I think that the device in your picture in an OTA ( over the air ) TV antenna aiming device. Try cranking up your antenna, and pointing it in the general direction of the nearest TV source. Then play with the button and see if it helps you fine tune the antenna.
BTW,...remember to crank down the antenna before you drive off.
2003 Rexhall Rexair, 3550 Ford V10 with Banks
2014 Honda CRV-AWD via Blue Ox
Travel with SWMBO and Golden Doodle co pilot