IIRC, all transponders have their own built in
GPS receivers. They may or may not require an external
antenna, depending on where they are installed and on the demands of the specific unit. Splitting will not
work for the gps portion. However, splitters can
work well for the
VHF tx/rx part. The best ones, like the Vesper unit, have built in preamps which compensate for any insertion losses. We are using a Vesper and with our masthead standard VHF
antenna get excellent range of
reception (typically 40+ miles with normal conditions) and have had reports of being received at as much as 15 miles.
Jim