Sorry Dave,
In the case of some World wide beams or Continental beams they do transmit all channels (transponders) on one antenna, you can multiplex more then one transmit signal into one feed then into one antenna, it is done all the time.
Now some satellites have spot beams meant for a local or regional location, these can be one or several transponders, into one or more antennas, these antennas are then directed at certain locations in the satellites foot print.
Here in the US, the main satellite signal suppliers use both, several transponders into a main or CONUS beam for stations like HBO or Showtime (and others) and spot beams for local TV channels, like one spot beam for Northern California, several for central Califonia, and several for Southern California, these spot beams can cover lets say the Los Angles area and a different one for the San Diego area, with the local channels for that area only.
