RF connectors explained

RF coaxial connectors have been around for so long that their beginnings are lost in the mists of time. Most people, however, are familiar with the traditional 50 Ω coax plug that went into the back of the family television set.

Today, there is a much wider market for these RF devices as frequencies have increased and the demand for high data rates has put pressure on makers to produce inexpensive but reliable connectors to suit a wide variety of applications. The original connectors were designed to work in the multi-MHz range and to effectively shield the signal core of coaxial cable. Top-of-the-tree models had little effect on the line impedance and the actual connector had a very low resistance offering little or no attenuation.

This brief overview covers only a few of the mainstream types, there being many variants on those discussed here, including precision types, flange connectors and quick lock devices.

The original Belling-Lee coax plug, as it was called, was the VHF/UHF domestic television cable termination and was only ever intended for medium frequency use where it was not vital for impedance matching. With the current 75 Ω cable there is a serious mismatch with this connector, leading to distortion on TV band signals.

Its successor, the BNC connector, has two bayonet lugs and mating is accomplished by a quarter-of-a-turn twist. Now widely used for television, test instruments and video signals, they can be made to match either 50 or 75 Ω cables, usually at frequencies below 4 GHz and voltages of less than 500. There is also a threaded version that can be used as an alternative to an RCA connector.

The 7-16 DIN connector is threaded and used as joiner for coax cables. It is widely used in network antenna systems where it is superior to N connectors or BNC for interference and intermodulation rejection. There is also the DIN 1.0/2.3 connector which has a push/pull lock and release and is suitable for use where space is at a premium.

One of the first connectors capable of carrying microwave frequency signals was the N device. It was developed in the 1940s to carry frequencies up to 1 GHz. Current types can handle 11 GHz easily and greater manufacturing precision has pushed the handling frequency to 18 GHz. It is a threaded and waterproof connector for joining coax. Available in both 50 and 75 Ω versions, the 50 Ω is mainly used in land mobile, wireless data and paging systems while the 75 Ω has been adopted by cable television.

The F connector is common for terrestrial television installations, satellite television and cable modems. It is a relatively inexpensive connector as it uses the solid core of the cable as the pin of the male connector. Good 75 Ω impedance match up to 1 GHz is possible and the bandwidth can stretch to several GHz.

The almost universal connector between a car radio and its antenna is the Motorola device where the sleeve is folded back over the cable while the centre core is soldered to the pin. Spring surfaces ensure a good electrical connection.

As well as the standard connectors, there is also a range of miniature devices, one of which is the U.FL, designed for use when space is critical as in portable computers and embedded systems where they can connect with a Wi-Fi antenna and mini PC card. The 50 Ω connectors will handle signals up to 6 GHz on as little as 3 mm² of PCB space.

The MMCX microminiature 50 Ω connectors have a lock-snap mechanism allowing them to rotate 360°. They are commonly used as antenna connectors on Wi-Fi, PCMCIA cards or for attaching GPS antennas.

A screw-type coupling gives the subminiature SMA a minimal interface with the cable. Designed for DC to 18 GHz, the male device has inside threads while the female internal threads.

The subminiature SMC is unusual in that the normal polarity is reversed with the centre pin being female and the socket being male. They have an electrical performance of DC to 10 GHz.

There are also special high-voltage types such as the SHV (Safe High Voltage) connectors which are normally used in laboratories, especially in Geiger-Muller tube detectors. Because the connectors can handle up to 5000 VDC (and in some cases up to 20 kV) at 5 A, the connector is designed to disconnect the high voltage before the ground contact to prevent operator shocks.

As a major electronics component, connectors are continually evolving as more specialised uses are found for them. This seems to be a trend that will continue well into the future