SMA connectors are semi-precision coaxial RF connectors with a screw-type coupling mechanism that were created in the 1960s as a minimum connector interface for coaxial cable. The impedance of the connector is 50. SMA was initially intended for usage from DC (0 Hz) to 12 GHz, but it has now been expanded to include variations for 18 GHz and 26.5 GHz. Microwave systems, hand-held radio and mobile telephone antennas, and, more recently, WiFi antenna systems all employ the SMA connector.