SMA (''SubMiniature version A'') connectors are semi-precision

coaxial In geometry, coaxial means that several three-dimensional linear or planar forms share a common axis. The two-dimensional analog is ''concentric''. Common examples: A coaxial cable is a three-dimensional linear structure. It has a wire condu ...

RF connector A coaxial RF connector (radio frequency connector) is an electrical connector designed to work at radio frequencies in the multi-megahertz range. RF connectors are typically used with coaxial cables and are designed to maintain the shieldin ...

s developed in the 1960s as a minimal connector

interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...

for

coaxial cable Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...

with a screw-type coupling mechanism. The connector has a 50 Ω impedance. SMA was originally designed for use from DC (0 Hz) to 12

GHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

, however this has been extended over time and variants are available to 18 GHz and 26.5 GHz. There are also mechanically compatible connectors such as the K-connector which operate up to 40 GHz. The SMA connector is most commonly used in microwave systems, hand-held radio and mobile telephone antennas and, more recently, with WiFi antenna systems and USB

software-defined radio Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in analog hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by ...

dongles. It is also commonly used in

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...

, particularly at higher frequencies (5 GHz+).

Connector design

The interface dimensions for SMA connectors are listed in MIL-STD-348. The SMA connector employs a inch diameter, 36-thread-per-inch threaded barrel. The male is equipped with a hex nut measuring inch (0.3125 inch / 7.9 mm) across opposite flats, thus taking the same wrench as a #6

SAE SAE or Sae may refer to: Science and technology : * Selective area epitaxy, local growth of epitaxial layer through a patterned dielectric mask deposited on a semiconductor substrate * Serious adverse event, in a clinical trial * Simultaneous Aut ...

hex nut. A ''standard-polarity'' SMA male connector has a 0.9mm diameter center pin surrounded by barrel with inside threads, and the standard SMA female connector has a center sleeve surrounded by a barrel with outside threads. The centre pin is the same diameter as the centre of RG402 Coax so that connections can be made with no discontinuity, forming the pin from the coax itself. As with most other connectors, the

gender assignment Sex assignment (sometimes known as gender assignment) is the discernment of an infant's sex at or before birth. A relative, midwife, nurse or physician inspects the external genitalia when the baby is delivered and, in more than 99.95% of birt ...

corresponds to the innermost electrical component. There are also ''reverse-polarity'' ("RP") SMA connectors in which the pin and sleeve are swapped so that the "male" RP-SMA has a center sleeve surrounded by an inside-threaded barrel, and the "female" RP-SMA has a center pin and an outside-threaded barrel. See below for a fuller description. Cross section of an SMA mated pair

The SMA connector uses a

polytetrafluoroethylene Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemo ...

(PTFE) dielectric that contacts along the mating plane. Variability in the construction and the mating of the connectors limits the repeatability of the connector impedance. For that reason and that they are just rated for a limited number of connection cycles, an SMA connector is not usually a good choice for

metrological Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fr ...

applications. SMA connectors are rated for up to 500 mating cycles, but to achieve this it is necessary to properly torque the connector when making the connection. A inch torque wrench is required for this, set to 3–5 in·lbf (0.3 to 0.6

N·m The newton-metre (also newton metre or newton meter; symbol N⋅m or N m) is the unit of torque (also called ) in the International System of Units (SI). One newton-metre is equal to the torque resulting from a force of one newton app ...

) for brass, and 7–10 in·lbf (0.8 to 1.1 N·m) for stainless steel connectors. Flats are sometimes also provided on the cable side of the connector assembly so that a second wrench can be used to prevent it from rotating and damaging the joint to the cable. It is also advisable to inspect and clean out loose debris from the internal surfaces with compressed air or a gas duster can before mating. SMA connectors must not be confused with the standard household 75-ohm type F coax connector (diameters: male inch (11 mm) circular or hex; female inch (9.5 mm) external threads), as there is only about a 2 mm difference overall in the specifications. Type F cannot be mated with SMA connectors without the use of an adapter.

Variations

The SMA connector is typically rated for mode-free operation from DC to 18 GHz, though some proprietary versions are rated to 26.5 GHz. For performance above this, other SMA-like connectors are used. These include the 3.5 mm connector, rated to 34 GHz, and the 2.92 mm (also known as 2.9 mm, K type, or SMK), good up to 46 GHz. These connectors keep the same outside thread as the SMA, so they can potentially be cross-mated, but the precision connector can be easily damaged when mating with low-grade SMA connectors. The precision versions use an air dielectric with appropriately scaled center conductors. Beyond 46 GHz, the 2.4 mm, 1.85 mm (also known as V type) and 1.0 mm (also known as W type) connectors exist. These are similar to the SMA connector, but with the geometries incompatibly scaled, and a metric thread to prevent accidental intermating. (But 2.4 mm and 1.85 mm connectors are compatible to each other.) These offer mode-free operation to 50, 65, and 110 GHz respectively.

Reverse polarity

Reverse-polarity SMA (RP-SMA or RSMA) is a variation of the SMA connector specification that reverses the gender of the interface, as shown in Figures 1 and 2. The term "reverse polarity" here refers only to the gender of the connector's contact pin, not in any way to the signal polarity. The female RP-SMA connector has the same external housing as a standard or conventional female SMA connector, which consists of an outer shell with the threads on the outside; however, the center receptacle is replaced by a male pin. Similarly, the RP-SMA male has threads on the inside like a conventional male, but has a center receptacle instead of the male pin in the middle. Normal SMA connectors are incompatible with RP-SMA connectors. SMAconnectors andRPSMAconnectors

Occasionally, the opposite convention is used to designate the gender of an RP connector: In this situation the female connector has a socket (or sleeve) contact and threads on the outside. Users should take care to check drawings and specifications carefully.

Wi-Fi Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio waves ...

equipment manufacturers have widely used RP-SMA connectors to comply with specific national regulations, such as those from the

FCC The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdictio ...

, which are designed to prevent consumers from connecting antennas that provide additional gain and therefore breach Part 15 compliance. However, by 2000, the FCC regarded the connectors as readily available, though delaying its ruling indefinitely. As of 2018, leading manufacturers such as Netgear and

Linksys Linksys is an American brand of data networking hardware products mainly sold to home users and small businesses. It was founded in 1988 by the couple Victor and Janie Tsao, both Taiwanese immigrants to the United States. Linksys products in ...

are still using RP-SMA connectors on their Wi-Fi equipment.

References

Sources

* ''Radio-frequency connectors. Part 15: R.F. coaxial connectors with inner diameter of outer conductor 4.13 ''mm'' (0.163 ''in'') with screw coupling – Characteristic impedance 50 ohms (Type SMA)''. International Standard

IEC The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...

60169-15, 1979. * CECC 22110/111 * ''Connectors, plug, electrical, radio frequency''. Military specification sheet MIL-C-39012.

External links