I’m new to IoT but we had exactly this with a Pi in a steel box in our hotel’s pool plant room. Our IT guy talked me out of soldering. A closed steel box is close to a Faraday cage, so the onboard chip antenna inside it will always struggle, and the Pi 3 is not designed for an external one.
What we did instead, cost about 20 dollars:
- A USB Wi-Fi adapter with an RP-SMA connector.
- A short RP-SMA extension cable through a cable gland in the box, antenna mounted outside on top.
- Onboard Wi-Fi switched off so the dongle is the only wlan0. Add
dtoverlay=disable-wifito config.txt and reboot. Then put your Wi-Fi settings on the dongle as usual.
Our signal went from about -84 to -58 dBm.
On the Hermetry side I also lengthened the offline alarm. The docs suggest an Open after time longer than a normal reconnect for devices that can drop briefly, we use 600 s, so a 30 second Wi-Fi dip no longer wakes anyone. Keepalive is 60 s in our script.