SMA connector

SMA connector

Figure 1. Standard male SMA connector: male body (inside threads) with male inner pin.
Type RF coaxial connector
General specifications
Diameter Male: 0.312 in (7.9 mm) HEX
Cable Coaxial
Passband Typically 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. SMA is designed for use from DC to 18 GHz, but is most commonly encountered with WiFi antenna systems and USB Software Defined Radio dongles.

SMA connectors can be visually confused with the standard household 75 ohm Type F Coax Connector (diameters: Male 7⁄16 in (11 mm) circular or hex; Female 3⁄8 in (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. Of course, this connection is possible using an adapter.

The SMA name is also used for a superficially similar Optical fiber connector.

Connector design

The SMA connector employs a 1/4 inch, 36 thread. The male is equipped with a 5/16 inch (0.312 inch) hex nut. This is not to be confused with a hex nut that accommodates a screw/bolt with 5/16 inch nominal outer diameter. This would result in an SMA connector that has larger minimum cross section dimensions than 5/16 inch. Instead, the dimensions of the SMA connector are such that the width across the flats is 5/16 inch. This is the same as the #6 SAE hex nut. Thus, the SMA connector requires a 5/16 inch wrench to fit across the flats. A 5/16 inch hex nut would require a 1/2 inch wrench to fit across the flats.

In SMA connectors, the terms "male" and "female" refer not to the male center pin and its female sleeve counterpart, but 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.[1]

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.[2]

SMA connectors are rated for up to 500 mating cycles,[3] 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.[4][5]

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 (also known as 2.9 mm, SMK, or K type),[6] 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.[7] 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

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.[8][9] Normal SMA connectors are incompatible with RP-SMA connectors.

Center pin Center receptacle
Internal thread SMA male RP-SMA male/plug
External thread RP-SMA female/jack[10] SMA female

Because they were not readily available, RP-SMA connectors have been widely used by Wi-Fi equipment manufacturers to comply with specific national regulations, such as those from the FCC,[11] which are designed to prevent consumers from connecting antennas which exhibit gain and therefore breach compliance.

The US FCC considered that the RP-SMA was acceptable in preventing consumers changing the antenna; but by 2000 it regarded them as readily available,[12] though delaying its ruling indefinitely.[13] As of 2013, leading manufacturers are still using RP-SMA connectors on their Wi-Fi equipment.[14]

See also

References

  1. ↑ "SMA Dimensions".
  2. ↑ "What Mates With What". Agilent.com. Retrieved 2014-06-02.
  3. ↑ "SMA Connectors" (PDF). AmphenolRF.com. Archived from the original (PDF) on March 31, 2010. Retrieved February 7, 2016.
  4. ↑ "Inspection". Agilent.com. Retrieved 2014-06-02.
  5. ↑ "Cleaning". Agilent.com. March 2, 2007. Retrieved 2014-06-02.
  6. ↑ "Terms - C". Web.archive.org. 2011-12-23. Archived from the original on July 30, 2012. Retrieved 2014-06-02.
  7. ↑ "Mating Rules for SMA, 2.92mm, 2.4mm, & 3.5mm connectors". Web.archive.org. 2013-08-05. Archived from the original on August 6, 2013. Retrieved 2014-07-31.
  8. ↑ "Reverse Polarity SMA Antenna Cable". DataPro. Retrieved 2014-06-02.
  9. ↑ "RP-SMA Coax Connectors :: Lighthorse Technologies INC". Coaxconnector.com. Retrieved 2014-06-02.
  10. ↑ "RF Connector – RP SMA Connector Introduction". Wellshow.com. Archived from the original on March 29, 2013. Retrieved February 7, 2016.
  11. ↑ "47 CFR 15.203". US Code of Federal Regulations. Retrieved February 7, 2016.
  12. ↑ "OET clarifies antenna connector requirements for Part 15 Unlicensed Transmitters" (PDF). FCC.gov. Federal Communications Commission. May 22, 2000.
  13. ↑ "OET extends effective data of antenna connector requirement indefinitely" (PDF). FCC.gov. Federal Communications Commission. September 28, 2000.
  14. ↑ "HGA7S Support | High Gain Antenna For SMA Connectors". Linksys.com. Archived from the original on 2014-01-16. Retrieved 2014-06-02.

External links

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