What are coaxial connectors?
A coaxial connector has an inner conductor that is enveloped by a concentric conducting sleeve, with the conductor and shield separated by an insulating material; hence the term “coax” or “coaxial” connector. (Not all RF [radio frequency] connectors are coax, but all coax are RF connectors.) They can be for cables or board-to-board or cable-to-board applications. Coax connectors transmit analog signals while minimizing RF signal losses and work from DC to high-megahertz frequencies. Cable coax connectors are used with specific coaxial cables to take advantage of common impedance and shielding. Coaxial connectors are available in hundreds of versions. Since they are in-line, single circuit connectors, adapters are available to connect between different series.
How They Work
As a carryover from radio applications, most cabled coax connectors have a pin/male center and are called plugs, while the mating receptacles have female/socket contacts and are called jacks. A coaxial connector, in its simplest version, consists of a signal pin, surrounded by a dielectric insulating material, which is then enclosed in a metal shield, usually the outside case of the connector. The center signal pin is usually a gold-plated, beryllium copper contact, which mates to a female contact with similar base metal and platings. These two contacts would then be soldered or crimped on to the center conductor of a coaxial cable. The center pins are surrounded by an insulating dielectric which is usually Teflon, although other materials can be chosen to provide the desired performance. The final component is the cable shield, or housing of the connector. This is designed to terminate to the shield of the coax cable, by solder, crimp, or clamping. The shield is used primarily as a barrier to outside interference.
Current microwave engineering grew out of the effort put into radar development in the 1930s. Waveguides pre-dated this period and received attention during WW II. After 1945, new equipment that required greater bandwidth, less weight, and smaller size, led to the development of the first standardized coaxial connectors. In the early 1940s, the primary coaxial connector in use was the UHF connector developed by E. C. Quackenbush of the American Phenolic Company (today known as Amphenol). UHF coaxial connectors were established for “low frequency” 0.6 to 300 MHz radio industry applications. UHF is an acronym for Ultra High Frequency because at that time 300 MHz was considered high frequency. Threaded UHF connectors became popular due to ease of mating/assembly. They are still used and produced worldwide today.
Joint military and industry standards committees were formed to define RF coaxial connectors. Currently, MIL-PRF-39012 and MIL-STD-348 are the primary specifications for most coaxial connectors. In addition, MIL-DTL-17 is the governing standard for MIL-Spec coaxial cables. There are equivalent IEC and other international standards. Precision coax connectors for critical test and calibration applications are per IEEE standard 287.
Popular Coaxial Connector Types
N, C, BNC, TNC, and other coax series have been standardized. The popular SMA started as the BRM (for Bendix Research Miniature) in 1958, followed by SSMA, SMB, SMC, and others. The Type N connector, with a threaded coupling nut with 5/8” UNEF 24 turns per inch thread, met the need for a durable, weatherproof, higher power, and higher frequency connector. Type N was named for work by standards committee member Paul Neill of Bell Laboratories (New York City). Originally for DC to 1 GHz, today versions are available for up to 11 and 18 GHz.
Neill and Carl Concelman of Amphenol jointly developed the quick-connect (bayonet coupling) BNC connector to meet the demand for smaller connectors to work with the new smaller coaxial cables. The “N” and “C” came from their initials, and “B” was either for “baby,” because of its smaller size, or for “bayonet.”
After CATV F and specials, the most popular coax connector in terms of dollar sales is the SMA. SMAs are threaded coupling connectors, available worldwide in many versions, including a variety of termination options, different levels of environmental sealing, and a range of electrical performance. The SMA is smaller than the Type N, with a 0.250” diameter threaded (36 threads per inch) barrel.
Higher performance SMAs usually use internal interference-fit discs to secure components in position and have field-replaceable termination. A slightly smaller version is available termed SMC, while a bayonet coupling version is offered by some suppliers, called SMB, for DC to 6 GHz. The SMB interface was adopted for use in automotive connectors called FAKRA and there is a smaller mini-FAKRA that operates to 17 GHz.
Why 50 ohm impedance?
The coaxial cable impedances in high-power, high-voltage, and low-attenuation applications were studied at AT&T Bell Laboratories in 1929. The best impedance is within 52 and 64 ohms, while maximum power handling is at 30 ohms. Minimum attenuation was with an impedance of 77 ohms. The arithmetic mean between 30 ohms and 77 ohms is 53.5 ohms and 50 ohms was chosen as a compromise between power-handling capability and attenuation. Today, the vast majority of coax cables and connectors are 50 ohms worldwide.
Why 75 ohm impedance?
The approximate impedance required to match a dipole antenna in free space is 73 ohms, which lead to 75-ohm coax as a standard for RF transmission applications where attenuation is important. 75 ohm impedance is used primarily for video and audio applications.
K Connectors (Millimeter Wave Connectors)
Most connectors for frequencies above 30 GHz can visually be identified, as they use air for the dielectric (insulator). A popular series is the 2.92 mm coax connectors, also called “K” for K-Band (“K Connectors” is trademarked by Anritsu). They mate with SMA, but are designed for use to 40 or 44 GHz. There is a performance overlap with SMA, as many applications above 20 GHz (such as the 5G band [n258] for 24.25-27.5 GHz, or 26 GHz local multipoint distribution systems [LMDS]), can be served by “better” SMAs. However, since most SMAs are used for low frequency, 6 GHz and below, many designers are not aware of high-end SMAs and unknowingly may select 2.92 mm connectors that are several times the cost of SMAs. For applicable higher frequency applications, K/2.92 connectors have become popular with multi-source availability including distribution.
Air dielectric coax connectors are called “precision” because higher frequency performance requires tighter manufacturing tolerances and better testing. Higher frequency coax connectors are metric-based. They are identified by the inner diameter of the outer coax path (housing).
As connector diameters become smaller, if production is done correctly, resultant connectors can operate at higher frequencies before moding. As connectors become smaller, they are subject to increased manufacturing and termination costs, plus damage from improper handling and mating. Successful application involves tradeoffs.
In a variation on the theme of Moore’s Law, one could say that a size reduction of a coax connector takes place every two years. Have we reached the limits of the coax connector? Can we build, handle, or terminate anything smaller than the 1 mm connector? Yes! The newest coax connectors in production include 0.8 mm and 0.6 mm. The 0.4 mm is under discussion for use/production by Anritsu.”
Sizes Standard, miniature, and micro
Common coax connector types
Mating: Plugs have a pin/male center and jacks (mating receptacles) have female/socket contacts. Reverse-sex versions also exist.
Fastening mechanisms: Thread, bayonet, braces, latching, and blind mate (detent).
Market Applications: Common types of RF connectors are used for television receivers, two-way radio, industrial and scientific measurements instruments, satellites, radar, Wi-Fi, and 5G “new radio frequencies.