Software shows Bluetooth antenna shortcomings

Research carried out by Flomerics has clearly demonstrated the negative effects plastic enclosures can have on the radio frequency (RF) signal performance of Bluetooth antennas.

Research carried out by Flomerics has clearly demonstrated the negative effects plastic enclosures can have on the radio frequency (RF) signal performance of Bluetooth antennas.

Using the powerful Micro-Stripes V6.0 electromagnetic (EM) modelling software, the research showed that placing a Bluetooth antenna against a plastic enclosure reduced its signal strength by 37.5% and shifted the broadcast signal to 2.159GHz - taking the device's performance outside the standards required for Bluetooth.

The study has huge implications for device manufacturers and demonstrates the potential hazards they could face if designers neglect to properly simulate and test their devices' RF outputs 'in-situ'.

David Johns, Vice President of Electromagnetic Engineering, Flomerics, said: "The research Flomerics has conducted is of consequence to every company that is developing a Bluetooth-enabled device.

The magnitude of the change in signal characteristics brought about by simply installing an antenna in a more realistic setting was huge.

Manufacturers cannot expect devices to operate optimally simply because a tried and tested antenna is being used.

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 antennas surrounded by plastic enclosures, therefore Flomerics' work centred 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 Micro-Stripes V6.0, a full microwave, RF and EM simulation showed that 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 centred around 2.428GHz - 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 centred around 2.158GHz - significantly outside the Bluetooth broadcast range.

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 in the near future.

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.

Antennas 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".

Flomerics research shows that all designers working with Bluetooth should be aware of the unique problems enclosures and casings may present to RF signal performance and the need to think about antenna installation very early in a product's design.

(This was Electronicstalk's Top Story on 2 July 2002).

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