The microwave and RF characteristics of FR4 substrates

FR4 epoxy glass substrates are the material of choice for most PCB applications. The material is very low cost and has excellent mechanical properties, making it ideal for a wide range of electronic component applications. As more and more microwave systems aimed at consumer markets are developed, there is a considerable interest in minimising the cost of these systems. Substantial cost savings could be realised by using FR4 in place of costly PTFE based substrates for microwave circuits and antennas. This paper describes a programme of work which has been undertaken at ERA Technology, as part of its Membership Research Programme, to evaluate the characteristics of this material at microwave frequencies between 1 GHz and 15 GHz. It is concluded that the use of FR4 is unlikely to be viable for antenna feeding structures due to its high losses. However, for high density microwave circuits where path lengths are short and for broadband antenna elements, where losses and absolute dielectric constant values are less critical, the material could be used in place of more conventional microwave substrate materials, offering significant cost savings.