Boy, I wish that there was a more correct way to say this. I use the word transponder the way you did, but it really bothers me when I do that, since it really isn't proper, but I don't know of a better way to phrase it. Often I just call them "signals" instead of transponders. Ie a transponder is a sort of repeater on the sat that retransmits everything within it's band, say 11700-11740. In most cases, on that transponder, we might find several, even dozens of signals, each of which we call transponders. Like for example, the 11729 LEO and the 11735 RTN signals, are both the same transponder, as are a half dozen other nearby signals, yet we refer to them all as separate transponders.
I know I'll continue to call these signals as transponders, but I wish there was a more accurate terminology.