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I have a length of RG-316 single shield 50Ω coaxial cable carrying a 5.8 GHz signal that I need to connect to a PCB. While cheap relative to most coaxial connectors, SMA connectors are still quite expensive, take up space and are relatively heavy. The PCB was designed for side mount SMA connectors.

  • Could I replace the original SMA connectors with direct soldering joints like this, without causing a large impedance mismatch?

Direct joint vs SMA

  • How could I improve the RF performance of the connection?

  • The mechanical strength of the joint is poor, and the teflon insulator does not bond well with common adhesives. Can I protect the joint with nonconductive adhesives (epoxy, hot glue) without significantly affecting its RF performance? What would be the best way to secure it mechanically?

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    \$\begingroup\$ I think the more important question here is if from a usability perspective this makes sense: are your users likely to want to change the cable, or is this pretty unlikely \$\endgroup\$
    – PlasmaHH
    Commented Jan 25, 2016 at 20:39
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    \$\begingroup\$ Good point, and it doesn't look too professional either. In this specific case I am the sole user, but I phrased the question in that manner since other people might find it useful in the future. \$\endgroup\$
    – jms
    Commented Jan 25, 2016 at 20:49

2 Answers 2

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Soldering directly to the pads of the SMA connector instead of using SMA connectors should work fine electrically.

The obvious problem is that the cable is no longer removable. If you're fine with that, then go ahead.

It would be good to have some mechanical strain relief if this has a chance of getting flexed. At only 2 inches wavelength, you do need to be careful about impedance changes over only a few mm. I'd stay away from adding any material around the solder joint or the wires outside the shield. However, you can do most anything you want back down the cable a bit from the solder joint and where the shield is still intact. Perhaps you can hold the cable down with cable ties or something further from the board.

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    \$\begingroup\$ Strain relief is particularly important for soldered wire, especially multi-conductor. The solder gets wicked up some distance into the cable, and the region where it stops is a very strong stress concentrator. In the long run, the conductors will work-harden and then fatigue right where the solder stops. \$\endgroup\$ Commented Jan 26, 2016 at 1:04
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Excellent advice in comments and answer thus far. Especially, yes I agree, nothing over the soldered joint itself. BTW, such a beautifully soldered joint as shown, IMHO, shouldn't add any more loss than the physical dimensions of the proper connector would otherwise do.

I would only add to please obey any advice you may come across from the providers of RG-316 cable. It is in my mind that now that you no longer have a large-ish connector on the end of the cable, there may be a temptation to thread this small diameter cable when re-installing, through nearby holes or bends which may kink it or put a sharp bend in it while also providing some strain relief.

Please, definitely no kinks or sharp bends in coaxial cables carrying RF. Do any strain relief you wish to, but nothing tighter than a gentle curve in the coax or you will lose RF in the cable.

Personal opinion: if I was doing something similar to any equipment of mine, if it came to a choice of kinking a cable or not having strain relief, I would definitely choose no strain relief. I mean, how rough do you plan on being with your own equipment?

Bear in mind that a cable tie is a good idea for strain relief, but if you pull it tight so it squashes the cable even a little, you will have altered the characteristic impedance right at that place in the cable, and you will lose RF due to setting up circulating currents by creating that very discontinuity.

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