In installations where two TDR-94D transponders are used, but space is not available to install four antennas, special relays can be used to operate the two TDR-94D transponders with only two antennas. This configuration can also be used to operate two TDR-94 transponders with one antenna. The diagram below shows a Coaxial Switch from Ducommun (P/N: D2-729B002) completing the configuration. The described PN has been replaced by a more recent version, D2-729B003. This new version is available from Southeast Aerospace.