It's mostly a function of how well your antenna is able to amplify the signal, how precise you can aim that antenna and how slow you transmit. If you have a very good antenna with lots of gain and a very precise mechanism of aiming it (at an object whose location is very well known, and Voyager is a moving target) it will take much less power than if either one of those elements is not optimal.
The idea here is that any radio energy that does not end up in the vicinity of the target was wasted and at 21 billion Km that gives you plenty of opportunity for mis-alignment.
Receiving the signal has similar challenges, with the added complication that this time the sender is sending with a power level that puts its signal under the noise floor by the time it reaches Earth.
Fun fact: this goes for the GPS satellites as well by the time their signal reaches your pretty little hand-held receiver and it takes nothing short of magic (to me, not to the people that design that stuff) to recover the signal.
The idea here is that any radio energy that does not end up in the vicinity of the target was wasted and at 21 billion Km that gives you plenty of opportunity for mis-alignment.
Receiving the signal has similar challenges, with the added complication that this time the sender is sending with a power level that puts its signal under the noise floor by the time it reaches Earth.
https://en.wikipedia.org/wiki/Noise_floor
Fun fact: this goes for the GPS satellites as well by the time their signal reaches your pretty little hand-held receiver and it takes nothing short of magic (to me, not to the people that design that stuff) to recover the signal.