Mailbox Sensor To Bridge: Poor Connection

I have the mailbox sensor installed. The sensor was installed in the mailbox (metal mailbox), the antenna is mounted outside of the mailbox. The bridge is 75 feet away in a direct line of site to the antenna. The connection from the sensor to the bridge is “POOR” and often drops out. I thought that if the antenna was mounted on the outside of the mailbox there would be no interference. What am I doing wrong? Thanks, Jim