SMA connector

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SMA connector
Male 50 ohm SMA connector.jpg
Figure 1. Standard male SMA connector: male body (inside threads) with male inner pin.
TypeRF coaxial connector
DiameterMale: 0.312 in (7.9 mm) HEX
CableCoaxial
PassbandTypically 0-18 GHz,
some up to 26.5 GHz
 
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SMA connector
Male 50 ohm SMA connector.jpg
Figure 1. Standard male SMA connector: male body (inside threads) with male inner pin.
TypeRF coaxial connector
DiameterMale: 0.312 in (7.9 mm) HEX
CableCoaxial
PassbandTypically 0-18 GHz,
some up to 26.5 GHz

SMA (SubMiniature version A) connectors are semi-precision 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 is designed for use from DC to 18 GHz.

Connector design[edit]

The SMA connector employs a 1/4"-36 thread. The male is equipped with a 0.312 Inch (5/16 Inch) hex nut.

In SMA connectors, the terms "male" and "female" refer exclusively to the male center pin and its female sleeve counterpart rather than to the threads that are used to hold the connection in place. The male connector has inside threads while the female connector has outside threads.

The SMA connector uses a polytetrafluoroethylene (PTFE) dielectric which will contact along the mating plane. Variability in the construction and the mating of the connectors limit the repeatability of the connector impedance. For that reason and the fact they are just rated for a limited number of connection cycles, an SMA connector is not usually a good choice for metrological applications.[1]

SMA connectors are rated for up to 500 mating cycles,[2] but to achieve this it is necessary to properly torque the connector when making the connection. A 5/16 inch torque wrench is required for this, set to 3–5& in·lbf (0.3 to 0.6 N·m) 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.[3][4]

Variations[edit]

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 (also known as 2.9 mm, SMK, or K type),[5] good up to 46 GHz. These connectors keep the same outside thread as the SMA, so they can all be cross-mated, but the precision connector can be easily damaged when mating with low-grade SMA connectors.[6] The precision versions use an air dielectric with appropriately scaled center conductors.

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[edit]

Figure 2. Female RP-SMA connector: Female connector body (outside threads) with a male inner pin contact. A male RP-SMA connector is the opposite in both respects — male connector body (inside threads) with a female inner sleeve contact.

Reverse polarity SMA (RP-SMA or RSMA) is a variation of the SMA connector specification which 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.[7][8] Normal SMA connectors are incompatible with RP-SMA connectors.

Center PinCenter Receptacle
Inside ThreadSMA MaleRP-SMA Male/Plug
Outside ThreadRP-SMA Female/Jack[9]SMA Female

Because they were not readily available, RP-SMA connectors have been widely used by Wi-Fi equipment manufacturers to comply with specific local regulations, such as those from the FCC,[10] which are designed to prevent consumers from connecting antennas which exhibit gain and therefore breach compliance. The FCC considered that the RP-SMA was acceptable in preventing consumers changing the antenna; but by 2000 it regarded them as readily available,[11] though delaying its ruling indefinitely.[12] As of 2013, leading manufacturers are still using RP-SMA connectors on their Wi-Fi equipment.[13]

See also[edit]

References[edit]

External links[edit]

 
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