A new generation of WSN communication transceivers are now available that support time-of-flight distance measurement. This measurement can be inseparably integrated with message transmission making it possible to authenticate messages based on distance. In this paper we present a practical implementation of Distance Based Message Authentication (DBMA) for WSNs using Nanotron NA5TR1 transceivers. We show that DBMA can be used to reject messages sent from outside a secure (trusted) area. With DBMA, messages can be authenticated without involving costly cryptographic algorithms. The DBMA implementation is evaluated in two different deployment scenarios. Distance measurement errors and their impact on the size of the required secure area are evaluated. Furthermore, we present methods to reduce the size of the required secure area.