Jim,
Here's the info on buddy boxes. For Futaba, Hitech and Airtronics the box is nothing more than 2 sticks and some switches. The buddy box does not need a batter or an antenna, heck you can even remove the antenna from the buddy box. For these brands the master radio does the transmitting and the antenna needs to be extended on it. When the trainer switch is pushed on these radios basically what is happening is that the sticks on the buddy box are given control and the master radio still transmits the signal.

Missing from the above list is JR/Spektrum radios. This is because they are a little bit different. For these radios the buddy boxes must have a charged battery in them, but the power switch is left in the off position. They still have the antenna down as the master radio does the transmitting. If the battery is not in the buddy box it will not function to control the plane.

Hope this helps

Ken