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I'm pretty good at repairing broken regular coax, but I recently had an RG-174 cut and it's too small for me to solder the center wire and keep the braid away from it. What's a good way to do this? Edits for clarity: this is for a feedline to an antenna, using for HF signals, so the repair method is important in keeping a clean signal.

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    $\begingroup$ By the way, the quality of repair you need to achieve depends on the power and the frequency of signals you want to transport – I'm just saying that because RG-174 is often commercially used over short distances to transport 2.4 and 5 GHz WiFi, and "a little thicker center conductor due to solder" matters more if your wavelength is centimeters than if it is meters. You say "regular coax", and that seems to be thicker, so I'm unfairly inferring that you're mostly working with more sturdy lower-frequency cabling,and the performance of repaired coax cable at GHz frequencies might disapppoint you. $\endgroup$ – Marcus Müller May 9 at 7:27
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    $\begingroup$ it's actually the feedline for my SOTA beams portable antenna, so I'm using 7, 14, and 28 MHz $\endgroup$ – scook May 10 at 19:14
  • $\begingroup$ oha! how long is that cabling? How much power do you push through? If I remember correctly, typical RG-174 has some 4 dB/m to 15 dB/m attenuation in the 5 MHz to 30 MHz range. $\endgroup$ – Marcus Müller May 10 at 19:41
  • $\begingroup$ It's a 10m cable, cut in about the middle, and I push 100W through it. I did successfully splice it, but I'm concerned about it long term. You got me thinking, though, so I went and found the Amphenol 112132, which is exactly what I need. Thanks! $\endgroup$ – scook May 10 at 21:51
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    $\begingroup$ I ordered the connectors so I should be able to repair it next weekend. I have an antenna analyzer so I'll check it out and post the results. $\endgroup$ – scook May 11 at 23:06
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At HF there are almost no limits on how bad your splice can be, as long as:

  • there's no short circuit
  • braid is connected to braid and inner to inner

You could even use a "chocolate block" connector :) chocolate
* photo from Tudor Barker, cropped

But to do a neat job:

  1. At each cut end, strip about 25 mm (1") of the outer sheath, and then about 7 mm (1/4") of the inner.
  2. open the braid out and lay it back on the jacket
  3. lay the inners overlapping and solder quickly and carefully. RG174 inner melts quickly, the trick is not to bend or force it until it sets again.
  4. insulate them with a 25 mm piece of electrical tape. (or more if you're < 200 MHz, less chance of a short circuit)
  5. fold the braids back, they will also overlap
  6. wind a bit of thin wire gently around the braid to keep it snug against the inner (but not so tight that it'll sink through the insulation
  7. solder the braids together carefully. Use a cool iron. Don't fully wet the joint - the RG174 insulation will melt and run out the ends! Just gently wet the bits of the braid where they meet, and tack the wrapping wire to the braid in a few places. The idea is to get just enough mechanical strength, not a solid tube of solder.
  8. insulate the whole thing with tape, or heatshrink if you remembered to put it on first...

I'm comfortable splicing coax like this up to 1.5 GHz for GPS without unreasonable loss. Of course it won't be a first class measurement cable, but for radio transmission a tiny, short impedance mismatch won't hurt. At HF the whole joint is 1/1000 of a wavelength, it will be fine no matter how ugly it is.

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    $\begingroup$ Great answer, Tom! However, IMHO your point #7 might lead to a cold solder joint. The the very ends of the braid should be quickly soldered with a hot iron before too much heat can transfer to the dielectric and melt it. On top of that, hemostats or tools meant for the the job can be temporarily clamped on the braid as heat sinks. And I don't use tape anymore, only irradiated polyolefin heat shrink tubing (like Alpha FIT-221). Much neater, less hassle, and it won't melt. :-) $\endgroup$ – Mike Waters May 14 at 20:26
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Hmm. I've done this many times. Would a magnifying headband and extra light help you? That's certainly helped me to do that. The same way as we would splice larger coax.

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  • $\begingroup$ my problems are keeping the center wire from the braid, and soldering the braid itself. On regular coax, I just terminate it with a bnc but in this case the bnc connector is too large for the cable. Maybe there's a different size bnc? $\endgroup$ – scook May 10 at 19:13
  • $\begingroup$ there are other connectors that you'd typically use with RG-174: SMA, for example $\endgroup$ – Marcus Müller May 10 at 19:42
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    $\begingroup$ actually found an RG-174 BNC connector. You still need a crimp tool for that smaller size! $\endgroup$ – Marcus Müller May 10 at 19:45
  • $\begingroup$ For those of us who are not in the loop what's the best way to splice larger coax? $\endgroup$ – user3486184 May 14 at 3:58
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Connectors.

Splicing without connectors will introduce an impedance "bump" which may or may not bother you. The magnitude of the bump may be considerable. The bump will cause reflections which may cause the spliced coax to act like a comb filter. It gets pretty messy unless you are able to splice the coax perfectly, and you can't do that.

Connectors aren't perfect either, but they have a lot of design work behind them to minimize the bump and the problems caused by the bump.

At DC, none of this matters, but I have to assume you aren't using coax for DC.

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  • $\begingroup$ Since you answered this, the original question has been clarified; it's now clear that he's using this at HF. What you said is certainly true at UHF+, but you might want to edit your question so it can be upvoted if someone chooses to. $\endgroup$ – Mike Waters May 14 at 20:34

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