Safe to have Bluetooth board over signals or ground plane?

[rea5245]

Hi,

I'm laying out a PCB for my latest project and it includes an HC-05 Bluetooth transceiver module, which includes a copper trace that acts as an antenna. My question is, is it OK (i.e. it won't hurt the RF reception or transmission) if there are signal traces running beneath the HC-05? How about if there's a ground plane under the HC-05? None of the signals would be high speed - less than 1 kHz.

Thank you,
   Bob

[rea5245]

Thank you, evb. The HC-05 datasheet - such as it is - says nothing about the antenna. But TI has an application note about Bluetooth antennas (http://www.ti.com/lit/an/snoa519b/snoa519b.pdf) and it says of printed antennas: "Printed and surface-mount antennas have certain common properties. Area around and beneath the radiating element must be kept copper-free. The ground plane must be placed on one side of the radiating element."

So that's my answer.

- Bob

[Jeroen3]

Two traces parallel to each other will form a capacitor.
Two traces/planes under each other will form an even better capacitor.
Two long strips with some insulator coiled up will form a radial capacitor.

On Vcc this won't be much of an issue, but at 2.4 GHz the effect is different.

[Howardlong]

In addition, even if you strip the copper, be aware that having extra the dielectric of any substrate like FR4 will affect the radiation pattern. As has been suggested, always follow the documentation for this sort of stuff, they will have had these modules in expensive RF anechoic chambers. There will often be recommendations on where to place the antenna WRT the board enclosures too. if not, always have it on the edge of the PCB, and that edge ought to be close to an RF aperture (ie, not in the middle of a conductive case, but at an end with plastic, that's what the black plastic areas are on iPads etc).

[Neilm]

I have used some modules that did not mention anything about the layout requirements in the datasheet. Hence, the first project too use in the company could barely talk to the PC beside it.

Later I found some app notes on the website that gave some patterns for where the module could not have any metal (trace, screw or what ever). The result being that the next project could happily reach over 8 meters.

Neil