An on board diagnostic (OBD) device having a radio modem is provided. An antenna of the radio modem is connected to a printed circuit board (PCB) housed within a plastic housing, where the PCB has a ground plane which is extended by a conductive extension. The conductive extension lengthens an effective length of a counterpoise of the antenna without necessitating an increase in size of the OBD device/plastic housing, resulting in maintaining a small form factor for the OBD device, while increasing antenna efficiency and/or bandwidth.