Having trouble locating any information on an 'Invisible Link' (I believe Infra-Red) that was advertised in a tiny magazine available to pirates in the early 90's. I remember it being on a page near 1kW amplifiers (that were a big thing back then).

Anyone?

Can anyone chime in here? Would love to know more about this kinda stuff.

Elektor did a long range infrared transceiver in the late 80's, they put the sender diode (LD271) into a car headlamp and used a 4 or 6 inch magnifying glass at the other end to focus the signal onto the receiver diode (BP104). The range was alleged to be 1750 Metres in ideal conditions, but you can bet it'd be more like 50 Metres in fog or heavy rain. Elektor Spain May 1988 - I have a PDF but it is in spanish, and I think I still have the English hardcopy, but I'd have to dig it out and scan it. It is mono though. You can buy stereo infrared headphones offa fleabay for €20, maybe using the same headlamp/magnifying glass arrangement would extend the range? There are some good webpages out there for making audio links using collimated lasers - this would be a more modern solution.

Back in the 1980s I used a pair of infra-red beams from one block to another - about 175m as I remember. The idea of a pair of beams was to try to improve the signal to noise ratio and prevent drop-outs when birds flew through the beams! It worked quite well, but required careful shielding from the sodium vapour lamps on the tops of the blocks that came on at night. When the lights came on, we got nasty hum in the received signal until I added a plastic tube to the receiving telescope to get rid of the orange lights. The link was horribly difficult to set up accurately, and the audio through it wasn't particularly good quality. We gave up on it after just one weekend.

The next link was up the mains at about 125kHz, and was able to get cleanly from one block to the next. It was mono, but that was all we broadcast at the time, and it worked very well. When the DTI chaps found the gear, they couldn't work out how the link receiver worked (it was connected to a dummy wooden dipole!).....

I did do a stereo mains link, but that was from a flat a couple of floors below the rig on the roof. It sounded great, and was used for a few months until the studio was raided because of poor site security and DJs who were a bit too casual.....

125kHz over the mains, wouldn’t that have to get through roadside transformers and power lines etc... to get to the next block?

The received level of the 125 kHz suggested that the mains wiring in one block was radiating enough to get to the other block! The 125kHz was very unlikely to get through the substation transformers at the bottom of each block!

Ordinarily, a mains link will only work on the same phase of a supply. I did one system that worked between Neutral and Earth, which gave great results and obviated the need for high voltage couplings to the mains. As you got closer to the sub-station, the range of the mains link decreased, since the Neutral to Earth impedance would reduce.