Originally posted Sept 10, 2015
K connectors, microwave measurements and careful plumbing
Over the years, I’ve heard several engineers speculate on alternative lives as plumbers. It’s a career that requires some technical knowledge and pays well, but can be shut off entirely—mentally and physically—at the end of the day and on weekends. One of the engineers lived next door to a plumber, so his wistful musings were probably well informed.
As a homeowner, I’ve done my share of amateur plumbing, and there is certainly satisfaction in a job well done—or at least one that doesn’t leak too much.
Of course, the plumbing that pays my bills is a rather different kind, and requires an even greater degree of care and precision. For example, the specifications for microwave and millimeter connector gauges show resolution better than 1/10,000 inch, or around 0.002 mm.
I’ve been looking into high-frequency connectors to make sense of something a friend said to me while discussing different connector types. When the subject of the 2.92 mm or “K” connector came up he said, “I have two of those and they’re both broken.”
I didn’t ask for details, but had heard elsewhere that 2.92s might not be as robust as their 2.4 or 3.5 mm cousins. One online source mentioned a thinner outer shell for the conductor, while another mentioned potential damage to the center conductor.
On the other hand, the K connector offers some distinct advantages in microwave and millimeter connections. It covers frequencies to 40 GHz or higher and is mode-free to about 45 GHz. It also intermates with 3.5 mm and SMA assemblies.
To help avoid damage, the 2.92 mm male connector is designed with a shorter center pin, ensuring that the outer shell is engaged before the center conductors make contact. The outer shell is thick and should be relatively strong.
The situation became clearer when I got a close look at two damaged 2.92 mm connectors. It helped me understand the dimensional requirements of a 40+ GHz connector that can mate with 3.5 mm and SMA connectors.
Damage to the collet or female center conductors of two 2.92 mm K connectors has rendered them useless. The fingers of the slotted contacts are bent or missing, likely from mating with a bad SMA male connector.
The 2.92 mm connectors should not be prone to damage when used with other 2.92 mm connectors, but intermating with SMA connectors—one of the benefits of this family—is more likely to be destructive.
For a brief explanation, start with the rule of thumb for determining the maximum frequency of coax:divide 120 GHz by the inner diameter D (in mm) of the outer conductor. The outer diameter d of the inner conductor is constrained to a specific D/d ratio to obtain the desired impedance. With a fixed d, the comparatively large center pin of a K connector results in very thin slotted contacts for the female center conductor.
Combine these thin contacts with SMA connectors that have looser tolerances, and which are more likely to have misaligned or projecting center pins. The result is a higher risk for damage to connectors that are otherwise robust and high-performance when mated with their own kind.
It’s logical to assume that a 3.5 mm connector, with larger d and thicker, stronger contacts, would be less likely to suffer damage from mating with an SMA. This appears to be the case, though insertion forces—and the chance of increased wear—may be higher.
It took a while for me to figure this out. One reason: some resources online were simply wrong, claiming, for example, that 2.92 mm connectors have thin outer walls (often true of SMA) and that metrology-grade versions are not available.
I now understand this small-scale plumbing a little better and can appreciate K connectors more fairly. They perform very well, are durable, and offer intermating advantages. Of course, you’ve got to take care when using them around SMA hardware, but that’s a good idea for 3.5 mm connectors as well.
SMA hardware also can be a hazard to 2.4mm and 1.85 mm connectors, and it’s worth paying close attention to the mating habits of the expensive plumbing on your bench. It’s an essential part of getting the performance you’ve paid for.