Underwater range of LoRa


#1

Does anybody know what the reach of LoRa is in sea water?


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(Richard Karst) #2

Attenuation of radio waves in water (and, in fact, in any conducting medium) increases both with increase in conductivity and increase in frequency. It can be calculated from the follow formula:
Attenuation (α) in dB/metre = 0. 0173 √(fσ) where f = frequency in hertz
and σ = conductivity in mhos/metre (Seawater is between 2 and 8 mhos/metre depending on salinity)
Therefore attenuation in sea water is very high and to communicate at any depth at all, it is necessary to use very low frequencies (10 to 30 kHz) where attenuation is in the order of 3.5 to 5 dB per metre.


#3

Thanks for the quick reply. So how would that work for LoRa. If you have a node which is able to transmit 5km through air with line of sight. How far would it go underwater?


(Richard Karst) #4
  1. 0173 √(868000000 2) = 720dB for one meter. given the link budget of LoRa this depends on environment but lets say its 138dB then we are talking centimeters.

#5

Thanks, that was very helpful.

Not for the one that wanted to sent sensor data from 100 meters deep. But this is clear :smile:


#6

And even "hoosjebootje" has a problem if the vessel sinks :wink:


#7

Like people have suggested the higher the frequency the less penetration that you get through water. I guess this is why microwave ovens work at heating up food that contains water.

We were looking at whether you could create flood monitors that sat at the bottom of a river/lake that used variations in the pressure of the water column to identify flood/drought etc. The only way that we reckon it could be done would be to have a buoy or other kind of floating antenna.

Read this blog on using 868 MHz in watery locations which was quite interesting. I can imagine that LoRa doesn't work that well in forests and also hints that 868MHz is quite vulnerable to rain. http://www.marcspages.co.uk/rtc/0161.htm


(Coeus) #8

I am also interested to know some basic principles about Lora radio applications under water.
So, will pressure make any difference? (I am assuming I have a Gateway at 10 metres under water and a node 50 metres under water: do I need to factor in the difference in pressure apart from the distance?)


#9

@coeus, well it was already determined earlier in this thread that you will not even come close to bridging 40 meters under water with a Lora signal, so I could assume that pressure does not make much difference…?