I include a few exerts, but I recommend the interested reader download the whole article.
Some Early History
With the beginning of World War II, and the emergency need for higher frequency applications above 300 MHz, it was determined that the UHF connector was not suitable and that new connector designs were required. A joint Army-Navy RF Cable Coordinating Committee (ANRFCCC) was established in the early 1940s to develop standards for RF cables, rigid transmission lines and connectors for radio and radar equipment. The task of this committee later was incorporated into the Armed Services Electro-Standards Agency (ASESA) when it was established in the late 1940s and eventually was reorganized into the Defense Electronics Supply Center (DESC) that continues the important work of connector standardization for the military today.
Under ANRFCCC guidance, the type N connector was born in 1942 and featured a threaded coupling nut for connection and an air coupling interface. The N derived from Paul Neill of Bell Laboratories (New York) who was on the committee and worked on the connector. This was followed by the HN connector that was a high voltage version of the type N and featured an overlapping dielectric interface. The type C connector followed next with a twist-lock coupling mechanism for quick connect and disconnect. It was named after Carl Concelman of Amphenol. Then, as smaller coaxial cables became available, the BNC connector was developed jointly by Neill and Concelman, hence the N and C and the B for baby because of its size.
Why 50Ohm Connectors
In the United States, the predominant impedance for coaxial transmission lines and connectors is 50 Ω. The theoretical impedance for minimum attenuation is 77.5 Ω and for maximum power transfer is 30 Ω; the average of these two impedances is 53.75 Ω or rounded off to 50 Ω (see Figure 2 at right). Therefore, 50 Ω is a compromise between minimum attenuation and maximum power transfer in a coaxial transmission line, and that is why it was selected. There are connectors available with other impedances, the next most popular impedance being 75 Ω (approximate minimum attenuation performance) that is in fairly wide use internationally and in long line communication systems.
In the early days because of the lack of knowledge and the unavailability of accurate RF measuring equipment, military specifications for connectors were based on detail mechanical piece part drawings, which were non-optimum designs from a microwave standpoint, with no performance specifications. This limited manufacturers from making improvements or risk noncompliance.
The SMA Connector
The SMA connector is the most widely used micro- wave connector in the world today. It originally was designed at Bendix Research Laboratories by James Cheal in 1958 and began life as BRM connector for Bendix real miniature connector. Its development was continued in 1962 by Omni Spectra (now a division of M/A-Com) when the connector became known as OSM for Omni Spectra miniature. It became popular under that name. In 1968, it was incorporated into MIL-C-39012 where it received its current designation of SMA for subminiature A. A more detailed description on the early evolution of the SMA connector is described in a previously published work (see paper - ed).
The SMA connector was designed to be a low cost miniaturized system connector. It is a coplanar pin socket connector with a dielectric interface. It is an excellent adaptation for use with 0.141 diameter semi-rigid cable in its simplest form. It found ready application for use with stripline and microstrip circuits because of its size. People have complained about their longevity in a measurement environ- ment, but it was not intended for this application. The APC3.5 connector is a well suited test connector for SMA and is being used for this purpose on a regular basis6. The principle of interface error correction using software and a VNA was advanced6 and later implemented.
SMA connectors will operate mode-free to 18 GHz and certain versions to 25 GHz (higher order modes can exist in the 22 to 24 GHz frequency range). There is also an improved version available from Amphenol and M/A-Com that operates to 27 GHz.
An interesting statistic is, "What are the most used coaxial connector types?" In 1985 based on shipments, the top seven coaxial connectors were SMA, BNC, N, UHF, TNC, SMC and SMB. (it would be interesting to see statistics for 2010 - ed)
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