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BTLE beacon strength

micallef25

Hello,

I will be using the NRF52 to send out advertising btle packets.

I am curious if anyone has any idea if the radio is strong enough to still send out packets if the NRF52 is enclosed a small water tight case and placed about a foot under water?

I think that this is high enough to be ok but am not 100% sure.

Thanks!

Eric Micallef

  • 0

    Water has a very high relative permittivity (dielectric constant). Since it is over 80 times that of air for RF waves it is highly reflective unless care is taken to match to the Er of water. This is same idea as light reflecting off of water. Also, water has a massive loss tangent. The loss tangent works out well to heat water in a microwave but for propagating RF waves, it's not so useful.

    Because of this, it is largely useless for propagating 2.4GHz RF waves. Out of curiosity I thumbed around on the web and the highest frequency I found any testing for was at 1GHz. So it will be much worse at 2.4GHz.

    Anyway, at 1GHz you can expect over 15dB/foot loss in fresh water and well over 30dB/foot in saltwater(depending on the article this number was shown to be as high as 300dB/foot). At 2.4GHz a rough guess would be to multiply everything by about 5. So, maybe 75dB/foot in freshwater.

    So if you have a beacon 1 foot deep in a pool of deionized distilled water and the receiver is precisely at the surface of the water 1 foot away you might get a few packets through once in a while.

    Enclosed is a nice paper I found on deploying 2.4GHz sensors utilizing 802.15.4 and 802.11b/g. They managed to send a signal about 18 centimeters (7 inches) after that it was 100% loss!

    Enjoy!

    sensors-12-04237.pdf

  • 0 in reply to AmbystomaLabs

    We haven't done any precise measurements, but as AmbystomaLabs allude to 2.4GHz works incredibly poorly when there is water around. You don't need many centimeters of water to block the signal completely.

    So the answer to your original question would be a resounding no I am afraid ;)

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