Good to know: Sacrificial Coupling Nuts

When I first traversed the contents of our RF lab, there was one component that I couldn’t figure out what it was for. Here it is:

So, you screw it onto a male SMA connector, and what you get is… another male SMA connector. Only, it’s 2-ish centimeters longer. What on earth?

If you go into RF from a theoretical DSP and software angle like I did, it’s likely no one gave you a thorough briefing in handling RF gear, and it’s mostly picking up stuff as you go along. Well, this is something I picked up.

An SMA coupling nut

If you want to buy one of those connectors, they’re called coupling nuts. And while there are varieties of these connectors that actually do something, for example, reverse polarity, this is a simple tip that uses regular coupling nuts.

And just in case it’s still not obvious what they’re for: You can use them to protect your SMA connectors. For example, here’s a prototype board I was working on: You can see coupling nuts screwed onto the on-board, soldered-on SMA connectors. The only purpose they serve is to protect the SMA connectors on the PCB itself. Because the coupling nuts now take all the damage instead of the original connector, we colloquially refer to them as sacrificial connectors.

An SMA coupling nut

Prototypes can be expensive, and a $1 connector can save you a lot of grief. So, what could harm an SMA connector? The number one thing for me is quick connect SMA cables, which, instead of screwing on, are simply pushed over the thread. For quickly setting up experimental setups, they are incredibly convenient, because tightening and loosening SMA connectors is just annoying. But in general, any usage of an SMA connector will cause wear and tear, so just grab a few coupling nuts and cover your SMAs.

More important than prototypes in terms of protection is expensive RF gear and high-end cables (the ones that cost upwards of three digits). So, don’t slap cheap cables directly onto expensive gear.