DME antennas are relatively a small blade or a short (2.5 inch) metal rod. They mount with two screws into the housing and require a hole for the antenna BNC connector. The easiest place to install one is just off center under the console forward of the spar )ideally under the ashtray area) where you have room to work. You will be leaving the outer skin on the fuselage and removing a small amount of the honeycomb for the antenna jack feed-through. Longer screws can be used so as not to have to remove any honeycomb, just drill through the 1/2 inch honeycomb. This also allows for a short antenna run that is out of the way. Placement to be determine not to interfere with the cables and trim.

W&B change would be negligible and should be under a pound. This is for the cable and antenna only. Radio is different matter.

Cheers

Roscoe