SMA connector

SMA ("SubMiniature version A") connectors are coaxial RF connectors developed in the 1960s as a minimal connector interface for coaxial cable with a screw type coupling mechanism. The connector has a 50 Ω impedance. It offers excellent electrical performance from DC to 18 GHz.

Connector Design

The SMA connector consists of a 0.250x36 thread. The male is equipped with a .312 inch hex nut. It is important that the nut be properly torqued with a 5/16 inch wrench, 3-5 in·lbf (0.3 to 0.6 N·m) for brass, and 7-10 in·lbf (0.8–1.1 N·m) for stainless steel connectors. When turning the nut, it is very important that the remainder of the connector does not rotate, otherwise premature wear of the connector will result. Furthermore, the connector should be carefully inspected before each use, and any debris cleaned with compressed air. When properly handled, a SMA connector should remain within specified performance through 500 mating cycles.

The SMA connector uses a polytetrafluoroethylene (PTFE) dielectric which will contact along the mating plane. Variablilty in the construction and the mating of the connectors limit the repeatability of the connector impedance. For that reason, a SMA connector is not a good choice for metrological applications.

The outer shell of the male or plug connector has an inside thread "and" an inner pin, which can be confusing. In conventional SMA, the terms "male" and "female" refer exclusively to the inner pin or socket of the connectors. The male connector is also commonly referred to as a "plug", whereas the female connector is also known as a "socket" or "jack".

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, SMA-like connectors are used. These are the 3.5 mm connector, rated to 34 GHz, and the 2.92 mm, good up to 46 GHz. These connectors keep the same outside thread as the SMA, so they can all be cross-mated, however they use an air dielectric, with the center conductors appropriately scaled. However, the life of the precision connector will be reduced, and can be easily damaged when mating with low-grade SMA connectors.

Beyond 46 GHz, the 2.4 mm, 1.85 mm and the 1 mm connector exist. These are similar to the SMA connector, but with the geometries incompatibly scaled. These have mode-free operation to 50, 65, and 110 GHz respectively.

Reverse polarity SMA

Reverse polarity SMA (RP-SMA or RSMA) is a proprietary variation of the SMA connector specification which reverses the gender of the interface, as shown in Figures 1 and 2.

There is much confusion caused by the terms 'male' and 'female' with reversed connectors. These are best avoided and instead the correct terms plug and jack should be substituted.

The RP-SMA plug connector has the same external housing as a standard or conventional SMA plug connector, which consists of an outer shell with the thread on the inside; however, the internal prong or pin is replaced by a receptacle or female inner pin. The RP-SMA jack connector as shown in Fig 2 has the same housing as a standard or conventional SMA jack connector, thread on the outside, however the female receptacle is replaced by a male prong or pin.

RP-SMA connectors are widely used by Wi-Fi equipment manufacturers to comply with specific local regulations, e.g. the FCC, which are designed to make it difficult for consumers to connect antennas with gain and thereby breach compliance.

The term "reverse polarity" might appear confusing to some non-WiFi RF engineers, as polarity elsewhere in RF literature can refer to the electromagnetic polarity, not to a change of gender (in this case replacing the prong for a receptacle and vice-versa).

ee also

*SMB connector
*SMC connector

References

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

External links

* [http://www.wa1mba.org/rfconn.htm RF Connectors from WA1MBA]