FROM EDN EUROPE: Dielectric resonators increase antenna versatility

Antenova claims that its antenna designs represent a step forward in small antennas. In most cases, says the company's engineering director, Roger Wise, conventional miniature antennas use radiating conductors in which dielectric material loads the conductor to reduce the physical dimensions of the overall device.

Antenova's designs, "high-dielectric antennas," are true resonant-dielectric structures. The company acknowledges just one other device on the Bluetooth market that uses similar principles.

The technology offers a number of advantages. Because wavelengths in dielectric materials are short, they are small; the dielectric usually contains the near field, leading to virtual immunity from loading effects due to the proximity of a user's hand, other antennas, or circuit structures or the effects of other dielectric structures, such as plastic cases. You can therefore design multiple antennas in close proximity—either for multiple services, such as a cell phone plus a GPS, for example—or for diversity reception for the same service. Even better, says Wise, you can design the dielectric antennas to support more than one band and service within one structure. The technology is therefore ideal for applications, such as the MIMO (multiple-input, multiple-output) principle (see "Module speeds 802.11a to 162 Mbps," this issue, pg 7).

Antenova has developed simulation techniques for the resonant modes of its designs, modelling materials that have been available only for the last two years. With a combination of mathematical modelling from first principles and empirical results and insights from prototypes, you can design the full range of antenna features, Wise says.

Design techniques, as with any other antenna, involve the shape of the device to support your chosen resonant mode, plus details, such as the feed point. The dielectric has no current in the conventional sense, but you can induce a displacement current, and the antenna then radiates energy. You can design for improved gain and efficiency at higher frequencies and achieve polarisation, nulls, and directionality to order; you can also design narrowband, wideband, or multiband structures.

Because of the high-dielectric coefficients of the materials, dimensions are on the order of millimetres at gigahertz frequencies. You can achieve efficiencies of 70 to 80% for entering electrical energy and exiting radiated energy. In contrast, says Wise, if you load a conventional antenna at cell-phone frequencies with a user's hand in proximity, its efficiency can easily fall to single-percentage figures. Arrays of the devices can provide beam-switched or steerable antennas in a small space. Nevertheless, Wise acknowledges, knowledge of the subject is still in its infancy, and he expects to be able to design more versatile products as experience increases.

The company has produced antennae for Bluetooth and 2.4- and 5-GHz WLAN that can be selective enough to eliminate a stage of bandpass filtering in the associated receiver. Physically, they are simply small pieces of shaped ceramic that are surface-mounted to pc boards. (See Picture.)

Antenova, +44 1223 810600, www.antenova.com.