SMA Connector Basics
The SMA connector is a subminiature coaxial cable connector whose name is derived from the Sub-Miniature A connector. It finds many applications for providing connectivity to RF components within devices that require coaxial connectivity. It is commonly used to provide RF connections between circuit boards, and many microwave components including filters, attenuators, mixers and oscillators use SMA connectors.
The connector has a threaded external connection interface with a hexagonal shape that allows for wrench tightening. They can be tightened to the correct tightness using a special torque wrench, allowing a good connection without over tightening. The torque required is typically 8 in-lbs.
Right Angle Male SMA Connector
SMA coaxial RF connectors were originally designed in the 1960s by Bendix Scintilla Corporation and Omni-Spectra Corporation. The first SMA connector designed for 141 semi-rigid coaxial cable. The original SMA connector can be called the smallest connector because the center of the coaxial cable forms the center pin of the connection, eliminating the need for a transition between the coaxial center conductor and the center pin of a special connector. It also has the advantage that the cable dielectric is directly connected to the interface without air gaps. The disadvantage of connectors is that they can only do a limited number of connect/disconnect cycles. However, for applications using semi-rigid coaxial cable, this is unlikely to be a problem, as the installation is usually fixed after initial assembly.
However, its use extended to other flex cables and introduced full connectors with center pins. These connectors are manufactured to a high standard and also allow a greater number of connect/disconnect cycles to be performed.
SMA connector performance
SMA connectors are designed to have a constant, 50 ohm impedance across the connector. SMA connectors were originally designed and specified for operation up to 18 GHz, although some versions top out at 12.4 GHz and some are specified at 24 or 26.5 GHz. A higher upper frequency limit may require operation with higher return loss.
The actual specification of a given connector is highly dependent on manufacturer and type - several different quality/performance levels are available. It is best to double check the connector specifications
Generally, SMA connectors have a higher reflection coefficient than other connectors up to 24 GHz. This is due to the difficulty of accurately securing the dielectric support, but despite this difficulty, some manufacturers have managed to overcome this problem appropriately and have been able to specify their connectors for 26.5GHz operation.
With flex cables, the frequency limit is usually determined by the cable, not the connector. This is because the cables accepted by SMA connectors are small, and their losses are naturally much greater than the losses of the connectors, especially at the frequencies they are likely to be used in.
SMA connector rated power
In some cases, the power rating of the SMA connector may be important. The key parameters that determine the average power handling capability of a mated coaxial connector is its ability to deliver high currents and maintain a moderate heat gain.
The heating effect is mainly caused by the contact resistance, which is a function of the contact surface area and the way the contact pads are held together. A critical area is the center contacts - these must be formed correctly and fit together nicely. It should also be noted that the average power rating decreases with frequency because resistive losses increase with frequency.
The power handling figures for SMA connectors vary widely between manufacturers, but some numbers suggest that some can handle 500 watts at 1GHz, dropping to just under 200 watts at 10 GHz, but this is much higher than Many people pass them with confidence.
