LoRa antenna

Description of tuning potted LoRa antenna on lion tracker

Picture 1: Potted only part where LoRa antenna is. When finished whole device is potted.

Schematic on bottom picture shows how is antenna wired with Murata IC and digital tunable capacitor PE64102 (DTC). DTC is wired in parallel with C7 to add capacitance and with that lower the antenna resonating frequency.

Picture 2: Schematic off LoRa transceiver and antenna

With recommended component values from datasheet we have antenna frequency that is far below 868MHz.

Picture 3: Component values from datasheet

Frequency response with component values from datasheet shows frequency center at 785MHz which is 83MHz to low.

Picture 4: Frequency response with component values from datasheet

After tuning with capacitor on antenna band select pin we managed to get right frequency curve. Below on picture you can see frequency curve with different capacitor values with the most suitable 3pF capacitor.

Picture 5: Antenna tuning process

Because the capacity is so small we can't fully use the potential of DTC. DTC has 32 possible combinations of capacity and we are operating at the beginning of this scale (at decimal number 4 witch is 2,98pF.

Picture 6: Antenna efficiency over DTC range at 868MHz

Picture 7: Device position for best signal strenght

Can we make efficiency of antenna better when potted?

Picture 8: PCB with EMC shield

Picture 9: PCB without EMC shield