Plastic enclosures may impede Bluetooth signals

Its research indicates that when a Bluetooth antenna is placed next to a plastic enclosure, its RF signal is impeded by as much as 37.5% and the broadcast signal can be shifted out of the official Bluetooth frequency range, of between 2.4 and 2.4835GHz, to 2.159GHz.

David Johns, vice-president of EM engineering for Flomerics said: "The magnitude of the changes in signal characteristics brought about by simply installing the antenna in a more realistic setting is huge."

Manufacturers cannot expect devices to operate optimally simply because a tried and tested antenna is being used, said Johns. "Every application will react differently meaning that simulation and testing of each antenna is required in-situ. If engineers are not aware of these issues before designs are initiated, significant time and money will be wasted building and testing prototypes before an acceptable RF solution is achieved."

Real world Bluetooth devices often use internal antennae surrounded by plastic enclosures, therefore Flomerics' work centered on a tab mounted, embedded Bluetooth laptop antenna supplied by a leading manufacturer. To create a realistic setting, a sheet of 3mm engineering thermoplastic material with the characteristics of a leading plastic was introduced that just touched the top of the antenna.

Using its Micro-Stripes V6.0 modelling software, Flomerics showed that with a full microwave, RF and EM simulation, the proximity of the thermoplastic shifted the broadcast frequency to outside the Bluetooth range and increased the reflection into the input port by 37.5% - severe enough to put the unit's performance outside the limits required for Bluetooth.

In free-space the Micro-Stripes simulated return-loss for the antenna showed a match of 22dB centered around 2.428 GHz - well within the Bluetooth range. When the same antenna was simulated with the resin touching the surface the match dropped by 6dB or 37.5% to 16dB and was centered around 2.158 GHz - significantly outside the Bluetooth broadcast range.

The company says that if this had occurred in a production device, the unit would require a significant redesign to bring it up to an actual sensitivity of -70dBm at 10m away from the antenna, adding significant costs and lengthening the product's time-to-market.

Rachid Aitmehdi, Head of the Electromagnetic Division, Flomerics, explained, "With the increase in devices requiring Bluetooth connectivity, this issue is going to cost more and more manufacturers valuable time and money.

"Micro-Stripes is designed to address these issues and is capable of simulating arbitrary-shaped antenna structures and accurately predicting the return-loss and radiation characteristics. Antennae are usually designed to operate in a free-space environment giving a certain "unloaded" performance over specific frequency bands. However, in reality it is very rare that they are actually installed in such an environment. If an antenna is installed within a system, the surrounding structure and materials can "load" and affect the antenna performance."

Lucas Gertenbach, a senior consultant at Bluetooth chip vendor Cambridge Silicon Radio (CSR), agreed that this is a real problem that has major implications for device manufacturers. "But most people involved in the antenna business are aware of the implications, and are able to compensate for the impact any dielectric material has on radio frequencies," Gertenbach said. And OEMs without RF knowledge "can get advice from the antenna manufacturers or consultants."

Gertenbach said chip vendors and system suppliers are well aware that "you can't just buy an antenna, put it on a pc board and think it will work first time. You have to test it over a period and get help to solve problems such as the one posed by plastic enclosures. The industry is more than able to cope with that".

He added the worst that could happen with a Bluetooth chip from CSR is the loss of some power. "The energy you put in to the antenna is reflected back to source, and the effective range could be reduced. But in almost all cases, the antenna is designed to overcome these effects".