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New for May 2025. This content was submitted by Davide, who totally wrote the book on connectors, you can buy a paper copy (760 pages) here. Get one for yourself, or if you are too cheap, have your company buy one for their library... Thanks again, Davide!
Coax connectors started in 1922 when Belling Lee developed a connector that was only functional at middle waves (the AM broadcast band). It was followed by the UHF connector in the 1930s, the N-type connector in 1942, the BNC in 1944, and the F-type in 1950. In the following years, the industry developed smaller and higher-frequency microwave connectors.
Microwave connectors started in 1958 when James Cheal of the Bendix Scintilla Division developed the BRM (Bendix Real Miniature) connector. Omni Spectra manufactured it in the 1960s as the OSM (Omni Spectra Microwave). In 1968, it got the name SMA (Sub-Miniature type A).
In 1974, Maury Microwave developed the MPC3 connector capable of 40 GHz. It was commercially unsuccessful because, back then, there were no applications for it. As test instruments capable of such high frequency became available, they had a use for this connector, which was codified by the IEC 61169-35 standard and manufactured by others as the 2.92mm connector.
Wishing to improve the performance of the SMA, engineers developed precision connectors that are nearly compatible with it but have a higher cutoff frequency and can handle more mating cycles (essential for test equipment).
In 1976, Larry Renihan of Hewlett Packard, working with Amphenol, developed the 3.5mm connector as a metrology-grade version of the SMA for use in its test instruments, as it can handle many more mating cycles than an SMA. It comes in two grades: the precision-machined LPC (Laboratory Precision Connector) for calibration standards and the GPC (General Precision Connector) for general use. It can operate up to 26.5 GHz, thanks to its air dielectric and precision machining. Six delicate spring leaves in the female socket accommodate pins of various diameters. Later, Amphenol sold it as the APC-3.5 (Amphenol Precision Connector 3.5 mm). It is also known as the 3.5-PC, PC-3.5, 3.5-PC, and OS-3.5. These connectors are used in vector network analyzers. As they are expensive, relatively delicate, and hard to replace, savvy engineers use a jack saver: a 3.5mm to SMA adapter that is semi-permanently mated to the network analyzers. That way, if the adapter is ever damaged, it’s relatively easy to replace. The 3.5mm has fallen out of favor, with the better-performing 2.92mm connector taking its place.
In the 1960s, engineers developed a microwave connector that was cheaper and smaller than the SMA connector and used a faster snap-on fastening. However, it was less reliable (vibrations could unmate it) and had a lower cutoff frequency of 4 GHz. By 1968, the connector became a standard and got its name of SMB (Sub-Miniature B). It is also known as the CN50. Unlike most coax connectors, the female is shrouded.
Bill Oldfield at Wiltron (now Anritsu) optimized the design of the 2.92mm connector and relaunched it in 1983 as the K connector (as it can operate in the K-band range). Other names include S292, SK, and SMK. The socket in the female has four spring leaves that accommodate pins of various diameters (sturdier than the six delicate leaves in the 3.5mm connector). Its shorter pin does not bottom out in the socket (as may happen in the SMA or 3.5mm). Oldfield also invented the V microwave connector for the V-band.
In 1986, Hewlett Packard’s Julius Botka developed the 2.4 mm microwave connector.
The SMB connector snaps on and is mated quickly, but vibrations can make it slip off. In 2002, the automotive industry overcame these limitations by adding a latched plastic shell to the SMB female plug to create the FAKRA (Fachkreis Automobil = Auto. Expert Group) connector.
Unhappy with the effort required to fasten an SMA connector, some manufacturers developed two non-threaded variants. They are smaller than an SMA as they lack a threaded ring. In 2003, the Quick Lock Forum Alliance (of manufacturers Amphenol, Huber+Suhner, Radiall, and Rosenberger Hochfrequenztechnik) released the QMA (Quick Sub-Miniature type A). It uses snap-on fastening, allowing the cable to rotate relative to the plug. The WQMA (Waterproof QMA) is a waterproof version of the QMA. Then, in 2006, Telegärtner released the QLS (Quick Lock Standard) male plug, which uses quick-lock to fasten itself to a standard SMA male. TE also makes some snap-on males that mate to the threaded female SMA.
Extracted from “The electronic connector book”.
Davide Andrea