LoPy buried in Compost!

[joearkay]

Hi all,

I've been trying to measure the internal temperature inside a wet heap of compost (don't ask!). I used a LoPy enclosed inside of a small Pelican box, connected to an internal antenna with a one-wire sensor connected to it, poking out of the enclosure.

The gateway is a Lard RG186, in turn connected to a large antenna, which is only about 2m away from the compost heap.

I can successfully receive messages via TTN when the compost box is outside of the compost heap, however when buried within the compost, I get nothing.

I'm guessing this is down to the wet compost essentially acting as a Faraday cage around my LoPy, feeding all of the signal into ground. Has anyone got any good ideas of how I could improve on this?

I thought about using a directional antenna on the gateway and pointing it at the compost heap, however it's not really the TX that's the issue, it's the RX (to the gateway).

Any helpful/silly suggstions always welcome!

Thanks,
Joe

[joearkay]

@jmarcelino - finally, I'm using the forums ;)

[gregcope]

I have seen this done before where the one wire temp sensor is in the compost and the microcontroller is outside.

[joearkay]

@gregcope thanks for the comment - I had thought of this previously. However, the enclosure and temperature sensor have to be a single unit.

I won't bore you with the technicalities, but I'm developing this for a site where they use plant machinery to move the compost heap around, so the boxes need to withstand this.

[marcozennaro]

Hi! You can check for papers on underground IoT (or WSN). For example: https://www.researchgate.net/publication/309397324_MoleNet_A_New_Sensor_Node_for_Underground_Monitoring

Cheers,
Marco

[joearkay]

@marcozennaro Hi Marco, I'll certainly give this a read - thank you.

[robert-hh]

@joearkay Polarization and placement of antenna is important. You have to ensure, that sending and receivung antenna have the same orientation and are in the best receive position. If the receiver is upright, the sender must be too. And you should consider, that the gateway antenna you use has a rather flat receiption characteristic. For better gain, the optimal volume looks like disk. The vertical beam width is +/- 18 degrees. So if your unit is close, but below the antenna, this "good" antenna might behave worse than a 4$ piece.

[robert-hh]

@joearkay In the box you might need a dipole antenna like this one: https://www.ead-ltd.com/products/169-433-868-915-antennas/morava-868-mhz-pcb-internal-antenna

[jmarcelino]

@joearkay
I find those flexible internal antennas are really sensitive to their surroundings. I'd suggest either an external antenna (could be IP67 well sealed to the enclosure) or if it must be internal try the ProAnt evaluation one, https://www.digikey.co.uk/products/en?keywords=Proant PRO-EB-472

[joearkay]

Thank you for the suggestions everyone. I'm looking at evaluating the PC81 868MHz from Taoglas as well as the one suggested by @robert-hh (this one)

@jmarcelino I couldn't seem to see the connector type on eval. unit you suggested - have you used one of these before? Could you provide some insight?

Thanks again all.

[jmarcelino]

@joearkay
The Proant eval kit comes with a standard male SMA connector (so you'll need a uFL to SMA pigtail for it, as the Pycom antenna kit comes with RP-SMA instead)

[MarcoBenini]

For lower attenuation it would better to go with 433 MHz, even better to 169 MHz that is also an ISM band.
The big problem with compost, if I remember well how is processed, it is its high water content.
There are several studies that can be found about "soil" and wireless sensors networks, search for WUSN (wirelss underground sensor network) and UG2AG (underground 2 aboveground).
Usually are studies made for agriculture, forestry, soil, pipelines, ... sensors deployment.