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New materials expand wireless options for engineers

Posted: 17 Jan 2007     Print Version  Bookmark and Share

Keywords:PCB antenna  PCB-based antenna design  PCB antenna vs. discrete antenna  antenna design  Bill Schweber 

Antennas usually don't get a lot of design attention until late in the cycle. Perhaps that's because they are just passive devices, apparently small players in the RF signal path. Or perhaps it's because designers hope they can always configure the antenna design and component selection to fit the remaining space in the box. Or maybe it's because they are not beneficiaries of Moore's Law.

Regardless of the reason, designers of portable wireless products now have new tools, approaches and components that provide fresh trade-offs in the quest for a perfect antenna. Along the way, designers need to make tough choices that involve true "make vs. buy" decisions. Unlike active components, where you have to add something to your BOM, it's entirely possible to get a free antenna that costs only a few square centimeters of PCB area.

In many cases, this is an attractive and viable option. In other cases, however, the cost of this apparent "free lunch" is too high. Now, some of the newer antenna designs and components give designers other options.

Small antennas
The general definition of an electrically small antenna is one whose elements are shorter than l/10. For a 300MHz signal, that definition threshold is 10cm; at 1GHz, it's just over 3cm.

Traditionally, small antennas offered only marginal performance. If you wanted truly effective antenna performance, you needed to stick more metal in the air and use multiple or complex-shaped elements to increase gain, control bandwidth, shape the field pattern or reject adjacent signals. And you have to make sure the antenna impedance is matched to the RF front end to maximise transfer of power. One major benefit of the push to higher-frequency RF operation, typified by cellular and Wi-Fi applications, is that a small antenna can be electrically viable.

Consider BOM
While the antenna "family tree" is complex, the branch containing small antennas has well-defined trade-offs. You either use a PCB-based or discrete antenna.

The PCB antenna, which can be a patch, loop, spiral or wire, has no BOM cost, but requires PCB space. Note that some PCB antennas are not part of the main circuit board, but are available as distinct devices, usually to be attached to the inside of the product's case. Its performance also depends on its layout, dimensions and placement with respect to nearby components. In addition, a user's hand, body or head usually has a detrimental impact on the antenna's performance. Any changes to the product's components or overall PCB layout will ripple into the antenna performance, so the design is constrained and pre-release revisions are not trivial.

On the other hand, revisions in the antenna?whether to accommodate specification changes or design shortcomings?can be done quickly once they are identified and will have no impact on the BOM. Changes also affect the antenna's impedance, and you may need to change the matching circuitry you have designed in.

In contrast, the discrete antenna has a parts cost and must usually be designed by its vendor for a specific band, bandwidth and other performance characteristics. In exchange, you get an antenna that consumes less board space than a PCB antenna and is much less affected (if at all) by PCB layout, nearby components or the user. Antenna impedance is fixed by its physical design, so the matching network is also fixed, regardless of placement. Those factors free the designer from some challenging constraints and re-spins of the board's layout and BOM.

Roll your own
There are many possible designs for small, often PCB-based antennas. The most common are the open-wire (also called open-ended) configuration such as the dipole and monopole; closed-wire designs such as the loop; and the solid patch.

The open-ended antenna is just a smaller version of larger antennas that have existed since the earliest days of wireless. In fact, the dipole is sometimes called a Hertz antenna, since it was used by Heinrich Hertz in his 1888 experiments. It is balanced with respect to ground and served faithfully as the rabbit-ear antenna of VHF TV before cable and satellite TV arrived.

In contrast, the monopole is single-ended with respect to ground and needs a ground plane. It has served as the whip antenna on many radios. It is also known as the Marconi antenna, since it's the configuration he used in his early experiments.

Loop antennas, used in mass markets such as UHF TV reception, also have a long history. They have a circumference roughly equal to the received wavelength.

Electrically, the rectangular patch is a wide piece of microstrip transmission line with a length equal to one-half its operating wavelength. The wavelength is measured not in a vacuum, but in the dielectric of the PCB material. The resonant band of the patch is fairly narrow and so is its operating bandwidth?about 5 per cent of the nominal centre frequency?which can be good or bad, depending on the application.

PCB implementation
All three classes of antennas can be implemented using the PCB, and a multi-layer PCB offers design options, including acting as the ground plane needed for some configurations. Products with modest performance needs?such as remote keyless-entry fobs and garage-door openers?have used these antenna designs.

Since the tangible cost of a PCB antenna is negligible, when and why is it not the preferred design choice? Several compelling reasons are related to up-front design and actual implementation.

First, antenna design is not easy. Even with a modelling programme such as the Numerical Electromagnetic Code, the electromagnetic world is a strange place for the circuit or system engineer. It's a world of electromagnetic fields rather than specific voltage and current points or channeled electron flows.

Second, as with most engineering designs, competing and conflicting attributes (such as centre frequency, bandwidth, field pattern, efficiency, and lobes and gain) create difficult trade-off choices.

Third, it is not easy to assess antenna performance, which requires special test equipment and an anechoic chamber or open field. It takes time, money and expertise. Proper test setups include a physical replica of the human hand or head, for example, to assess the impact of the user's hand on the antenna's performance, or conversely, the impact of the antenna radiation on the user's head.

And that's just in theory. In practice, other factors come into play. The antenna takes valuable PCB space, of course, and its performance attributes are greatly affected by nearby components as well as the user's hand, head and body. The relative permittivity of human tissue (er) is about 40, while the permittivity of PCB elements spans about 25 to 85, so the tissue loads the resonating elements and affects the fields.

In addition, where the design needs multiple antennas for either multi-band operation or for a diversity configuration, the interaction between several PCB-based antennas, as well as the antennas and the surrounding area, can make performance prediction very difficult and sensitive to slight layout changes.

There are regulations restricting the specific absorption rate (SAR) of the antenna's field. SAR is the rate at which mass?in this case, human tissue?absorbs RF energy; it is usually measured either by measuring the rise in temperature due to the absorbed energy or the electric field in liquid, which simulates human tissue. The antenna's near- and far-field performance have to be understood and analysed, and these may be closely linked.

Finally, an antenna does not live independently of the wireless devices' receiving front-end or transmitting power amplifier stage. The circuit designer must determine the impedance of the antenna and the impedance of the associated stage, and then design a matching network to maximise power transfer over the target bandwidth. This is usually a difficult design effort involving speciality calculations and measurements, and specialised tools such as the Smith chart are needed.

Seeking material gain
Fortunately, developments in materials science and antenna theory offer design engineers an alternative to both external and PCB-based antennas. These antennas maximise the volumetric efficiency of the antenna while overcoming or virtually eliminating layout impact and matching uncertainty. In contrast, patch and whip antennas are two-dimensional, and their efficiency is based mostly on their area, not volume. While discrete antennas do add tangible cost, they often can improve or guarantee product performance while reducing footprint size.

It may seem counterintuitive that dielectrics, as insulators, would have much of a role in antenna design and implementation. But that's not the case, and dielectrics have been part of antenna design for over 50 years, helping to shape and manage the electric fields of antenna patterns. Field energy is concentrated and stored within the dielectric with fairly high density, and thus external objects or field have relatively little effect and don't affect the inherent resonance of the antenna.

Of course, high relative permittivity is only one key parameter for a successful dielectric-based antenna. The material also needs low dielectric loss (high-Q material) and low temperature coefficient to minimise physical dimension changes, which would result in detuning.

Different approach
Not all of the new antennas use ceramics as their core. Barcelona's Fractus S.A. uses geometric patterns based on fractal geometry for its antenna-in-package designs. The multi-band antennas can be printed on a substrate or embedded in a chip. They offer a 1,575MHz GSM antenna with radiation efficiency of better than 70 per cent, peak gain of more than 1dBi and VSWR under 1.5:1. The device has a 50ohm unbalanced impedance and measures just 10-by-10-by-0.9mm.

As a comparison of patch vs. discrete antennas from a single vendor, Centurion Wireless Technology offers a discrete microstrip patch antenna that can be attached to the product's enclosure. It serves the 2.4-2.5GHz band and measures 43-by-43-by-1.65mm thick. This unit has gain of 5.1dBi and VSWR of less than 2.5:1 across the band. The company's BlackChip surface-mount antenna for the same band is 8-by-6-by-2.4mm with gain of more than 2dBi and VSWR of less than 2:1.

- Bill Schweber
EE Times

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