http://www.aea.net/Training/courses/...20Guidance.pdf

I've been trying to find out what antenna on the aircraft is used for the ADS-B Out. From what this says, the same S Mode transponder antenna can be used. It also recommends the use of 2 antenna, one on top and one on the bottom of the aircraft. The top mounted antenna is not mandatory, however in my opinion it should be, if they have plans to track aircraft by ADS-B satellites.