Pin protrusion in custom SMA cable assembly

[matthuszagh]

I made a custom SMA cable assembly using Huber+Suhner connectors 11_SMA-50-3-6/111_NH (https://source.z2data.com/2022/2/9/1/6/12/976592/PdfFile_180454.pdf). After I made this, I noticed about 1 mil of pin protrusion on each connector. At the time, I assumed it was my fault and that I must have been insufficiently precise with the procedure, though the results otherwise look quite good. However, after measuring a number of commercial assemblies (see my related post here https://www.eevblog.com/forum/testgear/pin-protrusion-in-commercial-sma-cable-assemblies/msg5723673/#msg5723673), I'm wondering whether the fault is with the connector and not my assembly process. The pin protrusion I experienced on this custom assembly was significantly less than the pin protrusion on some commercial cables. After all, the coupling nut does snap into place over the center pin. And it's not surprising that manufacturers would design these connectors such that the pin depth is determined by the manufacturing of the parts, not the assembly process. Thoughts?

[shabaz]

A special tool (called a locator tool) is required to assemble this, if you absolutely want to be certain the pin is at an acceptable depth, within the actual allowed tolerance. The tool usually costs quite a lot (it often comes in a set, along with other tools for cable assembly).

For this reason, if for whatever reason ready-made cables are not suitable, then often a better way to DIY cables (if that locator tool is not available) is to use a right-angle connector, because those usually have the pin pre-inserted, to the correct depth. The right-angle connectors can cost more though. Also, sometimes a straight connector is preferred of course.

Sometimes there are straight connectors with pre-inserted pins, designed like a turned-pin socket on the inside, for a friction-fit to the coax center conductor. I think I've seen those for larger RF connectors, I can't recall if that's a possibility with SMA or not. In any case, those sorts of connectors often cost a lot - more than a ready-made cable usually.

Also, it can be worth having an adapter (a good quality one) semi-permanently screwed onto test equipment, so that if there's a risk of a cable damaging things, then only the adapter is ruined, and not the test equipment.

EDIT: Incidentally, since you have a depth gauge, you could easily DIY yourself a locator tool. That's what I did, before I had a ready-made locator tool (although I guesstimated using other means, since I didn't have a gauge). To DIY a tool, you'd need to get a disposable SMA plug, insert the pin, measure it, and fix it in place (could bung the other end with epoxy glue. You may need to devise a way to keep it at the correct depth till the glue dries. Alternatively, replace the pin with say copper rod filed fully flush on the end, and solder it to the correct depth, using your gauge. Next, get a good quality SMA socket, torque on the shell of it to your SMA plug, and then insert a metal rod (again filed fully flush) into the SMA socket rear side, until it hits the pin, and then fix that in position (solder or epoxy glue). Now you can discard that plug. The socket becomes your locator tool.
Yet another approach is to get a machine shop to create a locator tool. But the DIY approach mentioned should work well I think.

[hp3310a]

A special tool (called a locator tool) is required to assemble this...

From your description I take it it's some sort of cylinder with a hole that fits the pin and allows it to protrude exactly the right amount. Is that it or is there more to it, do you have an example or link?

[shabaz]

Hi,
Yes, it's exactly that; simply a hole with the correct depth, photo attached. It is straightforward to DIY with a female SMA connector and a piece of metal rod.

[Miek]

One extra note on that tool: on my ones at least (Omni-Spectra T-2367 & Huber Suhner W14), the depth of that hole isn't critical. Instead, the diameter of the hole is sized for the narrow part of the centre pin, and the step in the pin sits flush with the top of the hole. That top surface also sits flush with the reference plane on the mating connector, so it automatically gets the pin in the right place with no critical measurements.

[matthuszagh]

Thanks Shabaz, that's exactly what I needed to know. That locator tool is what I was missing. Fortunately, the H+S W14 isn't that crazy expensive, so I'll probably just order that and not worry about making one custom.

[shabaz]

..the step in the pin sits flush with the top of the hole. That top surface also sits flush with the reference plane ..

That makes more sense. Thanks for the info!

Fortunately, the H+S W14 isn't that crazy expensive

Ah nice, I just saw the price. That is lower-cost than I thought!