I just bought a pair of these passive ethernet-over-coax converters. I was very skeptical, but they actually work! I just can't figure out how... they do not require external power, coax only has one pair of conductors (inner wire and outer shield) and ethernet needs at least 2 pairs. I hate to break them open - was hoping someone could point me to a whitepaper or other technical docs to describe the process.
I can't see any way around some sort of microcontroller that multiplexes the upstream/downstream channels onto the single coax pair, but where is it getting its power? I wasn't aware Ethernet (without PoE) could supply enough power to operate a mcu, but maybe?
Another thought: if ethernet transmits and receives at different frequencies, a couple of tight bandpass filters would allow multiplexing both signals onto the same coax pair and breaking them out on the other end. This would be a completely passive circuit requiring no power.
Thanks for shedding any light on this.