2.92mm Connector (K)
Understanding the 2.92mm Connector
By the 1980s, the classic SMA connector was ubiquitous, but it failed catastrophically above 18 GHz due to its Teflon filling causing higher-order waveguide modes. Engineers needed a connector that could reach 40 GHz (the Ka-band), but they also desperately wanted to be able to plug it into their existing millions of dollars of SMA-equipped hardware.
The solution was the 2.92mm Connector, popularized by Wiltron (now Anritsu) as the "K-Connector."
The 40 GHz Air-Dielectric Cavity
To reach 40 GHz, the internal geometry had to change.
- The outer conductor was shrunk to exactly 2.92mm, pushing the chaotic $TE_{11}$ waveguide mode cutoff up to roughly 46 GHz.
- The Teflon filling was completely removed from the mating interface. The 2.92mm is an Air Dielectric connector.
- The center pin was shrunk to 1.27mm to perfectly maintain a $50 \Omega$ impedance in air.
The SMA Compatibility Problem
While the 2.92mm shares the exact same 1/4-36 external threads as the SMA, mating them is highly dangerous. A standard SMA connector is filled to the brim with cheap Teflon. Worse, many cheap SMAs have center pins that protrude slightly too far.
| The Engineering Fix | How it Protects the 2.92mm |
|---|---|
| The Shortened Male Pin | The male center pin of a true 2.92mm connector is intentionally machined shorter than an SMA pin. This ensures the heavy outer conductors align and mate *before* the fragile center pins touch, preventing accidental bending. |
| The Deep Socket | The female 2.92mm socket is recessed deep inside the housing. If you thread a sloppy SMA male onto it, the SMA's overly long pin won't smash into the bottom of the 2.92mm socket, saving the expensive 40 GHz connector from destruction. |
Frequently Asked Questions
Should I regularly mate SMA and 2.92mm?
Absolutely not. While they are mechanically compatible, standard SMAs are built to terrible commercial tolerances. The cheap brass threads of an SMA will slowly grind away the precision steel threads of the 2.92mm. Over time, the 2.92mm port will be ruined. Always use a dedicated 'Connector Saver' (a cheap sacrificial adapter) when plugging SMAs into high-end 2.92mm VNA ports.
What happens to the VSWR when mated to an SMA?
It degrades severely. When the air-dielectric 2.92mm meets the solid-Teflon SMA, a massive capacitive 'step' is created at the exact mating plane. This causes a significant VSWR reflection. The joint will work, but its performance is limited entirely to the poor performance of the SMA (usually terrible above 18 GHz).
Why is it called the K-Connector?
Because it was designed specifically to cover the entire K-band (18 to 27 GHz) and Ka-band (26.5 to 40 GHz). Wiltron trademarked the name 'K-Connector' in 1983, but today 2.92mm is the open, non-proprietary industry standard.