Bulkhead connector to board connection - best practices?

[jmw]

I've been using die-cast Hammond boxes lately for a number of things; how do I make good connections from panel mount RF connectors to a PCB inside? With a bulkhead connector like this (https://www.digikey.com/product-detail/en/amphenol-rf-division/132139/ACX1237-ND/1011914), surely soldering hook-up wire from the solder cup to the board isn't going to give you a controlled 50 ohm impedance ... right? So, really basic plumbing question: what should the connection look like from the inside end of a panel mount connector, and how should it be terminated at the PCB to get a good match?

[OwO]

That connector isn't designed for very high frequencies; as you discovered there is no way to get a controlled impedance line to the board. There are similar connectors that have a teflon cylinder around the center pin with a specific diameter, which you need to precisely match with a drill hole on the enclosure wall, but my preferred approach is to simply have a u.fl or ordinary sma connector soldered onto the board and use a jumper coax cable to go from the board to the enclosure. You can find bulkhead sma to u.fl cables easily.

[matthuszagh]

I was thinking about this recently too. I generally agree with OwO, though I generally don't like using U.Fl - I'd typically go for an SMA connector as the PCB mount connector. But, I think the combination of PCB-mount coaxial connector plus jumper cable with one connector being a bulkhead connector is a good approach and should achieve optimal performance, including at very high frequencies. It also allows a lot of flexibility in terms of where the connectors are placed on the enclosure and PCB, and the PCB can be used outside the enclosure by just unfastening the jumper cables. The downside, of course, is cost, since it multiplies the number of requisite coaxial connectors by 3 (compared with just 1 PCB mount connector).

Another, cheaper, solution is to use a connector that can be PCB-mounted but also extend through the chassis panel. An example of such a connector is the Molex 73251-2201, which has a long, fully-threaded body. Though it wouldn't work with the connector just mentioned, you could use straight, through-hole SMA connectors on the bottom side of a PCB, and then place the PCB just under the top of the enclosure, with the components facing down and the SMA connectors facing up. A quick search didn't yield any straight, long-body connectors like the Molex I just mentioned, but you could probably get normal body-length connectors to work. This does reduce your flexibility, since things will need to work around where the connectors need to go. Also, many of the connectors that permit this probably won't work so well at high frequencies, though with some careful EM simulation and design it might be ok.

I'd probably go with the former approach in most cases, at least when I care about the finished result, which is usually the case when I put something in an enclosure.

[David Hess]

Where this matters, the solution that I always see is to have the bulkhead connector terminate into another connector for a coaxial cable which goes to the printed circuit board.

[matthuszagh]

Where this matters, the solution that I always see is to have the bulkhead connector terminate into another connector for a coaxial cable which goes to the printed circuit board.

I think we're saying the same thing. But, to confirm, does this image depict what you're referring to?

[David Hess]

Where this matters, the solution that I always see is to have the bulkhead connector terminate into another connector for a coaxial cable which goes to the printed circuit board.

I think we're saying the same thing. But, to confirm, does this image depict what you're referring to?

The bulkhead connector does not terminate into the coaxial cable directly, but into a connector and then a short coaxial cable with connectors on both ends is used.

Tektronix did this with Peltola connectors on the inside of the chassis.  They had a BNC to female Peltola bulkhead connector, and perhaps some other variation that I never saw.