Should I use multiple antennae per one NRF52811, or use the same number of NRF52811 as multiple antennae?

[Rolling Stone]

Dear Sir,

I am interested in the direction finding of Bluetooth 5.1.

I found your product NRF52811 SoC on the Internet.

Should I use multiple antennae per one NRF52811, or use the same number of NRF52811 as multiple antennae?

If I must use just one NRF52811, how can I connect multiple antennae to the SoC?

I'd be obliged if you'd tell me.

Best Regards,

Yoonseok Choi

[Andreas]

Hi,

 

If you want to operate as the locator unit (TX in AoD, RX in AoA), you need an antenna array with a number of antennas to switch between. If you want to make the tag (RX in AoD, TX in AoA) you do not need multiple antennas.

 

In order to use multiple antennas you need to add a RF switch system to switch between the antennas. You can use 1 SPnT switch (n = number of antennas), or you can use multiple smaller switches SPDT, SP4T etc.

 

Note that currently we do not have SW or tools supporting development of direction finding applications. You are free to make something yourself though, with the added difficulty that might bring. You might find this document interesting.

 

Best regards,

Andreas

[Rolling Stone]

Thank you for your kind explanation. By the way, what is the minimum distance between antennas in antenna array? Is it possible that antenna array exists inside one Bluetooth module? How can we separate and analyze the mixed signals came from multiple antennas?

[awneil]

Rolling Stone said:

Is it possible that antenna array exists inside one Bluetooth module?

You would have to consult the module manufacturer and/or the module documentation.

Rolling Stone said:

How can we separate and analyze the mixed signals came from multiple antennas

That's what the switches - which Andreas described - are for.

[Dmitry]

The optimal distance is 6.25 cm that equals to 1/2 of wavelength. There is no "minimal" distance, with lower distance you'll get lower accuracy.

The signal is not mixed - transmitter switches between antennas every 1 usec, receiver measures phase difference between signals received in different 1-usec timeslots.

[Andreas]

Hi,

 

As mentioned here you probably need in the region of a half wavelength spacing to avoid the antennas interfering, but this is probably a little dependent on the type of antenna, orientation etc. and I would highly recommend that you simulate this as it is more complex than regular nRF52 design.

I doubt that you will find modules that have build-in arrays, they are simply too large, but at some point down the road you might be able to find off-the-shelf arrays. We are also working on a development solution which comes with an array, this will probably not be sold separately, but I expect we will make the HW files available as with all of our HW designs.

How to make the IQ samples is specified in the nRF52811 PS, how you calculate angles and directions from this is not specified in the Bluetooth spec, so you will have to implement this yourself or wait until we have our solution ready.

 

Best regards,

Andreas

[Rolling Stone]

Thank you for your kind explanation. By the way, what is the reason for that the optimal distance between antenna elements in antenna array is 1/2 of wavelength? I'd be obliged if you explain it.