A pair of Homeplug power line networking adaptors arrived in the post today (no, they're not for use here!) and it reminded me of how far we've come with power line communications in the last few years - who would have imagined that we would have 500 megabit networking down mains cable (or over any other sort of cable)?
I tested the Homeplug (200 megabit) adaptors copying a large file, from one side of the house to the other and the real throughput was more like 35 megabit (around 4MB/s). This is still about twice as fast as WiFi (even using type-N speeds) over the same path though. Good enough for what they will be used for (extending an ADSL internet connection to a garage).
Back in about 1992, I was playing around with a pair of Radio Shack/Realistic/Tandy Plug 'n Talk intercoms. They used 230KHz FM to transmit down mains wiring. That was in the Long Wave broadcast band, and even though they used FM (narrow deviation), they could still be received on 225 and 234 KHz AM broadcast channels (by slope detection, being a few kHz off the centre frequency).
They had a locking PTT button, so could be used as a baby monitor and one of the ones I used had been modified to have a line in jack socket. This made it a really lo-fi version of today's iPod FM transmitters, I could broadcast to Long Wave radio receivers in the house. The signal actually extended beyond the house, either along power cables or direct radiation. It could get up to around 250 metres depending on where the power cables were coming from, also holding a portable radio by telephone poles improved the signal greatly.
At that time there were also some intercoms that used lower frequencies of around 80 - 140KHz (a 4 channel model with 20KHz spacing?) but these would have been no use for my microbroadcasting activities, being below the Long Wave band.