75 Ohm cable
I should add that at one stage (early 80s) I used a network which was designed to be wired up with 75 Ohm TV coax. It consisted of small boxes, allegedly each contained a Z80 with an RS232 connector, a TV connector & a small stub of TV coax with another connector on it. These were daisy-chained with more TV coax. I can't remember what the head end was like but it must have broken out a batch of RS232s to connect to the host, a Z8000 box.
A few years later, and another job not a million miles from Euston, I came across a very much grown up version, again strung together with coax but definitely not TV coax, doing much the same job. In that case the head end had a room to itself but still fed the serial lines through to a server. And that gig also had some of the original hose-pipe sized Ethernet as well.