Developing a Transfer Function for FM Band Transmitters

Low power, license-exempt FM transmitters are used in vehicles to transmit a weak signal onto a vacant channel in the FM Broadcast Band (88-108 MHz) for the purpose of receiving Sirius-XM broadcasts on the vehicle’s FM Radio. The only method for determining compliance of these transmitters according to the FCC Rules and Regulations is to measure the field strength from the transmitter installed in three typical vehicles. This article describes an alternative method for determining compliance by measuring conducted RF power output and using a transfer function derived by measuring the radiated field strength from transmitters installed in a number of vehicles.