Open hardware for a Zigbee nrf52833 soil moisture probe, It is designed for using M3 stainless steel rods as probes screwed into the headers H1,H2,H3. It is designed to run on a 3.6v AA SAFT battery, average current is in the sub uA range when sampling 5 times per hour. So battery life should be in the range of 5-10 years. It has one 75mhz probe, one 1mhz probe (Variable frequency, so the plan is to do a frequency sweep with this to determine electrical conductivity in the medium) and a ground probe. It allso features a NFC coil, to pair the sensor with a gateway. And a temp sensor.
Check out the ibom tool for component placement and hand soldering Assembly IBOM
The antenna is simulated and optimized in matlab with optimization script a theoretical -44dB S11, Actual measured S11 on the RF blanks is -26dB with a 0.5pF shunt capacotor soldered to L7 and a 0R resistor at C23. I left the antenna 1mm too long for tuning, but -26db is good enough for me.
Prototype produced at Eurocircuits