I have a feeling this is normal, but wanted to verify. I've been checking all my antenna cables as I make them and noticed that if I test both ends of the NAV cable with it disconnected it's open between center conductor BNC back shell. Once I plug in the NAV cable to the 255 and check the other end, the center conductor has 0.3 ohm resistance to the BNC back shell. This is the only cable that acts this way, all the others remain open between conductor and back shell(comms,gps,transponder).