Analysis of RF coaxial SMA connector(1)

SMA connector is a widely used semi-precision ultra-small RF and microwave connector, especially suitable for RF connections in electronic systems with frequencies up to 18 GHz or higher. SMA connectors come in many forms, male, female, straight, right-angle, partition fittings, etc., so that they can meet most requirements. Its ultra-small size also allows it to be used, even in relatively small electronic devices.

1. Introduction of SMA connector

SMA is commonly used to provide RF connections between circuit boards. Many microwave components include filters, attenuators, mixers, and oscillators. The connector has a threaded external connection interface, which has a hexagonal shape and allows tightening with a wrench. They can be tightened to the correct tightness with a special torque wrench, so that a good connection can be achieved without over-tightening.

The first SMA connector is 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, and there is no need to transition between the coaxial center conductor and the center pin of the special connector.

Its advantage is that the cable dielectric is directly connected to the interface without an air gap. The disadvantage is that it can only perform a limited number of connection/disconnection cycles. However, for applications using semi-rigid coaxial cables, this is unlikely to be a problem because the installation is usually fixed after initial assembly.

2. SMA connector performance

The SMA connector is designed to have a constant, 50 ohm impedance on the connector. SMA connectors were originally designed and designated for work up to 18 GHz, although some versions have a top frequency of 12.4 GHz and some versions are designated as 24 or 26.5 GHz. A higher upper frequency limit may require operation with higher return loss.

Generally, SMA connectors have a higher reflection coefficient than other connectors up to 24 GHz. This is due to the difficulty of accurately fixing the dielectric support, but despite this difficulty, some manufacturers have managed to properly overcome this problem and are able to specify their connectors for operation at 26.5 GHz.

For flexible cables, the frequency limit is usually determined by the cable rather than the connector. This is because the cables accepted by SMA connectors are very small, and their loss is naturally much greater than the loss of the connector, especially at the frequencies at which they may be used.

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