Antenna Systems and Technology Magazine and Conference

Antenna Systems & Technology

New Developments in Antennas for Small Satellite Terminals
By Vladimir Stoiljkovic and Martin Shelley • Cobham Technical Services

Public safety and emergency agencies are increasingly reliant on satellite communications (satcom) to provide constant access to voice, data, images and video, when terrestrial communication network services are not available. An ability to deploy small satcom terminals is now seen as an essential operational requirement to provide a reliable emergency communication network. In case of marine applications, compact terminals are required for easy mounting on small boats for a range of applications.

In order to offer a truly global reach, broadband terminals that can be readily transported on-board commercial aircraft without the need for special transportation arrangements are also required. This puts stringent constraints on the terminal size and weight. Furthermore, terminals should be easy to deploy and must have low power consumption.

Reflector Antennas: The Traditional Choice for Portable VSATs (Very Small Aperture Terminals)
When designing a portable satcom terminal, the antenna size is very important since it has an impact on data rates that can be achieved, as well as power consumption requirements of the associated RF electronics. Traditionally, reflector antennas have been used due to their high gain. Although reflector technology is considered to be mature, the improvements in antenna system performance are still being made.

As an example of a recent development, Figure 1 depicts a front-fed circularly symmetric reflector mounted on a three-axes stabilized positioner used in marine applications. This is a Ku-band VSAT antenna system, designed and tested to meet the most demanding sea-state conditions, with an emphasis on reliability, simplicity and maintainability. The system is also available in X-band.

Key features of the VSAT system include:

Significant enhancement of the RF performance with a new patented radome design;
Considerable improvement in vibration and shock dampening;
Advanced design of the electronics, packaged in two easily replaceable housings.

The antenna system offers lower cost of ownership, due to a design that allows for easy maintenance and servicing, which results in increased reliability over the life of the product.

Reflector antennas are also used in land applications, where a complete satcom terminal is packed and transported in a back-pack or a suit-case. In such systems, the dish is usually split into petals and a mechanical fastening system is used to assemble the piece parts, using camlocs or over center fasteners. A splash-plate feed (like the one depicted in Figure 1) is typically employed for compactness, but it takes a trained user to assemble and deploy the antenna. Furthermore, due to its inherent depth, the reflector antennas are not easy to integrate as part of ultra-compact satcom terminals.

Next Generation of VSAT Antennas – Flat-Plate Arrays
In order to provide a man-pack antenna, which does not require assembly and is easy to integrate in a suit-case type satcom terminal, Cobham Technical Services has investigated the use of array-based antennas. Having experience with the design of waveguide-based and printed circuit-based flat panel array antennas, a number of designs were originally considered for this requirement based on existing technologies. The conventional printed circuit antenna technology was ruled out due to the very high losses that are realized using any form of micro-strip or strip-line beam-former structures at higher frequencies, such as X, Ku, Ka and EHF bands. Likewise, conventional waveguide designs were considered to be unattractive because, while losses are considerably lower than for printed structures, they are more bulky and the cost of manufacture of very intricate waveguide structures with the tolerances needed to operate at high frequencies is much higher.

Hence, a family of new designs, based on a number of unique low loss printed structures has been conceived. The X-band Diamond flat-plate antenna consists of a two-dimensional array of novel, Cobham-proprietary, broadband, dual-polarized printed elements, combined using beam-formers implemented in low loss strip-line transmission line technology. The antenna aperture measures 430 mm by 430 mm and is constructed using a number of very thin printed circuit boards and ground planes, bonded into a rigid “picture frame.” A printed polarizer is placed on the top of the antenna to achieve circular polarization. Polarization switching is achieved by mechanically rotating the polarizer; the outermost polarizer layer acts as a weather-proof cover for the antenna.

A foldable dual panel flat-plate X-band design has also been developed. This antenna is even more compact and consists of two panels, which can be folded together to minimize the storage volume required while, at the same time, realizing an aperture large enough to achieve high data rates when deployed. The panels are mechanically joined to form the complete aperture, but there is no RF connection between them. The antenna operates in dual CP. In this case, the CP is generated using an external network, with each hand of polarization available over the full 7.25 to 8.4 GHz band. This allows the system to be easily configured to operate with satellites that transmit with RHCP and receive with LHCP, or vice versa.

Apart from X-band designs, Cobham Technical Services have also developed Ku and Ka band variants.

The Ku-band antenna aperture consists of an array of dual-polarized radiating elements, arranged in a series of sub-arrays. The array is designed to be used in a diamond configuration, with polarizations aligned to the diagonals of the array. This makes it possible to achieve excellent side-lobe performance in the geostationary arc and at the same time offers exceptional efficiency.

An efficient, low-sidelobe design operating at Ka-band has been developed and is subject to patent application. The design is developed for high-volume manufacturing, where tight tolerances have to be met.

Current development is underway for applications at even higher frequencies, at around 40 GHz and above.

Small Antenna Aperture Effects
Smaller antennas have larger beam-widths and significant potential to interfere with adjacent satellites. Strict regulations exist to limit this interference. These limit the amount of power the user is allowed to radiate outside the main beam of the antenna. This is defined in terms of power density (effective isotropic radiated power/bandwidth), using side-lobe templates. This limit on the operation of the system will depend on the antenna aperture size and shape, and the relative locations of the user, the satellite and the orbital locations of the nearest neighbouring satellites. Generally, the limit is only applied near the geostationary arc and antenna performance outside this region is unconstrained. Different operators apply different limits, and in some cases the performance in the non-geostationary arc is also defined.

Where the templates only apply along the geostationary arc, a rectangular aperture solution can have significant benefits when compared to an equivalent circular reflector in many situations.

Generally, unless the satellite and user have very different longitudes and/or the user is very close to the equator, a mobile terminal will need to look in a southerly direction (in the Northern hemisphere) to form a link with a satellite in geostationary arc. In this instance, the geostationary arc will be broadly aligned to the azimuth radiation pattern of the antenna and, in order to maximize the power that can be radiated, the antenna needs to have the largest possible aperture dimension in the azimuth plane. For a given aperture area, this is achieved best by using a rectangular aperture with a large azimuth dimension and a small height, rather than a circular reflector.

Portable Terminal with a Flat-Plate Array to be Carried as Hand Luggage on the Commercial Aircraft
An example of a flat-plate array integrated into a portable satcom terminal is shown in Figure 3. The terminal comprises of a compact lightweight case that all of the RF electronics, filters, GPS, compass and modem are integrated. In such ultra-compact configurations, it is important to ensure that the electromagnetic compatibility (EMC) and thermal design issues are considered at the outset of the design process. In the Cobham design, the heat sink has been designed to ensure that optimum cooling is provided, while maintaining a hermetic enclosure. A moulded plastic cover, which is fully IP67 rated, is used, which also acts a stabilizing base when the antenna is deployed.
This antenna configuration provides a symmetrical arrangement of the aperture relative to the case unit in its deployed configuration. The mechanism for deploying the antenna ensures ease-of-use, accurate positioning and fast set-up time and is also configured in conjunction with rubber/foam bump stops to ensure that the stowed unit meets the environmental requirements. An aircraft carry-on luggage style telescopic handle, together with wheels and other handles, aids transport of the complete unit. The complete terminal is compatible with the IATA hand luggage requirements.

Conclusion
Cobham has developed a range of flat-plate array antennas for operation at high frequencies, suitable for integration into portable satcom terminals. In spite of stringent manufacturing requirements, the antennas are cost-effective to produce. The antennas require no assembly, and can be easily and rapidly deployed. The array based antenna topology provides more degrees of freedom which make it possible to optimise the antenna packaging, while maintaining excellent RF performance.

Vladimir Stoiljkovic is a Business Development Manager with Cobham Technical Services. He has been involved in the field of telecommunications for more than 20 years. He has experience of a range of technologies for satellite communications and navigation, spanning commercial and military land, airborne and naval applications. Vladimir has undertaken a wide range of R&D projects in the field of microwave and millimeter-wave hardware, including the design of active circuits and antennas. For more than 10 years he has been working in research and development of very high frequency transistor and diode oscillators and wide-band antennas.

Martin Shelley is a Project manager with Cobham Technical Services. Mr. Shelley has contributed to projects extending from feasibility studies through to the development of antenna products for high volume consumer applications. He has an expertise covering an extensive range of technologies including most types of arrays, reflectors and low gain antennas, together with printed and waveguide power dividers and high pulsed power solid state amplifiers. These skills have been applied to applications including spacecraft payloads, military systems and commercial wireless access systems.