This paper investigates the advantages, disadvantages and tradeoffs of applying directive antennas to implant - on-body ultra wideband (UWB) communications. Our study concludes that directive antennas can reduce unnecessary exposition of human tissue to electromagnetic radiation, reduce exposition to narrow band interference, save energy and reduce the possibility of eavesdropping. However, when using directive antennas, receive/transmit beam direction needs to be guaranteed. We have also analyzed on-body node positioning, the best aperture angle and its tradeoffs. We further conclude that, at least, directive antennas with 180? aperture angle should be used at the onbody nodes. ©2010 IEEE.