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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.

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  • \$\begingroup\$ This says it requires something connected via "poe active coax adapters". I'm not sure what that is, but that sounds like some kind of power over ethernet injector to me. \$\endgroup\$
    – Hearth
    May 29, 2021 at 19:17
  • \$\begingroup\$ @jsotola no you are mistaken. I know what you're referring to, but these are actually ethernet extenders using coax cable. \$\endgroup\$ May 29, 2021 at 19:19
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    \$\begingroup\$ I'm not too familiar with ethernet; is there a single-pair mode that they could be using? I know that gigabit and up requires all four pairs, but there may be some mode that just uses a single pair, and then just use a balun to send that over coax instead of twisted pair... \$\endgroup\$
    – Hearth
    May 29, 2021 at 19:22
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    \$\begingroup\$ What if it just uses a hybrid like how telephones are able to have transmit and receive over a single pair of wires? \$\endgroup\$
    – Justme
    May 29, 2021 at 19:36
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    \$\begingroup\$ 10-base-T 10Mbps and 100-base-T 100Mbps use two unidtrectional pairs, there's no single pair mode. 10-base-2 uses a single coax bus, but is half-duplex. I am not aware of any passive 10-base-2 to 10-base-T media converters \$\endgroup\$ May 30, 2021 at 0:52

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There's probably a hybrid transformer for the ethernet and a bias tee for the power in there

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  • \$\begingroup\$ Thanks, those are interesting components that I didn't know about! Learning! \$\endgroup\$ May 30, 2021 at 0:57
  • \$\begingroup\$ So with this setup, what are the downsides? Why not just design all ethernet this way and reduce the number of conductors needed? \$\endgroup\$ May 30, 2021 at 0:58
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    \$\begingroup\$ the main disadvantage is that coaxial cable is more expensive than twisted pair (unless you already have coaxial cable installed) \$\endgroup\$ May 30, 2021 at 1:01
  • \$\begingroup\$ Also curious: how much wattage is available in the ethernet signal itself (not PoE )? Could a bias tee extract a DC signal with enough power to operate a MCU? \$\endgroup\$ May 30, 2021 at 1:03
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    \$\begingroup\$ the signal is 3V, 3V into 100 ohms is 90mW and you will loose half of the power in the dummy load at each end (if using the hybrid transformers), there is no power when the terminal is not sending. \$\endgroup\$ May 30, 2021 at 1:07

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