SMA connector

- details and information about the SMA RF connector, including the SMA socket and plug, SMA adapters and links to suppliers.

The SMA connector is a sub-miniature coaxial cable connector and it takes its name from the words Sub-Miniature A connector. It finds many applications for providing connectivity for RF assemblies within equipments where coaxial connections are required. It is often used for providing RF connectivity between boards, and many microwave components including filters, attenuators, mixers and oscillators, use SMA connectors.

The connectors have a threaded outer coupling interface that has a hexagonal shape, allowing it to be tightened with a spanner. Special torque spanners are available to enable them to be tightened to the correct tightness, allowing a good connection to be made without over-tightening them. The torque required is typically 8 inch pounds.

The SMA coaxial RF connector was originally designed in the 1960s by the Bendix Scintilla Corporation and Omni-Spectra Corporation. The first SMA connectors were designed for 141 semi-rigid coax. The original SMA connector was what could be termed a minimal connector because the centre of the coax formed the centre pin for the connection, removing the necessity for a transition between the coax centre conductor and a special connector centre pin. It also had the advantage that the cable dielectric was take directly to the interface without air gaps. The disadvantage of the connector was that only a limited number of connect / disconnect cycles were possible. However for applications where semi-rigid coax was used, it was unlikely that this would be a problem as the installations were normally fixed after initial assemble..

However its use extended to other flexible cables, and full connectors with centre pins were introduced. These connectors were manufactured to high standards and also allowed greater numbers of connect / disconnect cycles to be performed.

SMA connector performance

SMA connectors are designed to have a constant have a 50 ohm impedance across the connector. The SMA connectors are designed and specified for operation up tom12.4 GHz, although many high quality versions are useable up to 18 GHz. Also it may be possible to sue them to 24 GHz with a greater level of loss and a lower return loss.

In general, SMA connectors have a higher reflection coefficient than other connectors used up to 24 GHz. This arises from the difficulty in accurately anchoring the dielectric support, but despite this difficulty, some manufacturers have managed to suitably overcome this problem and are able to specify their connectors for operation to 26.5 GHz.

For flexible cables, the frequency limit is normally determined by the cable and not the connector. This is because the cables accepted by SMA connectors are small and their loss is naturally much greater than that of the connectors, especially at the frequencies at which they are likely to be used.

Connector formats

The SMA connector is available in a variety of forms. The plugs are available in straight and right-angled formats and the sockets are available as cable entry or single pin centre solder contacts. Typically the cable entry types have a single nut fixing to enable them to be attached to a panel. They may also be free connectors. The centre pin types either have a two or four screw fixing capability to enable them to attach to a panel.

By Ian Poole

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