What Is an SMA Connector?

An SMA connector, short for SubMiniature version A, is a small coaxial RF connector with a threaded coupling nut. It was developed in the 1960s for microwave equipment and is now the most common antenna connector on wireless devices, from Wi-Fi cards and cellular modems to GNSS receivers and test equipment. Its interface is defined in IEC 61169-15 and the US military standard MIL-STD-348.

SMA connectors are 50 ohm, matching the coax cables and radios used for wireless work. Standard versions are typically rated from DC to around 18 GHz, and precision versions higher, which covers every Wi-Fi, 4G, 5G sub-6 GHz and GNSS band with a wide margin. The threaded nut holds the connection firmly, which keeps the impedance stable and stops the plug working loose.

Gender is set by the centre contact. An SMA male plug has a centre pin and a nut with internal threads. An SMA female jack has a centre socket and external threads, and is the usual connector on the equipment.

RP-SMA, or reverse-polarity SMA, keeps the same body and threads but swaps the centre contacts. An RP-SMA jack has external threads like an SMA female, but a centre pin instead of a socket. An RP-SMA plug has internal threads with a socket in the middle.

The variant appeared because US FCC rules required consumer wireless devices to use antenna connectors the public could not easily swap, to stop higher-gain antennas pushing transmit power over the legal limit. Wi-Fi equipment adopted RP-SMA, and it became so common that it is now the usual Wi-Fi antenna connector. Cellular and GNSS equipment mostly kept standard SMA.

The two screw together, because the threads are identical, but the centre contacts do not meet: an SMA antenna on an RP-SMA port connects the shield and nothing else. Always check the centre of both connectors before fitting an antenna.

SMA is one of several RF connectors you will meet on wireless equipment. All of those below are 50 ohm unless stated.

BNC uses the same coax principles with a quick bayonet lock, and comes in 75 ohm for video as well as 50 ohm.

Inside an industrial computer, the Wi-Fi card or cellular modem has tiny U.FL or MHF4 sockets. Short pigtail cables run from these to SMA or RP-SMA bulkhead jacks on the case, where the external antennas screw on. A system can carry several antenna ports, and each one has a job.

A metal enclosure blocks radio signals, so antennas belong outside it. Small dipole antennas screw straight onto the computer. In cabinets, vehicles and machines, the antennas usually mount remotely, on the cabinet roof or vehicle roof, connected by coax extension cables. Combination roof antennas put Wi-Fi, 4G or 5G and GNSS elements in one housing.

Every metre of cable loses signal, and the loss grows with frequency, so thin cables such as RG-174 suit only short runs at 5 and 6 GHz. Longer runs need low-loss cable of a larger diameter, as our explainer on how coaxial cable types differ covers. Tighten SMA connectors by hand and then with a torque spanner to the connector maker’s figure, and seal outdoor joints against water.

Our in-vehicle computers and industrial IoT gateways include many models with Wi-Fi, 4G or 5G and GNSS options, which bring their antennas out to connectors on the case. For positioning, our comparison of GNSS and GPS explains the antenna choices, and our white paper on integrating 4G, LTE and 5G connectivity covers cellular antennas.

We fit the Wi-Fi and cellular modules, pigtails and SMA antenna ports each system needs as standard, and test every radio with its antennas before the system ships.