied.

This material does not comply with Foster’s theorem as it is active and is implemented by introducing negative inductors and capacitors into its structure. Negative inductors and capacitors can be obtained by using an active circuit called a negative immittance converter (NIC). Under the AMULET programme, several negative inductors and capacitors based on semiconductor NICs have been designed, manufactured and tested, operating at 20-250MHz with high Q-factors, typically >100 across this band. These materials compensate for the reactive energy surrounding an antenna, leading to inherently broadband radiators. A demonstrator of this antenna is being developed to establish an electrically small antenna with high efficiency broadband.

Non-Foster material cannot deal with a modest amount of transmitted power, since reactive stored power can be large and beyond the capabilities of semiconductor devices. As a result, devices have been limited to receive applications handling low amounts of power. As this is a major limitation, high power NIC devices are being investigated for power non-Foster materials.

On the move

Directional antennas are used in satellite communication applications to provide sufficient signal-to-noise ratio to transmit high data rates. There is a trend towards smaller satellite communications terminals, operating at higher frequencies (X-, Ku- and Ka-band) in the microwave spectrum with data rates in excess of one megabit per second. However, conventional antennas do not lend themselves to compact terminals, which can be easily carried and instantly deployed. There is a high demand for the emergency services, disaster relief agencies and the media, who increasingly rely on broadband communications to co-ordinate operations.

Traditionally, flat panel antennas suffered from much higher losses than conventional dishes, particularly at Ka-band. They are usually based on several layers of printed circuit and dielectric substrates, which introduce losses and can excite surface waves and parallel plate modes that trap the electromagnetic energy in between the surface of the layers rather than radiating it. Electromagnetic band gap material concepts combine the suppression of surface waves with low loss multilayer assemblies of printed circuits, making it possible to realise a high performance antenna suitable for portable satcom applications. Using these ideas, a multilayer low loss flat panel array of 430mm2 has been developed under AMULET, operating at X-band (7.25-8.4GHz), which is fully compliant with very small aperture terminal (VSAT) radiation pattern requirements (see p29). It controls the amount of energy transmitted towards adjacent satellites to avoid unwanted interference. The result is a flat panel array which is competitive in terms of efficiency at ~85% to a VSAT reflector, but with superior packaging. Despite its reduced size, it complies with pattern interference standards such as MIL-188-164A, which is difficult to achieve with a conventional VSAT.

Antennas, unlike electronics cannot be readily miniaturised without adversely affecting their performance. However by understanding and harnessing the macroscopic properties of advanced materials, designed using sound physical principles, it is possible to herald a new era in communication, navigation, avionics and surveillance technology.

Further information
Cobham Technical Services