Post
by Albert H » Thu Feb 29, 2024 10:35 pm
I did one carrier current transmission system when I was at University (a very long time ago!). To get reasonable results, we found that we had to run about 20 Watts into the coupler, and even then our signal couldn't be received more than a couple of metres from the mains wiring! It also hummed (not too badly, but enough to be annoying). We used a valved rig in those days, with a couple of 6146s in the final, and used a repurposed guitar amplifier as the modulator!
After a couple of months, we moved to a "leaky feeder" system (as used in tunnels), which used a coaxial cable with slots cut into the braid (which was horribly expensive as I recall). The cable was run in a loop around the campus, with a couple of further loops around the halls of residence. Each feeder had a terminating resistor at the far end. We ran the same valved rig into the leaky feeder, and found that we could receive it almost 2 km from the campus! Obviously the power was reduced, and the coverage on the sites was excellent, and there was no hum.
After a couple of months of operation, it occurred to me that perhaps we should try other frequencies down the cable. The original AM effort was on 1 MHz (later 999 kHz), and a couple of us built a valved VHF FM rig that was on ~94.2 MHz. The VHF rig did around 9 Watts (using a QQE03/10 output bottle). The oscillator was a free-running job (no PLL in those days) and tended to wander a bit in frequency, but that didn't matter in those days, since receivers were also free-running and used "AFC" to lock on to the signal you wanted to receive. In those days, Band II was mostly empty (apart from the three BBC national services, and the Police around 100 MHz), so our little FM signal was easily received in the area. We were delighted to discover that the leaky feeder radiated the VHF signal really well - possibly better than the MW signal!
Periodically, we'd have an inspection of our gear by the Post Office, so when they were coming, we turned the concealed VHF rig off!
A couple of years later, I built my very first stereo coder for that system, and we replaced the mono mixer in the studio with a homebrew stereo one. We discovered that virtually none of our listeners listened on MW, because the VHF FM stereo sounded much nicer! Unfortunately, since we were only licensed for the MW output, we had to keep it going!
"Why is my rig humming?"
"Because it doesn't know the words!" 
