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Twinax (shielded controlled impedance balanced line pairs) is said to be used in 10G ethernet signaling for greater noise immunity that other cabling types. Why isn't twinax (or similar) commonly used for amateur radio antenna feed lines, where noise immunity on receive is also quite important? Rather than unbalanced coax or unshielded twin-lead?

(Added: Especially when dealing with a balanced antenna (dipole), balanced differential inputs, or balanced push-pull final RF amplifier stage.)

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    $\begingroup$ * citation needed. Is twinax is less noise-immune than coax? And is noise immunity really important on receive, given that there's an antenna at the end of the cable? Of course there are special cases. $\endgroup$
    – tomnexus
    Feb 19 '20 at 10:09
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Why isn't twinax (or similar) commonly used for amateur radio antenna feed lines

It's always hard to say why something isn't done, but after looking around at the options I'd say cost has to be one factor. A 100' spool of 18AWG or 20AWG twinaxial cable costs around $220, which is five or six times the cost of coax or ladder line. The connectors are also much more expensive than typical coax connectors.

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Why coax or twin lead or ladder line?

  • Because the others are traditional.
  • Because twinax is relatively rare and expensive.
  • Because unless you are dealing with high frequencies (>5GHz) twinax is overkill. Most amateur radio is <1GHz where coax and twin lead make more sense. Above 5GHz, more commonly I see wave guide and hardline used.

Having said that, I have seen twinax used for short runs in situations where twin lead was inappropriate — like passing through a metal window frame.

Also, it has been pointed out (in another question) that twinlead is not better (lower loss) than high end coax, so I suspect that twinax may also not be better, and possibly waveguide and hardline are better than twinax already.

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  • $\begingroup$ The other question that I'm thinking of was about ladder line vs. twinax. I think Phil Frost pointed out that ladder line has considerably lower loss than twinax. Same is true with coax, twinax has twice the loss of a single run of coax. $\endgroup$
    – Mike Waters
    Feb 20 '20 at 16:17
  • $\begingroup$ It sounds like the advantage of twinax is low noise, where in AR we care more about low loss. $\endgroup$
    – user10489
    Feb 21 '20 at 12:41

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