The premise in question is that a receiver entails a transmitter. I worded this a little differently than the original argument. Here is how it is worded in the Summa:
"But this cannot go on to infinity, because then there would be no first mover [transmitter], and, consequently, no other mover [receiver]..."
The first mover is the cause, and the final mover is the effect, with the intermediate movers as "conduits" through which the cause travels. If the string of intermediate movers is infinitely long, then there is no cause and hence no effect. My argument for this will consist of the term "receiver" meaning it is a receiver of some effect, and "transmitter" meaning it is the source of that effect.
1. If there is no transmitter, then there is nothing being transmitted and hence nothing being received
2. But there is a receiver
3. Therefore, there is a transmitter