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I have a run of TV/DVB-S coax in my house and I think about abusing it for ethernet.

I am aware of the "good"/modern solution (ethernet over coax, fancy modulation an all, something like 270 Mbit/s), but since I don't need the fast data rate and have some 10base2 (thinnet) equipment idling around my house, I have the idea of running 10base2 over the coax segment.

Of course, there will be an impedance mismatch between the 10base2 transceivers (50 Ohms) and the TV coax (75 Ohms). Some energy of the signal will be lost due to reflection (that's why people usually use 10base2 with proper 50 Ohm coax like RG-58). However, I still wonder how my chances are if I try it the redneck way ("There, I fixed it!"). The length of the "wrong" coax is only something like 15...20 m (approx. 50 ft), after all - and the 10base2 standard accepts up to 185 m (606 ft). My hope is that trading a shorter length for the higher/mismatched impedance might make my hack work.

How robust is 10base2? Has anyone some experience about 10base2's tolerance for an impedance mismatch? Has anyone ever tried something like an unterminated run of 10 m of 10base2 - the effect may be similar to what I'm up to...

I know it's all outdated and stuff, but if it does the trick, I would be a happy person!

Note: There is a related question already, but the exact detail I'm curious about is ruled out over there. I'm all in for the major style points, accepting the vintage tech.

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  • Banjos start dueling..
    – Lynn Crumbling
    Apr 15, 2014 at 13:37
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    @zebonaut The song "Dueling Banjos" is a song tradionally associated with the southern United States "hillbilly" culture, the hillbilly culture is a regional subset of the redneck culture, you used the term "redneck" in describing your setup, therefor Lynn made the comment referring to the song :)
    – Scott Chamberlain
    Apr 15, 2014 at 13:55
  • @ScottChamberlain Hahah. Thanks Scott. Perfect explanation.
    – Lynn Crumbling
    Apr 15, 2014 at 14:02

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An impedance mismatch like that is generally a bad idea. It's not just the energy loss, but the reflections rattling all over your circuit can lead to interference. I haven't used 10base2 myself, but it sounds very sensitive to interference, and I would guess that the impedance mismatch would be a killer.

Now there are a few "redneck" ways around it.

  • Try pulling through RG-58 using the 75-Ohm cable. I have done pulls here and there - it really depends on how the installation was done. There is a risk of getting stuck or tearing a cable.

  • Adapt the signal over the impedance transition. There are a few ways to do this:

    • There are commercially available Impedance Matching Pads. They have an internal circuit which should do the job:

    http://www.minicircuits.com/products/MatchingPads.shtml

    • You can could try a DIY circuit, e.g.:

    http://www.ham-radio.com/k6sti/match.htm

    • Also, reflections are minimized if you gradually change the impedance of a waveguide. You could make your own cable where it gradually changes from 50 to 75 Ohm. Unfortunately, this transition cable might need to be longer than you like, and adjusting the impedance by hand is tricky.

Easiest test is to try it! (if you already have all the equipment)

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  • Excellent answer, you link to some of the sources I have read and considered trying since I first posted the question. As far as the DIY circuits go, I am pretty sure it would have to be the resistive divider, because the others (using L and C), while lossless, would work at a specific frequency only. Also, I am happy to announce that the dirty hack of 10base2 over 75 Ohm coax (without impedance matching networks) works well enough, and even shows a fairly good eye pattern on the oscilloscope. I still have to compare the current setup to to using a resistive matching network...
    – zebonaut
    Jun 20, 2014 at 8:44
  • That's an important result. Congrats!
    – sudoodus
    Jun 24, 2014 at 18:49
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Seven years on: In about 1985 I added a short stub of 75 Ohm cable between the 10BASE2 LAN proper and a workstation. The main LAN uses 50 Ohm cable. This short stub crippled the network, reducing throughput and reliability to a small fraction of usual.

Use of all 75 Ohm cable may well work - or work better. If there are on-card terminators they may cause problems.

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