Just to expand on Norman's response... DME and transponder are two completely separate systems, with no interconnection whatsoever. The operate in approximately the same frequency band, which means their antennas look the same. DME works by talking to a DME groundstation, transmitting and receiving as necessary to carry out that function. The transponder on your airplane responds to interrogations from ground-based Secondary Surveilance Radar (SSR). There is no connection between the ground-based SSR and any DME station, so both transponder and DME on your airplane may be transmitting at the same time or receiving at the same time, or any combination of transmitting and receiving at the same time. It is for this reason that you must have totally independant anntennas for the two systems.