Sensors Mag

Getting into Pockets and Purses

November 1, 2005 By: Brad Hurte, Oliver Leisten

Antenna Counters Sensitivity Loss in Consumer Devices

A basic lack of consumer awareness of the limitations of GPS has acted as a barrier to true mass adoption of the technology. Continued improvements in receiver size, performance, and cost have fueled an explosion of consumer GPS products during the past several years, and public awareness of the potential utility of GPS has similarly increased. Telematics systems, locatable mobile phones, GPS-enabled PDAs, and more novel GPS products such as pet finders have flooded the marketplace, with new products and applications announced almost daily. Yet many consumers are dissatisfied by the low position accuracy their devices furnish under some circumstances — if indeed they can obtain a position at all.

Consumers expect a product simply to work, irrespective of the conditions to which they subject it. A GPS receiver should tell them where they are — whether they're inside a car or building or whether the receiver is buried inside a pocket or purse. The consumer shouldn't be asked to shoulder the burden of ensuring the receiver has a clear line of sight on a cloudless, sunny day, while holding the receiver at arm's length.

In fact, it is a sure sign of a nervous GPS device demonstrator when he or she holds the device between thumb and ring finger in the genteel manner in which one might hold a teacup! Such a delicate grip betrays an attempt to preserve antenna efficiency, reducing the effect of body loading by minimizing the mass of human tissues in close proximity to the device. Body loading constitutes a critical issue in the design of portable GPS products, because it accounts for much of the loss of receiver sensitivity and can lead to unacceptable performance even when a device is used in the manner intended by the designer.

 Antenna Terms
Antenna Terms

The loss of electromagnetic resonance energy as it dissipates into the lossy body impairs receiver sensitivity. The antenna's function is to resonate an electromagnetic cloud in the space surrounding the antenna. The resonance of that electromagnetic near field actually causes radiation of energy to the far field, which is the field perceived as the doing the antenna's work. GPS receivers, of course, deal with the inverse of this process — energy from the far field causes a resonance in the near-field cloud surrounding the antenna. If a user presents his or her body into the space occupied by the near field, the energy that ordinarily would be available to the receiver dissipates into the user, and sensitivity decreases. With conventional antennas, the near-field region is large (as much as three wavelengths in radius or almost 2 feet for GPS), and the loss of receiver sensitivity to loading can reach as much as 10 dB. Therefore, designers must face the key challenge of antenna efficiency in a body-loaded environment.

Although many advances have been made in receiver design, increasing receiver sensitivity to very low levels with or without network assistance, the drive to make antennas smaller runs counter to the need to make them more efficient. After all, with antennas, bigger is better. Expressed simply,
(gain × bandwidth)/volume = constant

If the volume decreases, then (gain × bandwidth) also must decrease. Most device designers try to squeeze the maximum amount of gain from a small antenna by recognizing that a conventional antenna must resonate against a ground plane, which when coupled with the device effectively increases antenna volume to the volume of the device itself. Unfortunately, user interaction with the device in applications in which the device is to be held, worn, or sit in a user's pocket causes the antenna to de-tune and lose energy to the lossy materials surrounding it — a vicious circle.

Consumer applications require an antenna large enough to provide sufficient signal to the receiver, small enough to attractively fit small products, and isolated from the device so it doesn't interact with its environment — except to resonate with received GPS signals. Only then can we can consider products that will work when sitting in a user's pocket or purse. The dielectrically loaded quadrifilar helix antenna provides such a structure.

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