Antennas in the New Age
by Ofer Givati, Sinclair Technologies, Inc.
The market for RF components and systems has undergone many transformations in recent years. Increased demand for RF applications and equipment has led to an equally compelling demand for lower cost products. Antennas are of course, an integral part of any RF communication systems. In recent months there have been considerable developments in the area of antenna design that address a number of evolving needs, from concerns about space constraints and aesthetics to reduced costs and improved signal quality. In addition, we are seeing advancements made in the area of intelligence in radio systems - all of which impacts the cost of development and assembly.
One of the main drivers behind antenna research and development today is the need to improve communications as well as extend the life of the equipment without adding significantly to the cost. This has led to a renewed focus on new structures, different materials and fabrication techniques. Modifications and/or innovations in these areas have been instrumental in introducing cost effective, ergonomic, aesthetically pleasing, electrically and mechanically suitable products that address the specific performance needs of the RF marketplace.
The task for antenna and component developers is not without its challenges. The industry is finding itself shifting from one that is driven by low volumes and large margins, to one of high volumes and low margins - a factor that is placing ever-increasing pressure on suppliers to reduce costs while maintaining performance levels. At the same time, the performance demands that come with expansive RF applications mean that equipment must offer superior quality on all fronts - from materials to signal integrity.
While the drive for economy seems to run counter to the drive for quality, there is no question that superior design extends product life to deliver a more attractive return on investment. The need for application-specific design elements therefore is an essential part of addressing the installation and signal challenges faced by today's RF application engineers.
The Design/Cost Challenges
Cost has been a major deterrent in a number of areas, such as the widespread implementation of RF Identification (RFID) programs. There has been considerable pressure to reduce the cost of any equipment and/or related technology used in these applications. It stands to reason that if every item to be tracked - from automobiles and warehouse cases to single items on store shelves - is to be tagged with an RFID, the cost of the tag - comprising the Application Specific Integrated Circuit (ASIC) and the antenna to which it is attached - needs to be economically feasible.
Aesthetics and size are also challenges. High sites for example, are faced with an increasing demand to accommodate more services within the same real estate. This poses a technological challenge to ensure that such services can co-exist in close proximity to one another without compromising performance. In addition, it is important that antenna supporting towers and masts endeavor to be seen as "environmentally friendly" (in other words, as unobtrusive and aesthetically pleasing as possible), and antenna design therefore, must follow suit.
Whether we are talking constraints relating to cost, size or location, a number of challenges are being addressed through new developments in antenna assembly, production and implementation, that are helping to improve performance while reducing costs.
Methods and Materials
With mobile and in-building antennas, the general tendency is to design conformal antenna structures. There are both practical and aesthetic reasons for this. Mobile structures seek conformal antennas in order to reduce their footprint and thereby increase the scope and flexibility of applications. Users of hand-held and in-building systems tend to desire more conformal antennas for aesthetic reasons. In addition, the high level of integration for RF applications has recently led to the introduction of surface-mount antennas.
Typically, conductive ink is applied onto a flexible substrate in forming antennas for these types of RFID applications. Taking this approach a step further, the substrate can be shaped by removing and folding regions to form a three-dimensional antenna structure if so desired.
Alternatively, antennas can be formed by printing on existing enclosure surfaces of wireless devices or onto surfaces of components within the device enclosure. This printing of conductive ink onto flexible substrate material reduces the labor cost associated with handling, thus enabling the design and manufacture of cost-effective antennas for a wide range of wireless applications such as RFID tags, Bluetooth devices, mobile telephone, pagers and the like.
In addition to cost reductions associated with mass production and reduced handling, further savings are possible through the selection of material and fabrication process. For example, using pure copper traces instead of high silver content conductive ink will eliminate many of the costs associated with etched wastage. Doing away with the subtractive method eliminates the squandering of useful material, which is a significant contributor to cost reduction.
Some wireless devices incorporating surface-mount antennas are produced using Low Temperature Co-Fired Ceramic (LTCC) multi-layer technology in order to reduce antenna size. The high dielectric constant associated with ceramic material and the ability to form multi-layered structures provide an elegant response to the quest for high level of integration without occupying a lot of real estate.
On the other hand, while the high dielectric constant helps to reduce antenna size, the trade-off can be reduced antenna bandwidth and radiation efficiency. Resolving this involves forming multi-layered structures - a method that opens the door for the design of complex structures comprising antenna arrays and the associated beam forming networks.
This has been aided by the evolution of Micro-Electro-Mechanical Systems (MEMS) fabrication techniques - a process that has been instrumental in improving performance of smaller antennas. The development of MEMS fabrication techniques has allowed antenna structures (such as patch antennas) to be developed using a surface micromachining process, thus realizing complete systems on a chip. The sheer volume of anticipated production for RFID applications, coupled with the high level of circuitry integration, should result in much more cost-effective mass production processes.
Further cost reduction in antenna production can be achieved through the selection suitable materials. Antennas embedded or housed in plastic structures have been desirable for various fixed and mobile applications where there is a demand for rugged temper-proof structures. One way in which antennas are manufactured within plastic structures is the solder-in-plastic method. The solder-in-plastic method involves the use of a plastic molded item with an antenna artwork (represented by channels) on the surface of the plastic. Such solder-in-plastic processes involve a special purpose machine with a closing plate, which is pushed against the plastic item to close off the channels. Molten solder is then injected into these channels to form the antenna and its feed network in a single mass production process. It is even possible to connect the feed cable during this process.
Using current injection molding technology and infrastructure to fabricate antennas is yet another factor that will probably have a significant impact on antenna manufacturing costs in the short term. Injection molding technology has matured over the past few years to offer cost-effective products in very competitive markets. At the same time, more recent developments in materials have led to the use of a conductive composite recipe in some antenna assemblies. The advantage to this is that it can be blended with resins and be injection molded or extruded to form any shape or size of antenna structure. While there are other, newer antenna fabrication technologies, many are still in their infancy and do not have the cost benefits associated with a matured technology such as injection molding. With time this imbalance will likely change.
Conformal Antennas for Mobile Applications
Conformal antennas are the desired design for mobile applications, such as antennas mounted on moving objects such as aircraft or land vehicles. Recent advances in composite material have once again come into play here. Today, these composite materials can offer the mechanical rigidity to form part of the fuselage with a variety of different electrical properties. In addition, such composite materials typically allow the mixing of conductive and different dielectric material additives to render the final structure the desired electrical characteristics. For example, metallized carbon-fiber composite material such as copper or silver plated carbon-fiber-reinforced plastic can be used to fabricate antennas and other microwave devices to reduce weight, improve stability and deliver good electrical performance.
Another antenna fabrication technique used is the adhesion of copper to carbon-fiber-reinforced plastic of cyanate ester or epoxy matrices. Resistive surface properties of amorphous metals (also known as metallic glass alloys) have also been viewed as suitable materials for fabricating conformal slot type antennas onto the fuselage of mobile structures - especially those in the HF and low VHF frequency ranges.
One should bear in mind that a very low dielectric composite material is desirable when the structure acts as a radome housing the antenna. This concept can be extended to fabricate integrated mast (or tower) antennas. The radome conduit acts as part of the support structure and the antenna embedded within does not pose an environmental "eyesore". Beyond the visual component, it has the benefit of not being supported by a conducting pole, which interacts with the antenna system.
Antenna glazing, which is specifically designed for vehicles, is fabricated using two glass sheets separated by a dielectric spacer onto which an antenna microstrip is formed. Any one of these techniques can lend itself to custom antenna design that is particular to the geometry into which the antenna fits. A large variety of composite material products have been fabricated for years using contact (or open) molding technique. That same technique can also be utilized for the fabrication of these types of conformal antennas. However, the labor-intensive nature of most composite material product fabrication makes the economy of the solution highly dependent on labor cost.
Even the more traditional panel type of antenna arrays have been undergoing a significant transformation in recent months. In the past, such antennas have been manufactured using aluminum back plan reflector systems to which a plastic radome is riveted. However, this technique has lent itself to Passive Intermodulation (PIM) - a growing phenomenon in the crowded signal space of the wireless world. Current demand for low PIM performance is doing away with pressure contacts in general, but especially with respect to dissimilar metals. As a result, replacing the aluminum reflector housing with plastic housing has opened the door to more elaborate reflector system designs, making use of material such as conductive paint, copper foil, resistively loaded dielectric and numerous hybrids to achieve better control over the radiation pattern.
Moreover, the void in the panel antenna housing is sometimes replaced by Styrofoam material to improve the structure's mechanical rigidity. In addition, wire antennas, such as HF loaded dipoles, are replacing their traditional resistive loads by ferrite beads that slide over the wire, and thereby avoid the need to break the wire. Teflon surface coatings for exposed dipole antennas have also been effective in reducing the accumulation of ice. While these are highly effective methods to improve rigidity, extend the life of the product and increase the lifespan of equipment in harsh environments, changing the material content in manufacturing not only represents an opportunity to developers, but also the ever-present cost challenge.
The nature of any market is that it is always receptive to product improvement at no additional cost. Likewise, it gladly embraces existing products at reduced cost. The more difficult task, however, is to accept product offerings at a higher cost.
All antenna designs have a set of electrical and mechanical specifications, which the design aims to meet. Also, all designs have a cost target, which is forever under market pressure to be further reduced. Lowering the design cost without compromising performance or quality therefore, is the engineering challenge at hand.
Note: This article does not attempt to cover all materials or fabrication techniques of antennas employed today but rather to highlight some key trends and directions in today's wireless market.
Ofer Givati has been designing products for Sinclair Technologies, Inc. and headed its Product Engineering Department. Dr. Givati has worked on projects involving the designs of antennas, antenna systems on large and complex platforms and antenna simulation software to name a few. Dr. Givati holds BSc(Eng) electrical, MSc(Eng) and Ph.D. degrees, in Electrical Engineering, all from the University of the Witwatersrand, Johannesburg, South Africa. For more information on Sinclair Technologies, visit www.sinctech.com.