A contact sensor inside my steel mailbox of forty years would not work. Even though the bridge is a direct line of sight at about 35’. I worked up several designs for keeping the contact sensor external while actuating it with a linkage attached to the mailbox door. This was doable but I wanted something simpler. WARRANTY VIOLATION AHEAD! I decided to modify the sensor so it could be mounted inside the mailbox while its antenna would be outside the mailbox. This was done by un-soldering the original antenna from the circuit board and re-soldering it to the end of a 25” long extension wire. The other end of the extension wire passed through a hole made in the side of the sensor case and soldered to the old antenna attachment. Signal strength before the project was 2 bars and after the modification 2 bars.
The first two pictures show the front and back side of the circuit board. With some labeling.
The third picture show the antenna connected to the end of the extension wire.
The fourth picture shows details of connecting the extension wire to the circuit board. I explain here that the detail put forth is better than what was actually done. It was difficult to get the board back in the case. The shown detail should be much easier to work with.
The Fifth picture shows the route I would use for the extension wire in and outside the mailbox. The 3/16” slotted hole in the back of mailbox a gentle entry and curving of the extension wire.
The last picture shows the finished installation. Later I will put better outside tape and paint it with a matching paint. The sensor unit and magnet installations may have to hardened later. It works very well.
This approach of placing the antenna remote from the sensor unit may have applications with steel doors.