I had to look it up as well. I am amazed that voyager 1 is running on about 470 watts of power and is still able to transmit an intelligible signal to earth. Im also wondering how much a transmitter that was capable of the Ghz range cost back in the mid 70s when it was built. Incredible stuff.
The two Voyagers are often seen downloading data through NASA's Deep Space Network. Here's a real time DSN status page: http://eyes.nasa.gov/dsn/dsn.html
GHz transmission in the 1970s is not special. The cavity magnetron was invented during WWII and used for RADAR purposes and not long after microwave connections were being used for long distance telephone connections.
The magnetron is a good point! But for RADAR applications you just need a big source of emissions that can be pulsed on and off. The magnetron does that well enough. I have never heard of anyone using a magnetron modulated in some scheme to transfer information at any some useful rate. It's definitely possible with gunnplexers. You could certainly do more code with a magnetron, but even that would be difficult given their unstable output frequency.
Microwave towers were pretty common in the 70s, and were used before then for long distance telephone links. I can't imagine that they were all that more pricey than any other transmitter.
As for signal transmission, consider that the radio noise is 100 times stronger than the GPS signal your phone's nav software is trying to decipher. Now imagine a humongous antenna with much more processing power than your Samsung Galaxy. Though amazing that we can still make sense of Voyager's signals, it's not quite to the "miraculous" category.
There's still inverse square law at play, although it's much more forgiving than atmospheric attenuation (logarithmic db loss with distance versus linear).
