Can't understand XPDR/DME antennas

Hi. I can't understand if I need two different antennas for having at the same time DME and transponder capabilities.
Antenna handles both. But at the same time or not?
Thanks.
Camillo

You will need two antennae. One for the DME and another for the Transponder.

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.

Ciao Camillo, saluti da Sun 'n Fun.

To expand a little further what has already been said, let me also add that, albeit the two antennas must be spaced one from another on the plane fuselage, there are high chances of interference between DME and transponder as the two systems use frequencies which are close to each other, and that's why both systems provide with two pins, called "suppression in" and "suppression out", so that when DME is talking the transponder will not start to transmit, and viceversa.

Found an old but still very interesting article mentioning also this issue here:

http://www.airsport-corp.com/atcrabs.html

Ciao, Luca

Ciao Luca and hallo to others!
Thank-you all for your answers. I'm about to order my antennas and don't want to waste money.

I'm planning installing two of them under the fuselage, just behind the rear baggage bulkhead.

Thank-you again.
Camillo