Why 6mm clearance from Bluetooth Module Antenna ?

Hi, I am just wondering why for all bluetooth modules that has a ceramic antenna there is a 6mm clearance from all metallic objects (traces, planes etc) ?

I'm not an RF expert so it would be great if you gurus out there can shed some light on how this magic number 6mm was obtained.

In addition, if there was a ground plane underneath the antenna, what would happen ?

Hello,

Large metal constructions close and perpendicular to the antenna construction causes the impedance (as seen by the RF module) to change significantly. This results in less radiated power. The reflected field from the metal tends to cancel the radiation from the antenna.

"6mm" is not a special number. Very likely that some manufacturer published a datasheet where it mentions this minimum distance. If something does function, others will copy it. A quarter wave whip antenna 6mm from large metal surface will result in significant degradation of radiated power (if the antenna is not specially designed for that situation).

You can optimize an antenna so that it functions close to metal, but when (distance to metal) < 0.1lambda, it becomes more difficult to design an antenna that can easily be integrated into products.

A PCB ground plane under an antenna will degrade performance significantly, unless specially designed for that. You will notice bad VSWR and the FR4 material may absorb significant part of the power. VSWR is measure of how well the antenna accepts the transmitter power. 1 is best situation possible, above 3 will result in significant power being reflected back to the transmitter.

Thanks, this is good information.

However, I'll be also interested to know if there is a way to calculate a minimum clearance from a "ceramic chip antenna" ?

I've searched the net but couldn't find anything.

Hello,

As you don't know what was in the mind of the designer, I think trying to calculate a minimum clearance is virtually impossible. If you want to know the behavior close to other materials (both conducting and non-conducting), you should measure it (that means both VSWR/reflection coefficient and radiated power or gain). .

It can be done with relative primitive means, but if you are not into RF it will take significant time to acquire the knowledge, especially when you don't have somebody at hand who knows what to do. Having an antenna analyzer or (vector) network analyser and a signal source (your bluetooth module?) makes life easier, but still things can go wrong.

Just basic antenna knowledge isn't sufficient. You need to assess the influence of reflections and influence of the cable that feeds the antenna (if you use a VNA or (scalair) antenna analyser).

In my opinion, even if you follow application notes or datasheet guidelines, you should check your antenna.

If this is a once only problem, and you don't have lots of time, hiring an expert may commercially be attractive.

Hi WimRFP,

Thanks for the good advice.

Do you have any good books to recommend regarding RF ?

I have only very basic knowledge in RF but I'm keen to learn.