As Walt mentioned above, planning the locations for all the needed antennas is important to insure reliable radio performance.
With that in mind a few additional things to consider,
Transponder: Because of the frequency of operation, 1030 / 1090 MHZ, the coax from the transponder to the antenna will have the highest loss. Mounting the transponder antenna near, rather then far from the transponder will minimize the line loss. Modern transponders output 150 to 200 watts of energy, high quality coax, good connectors, and short runs insure the max. energy to the antenna enhancing performance.
UAT ADS-B:
You may install UAT in the future, planning a location now for the UAT antenna would save having to shift other antennas around to get the needed antenna spacing. Most UAT makers require a min of 60" from the transponder antenna to the UAT antenna. The UAT operates at 978 MHZ and the spacing insures the transponder does not overload the UAT receiver.