RF Coax Cable Connectors

- an overview of the different types of RF connectors that can be used with coax cable or feeder including the UHF (SO239 / PL259), BNC, TNC, N-type, SMA, SMB, SMC, MCX, etc..

Coax cable connectors, often called RF connectors are in widespread use. Wherever radio frequency or RF connections need to be made there is the possibility of using coaxial connectors. Where signals reach frequencies above a few million Hertz, these coaxial connectors need to be used. The need for their use arises because it is necessary to transfer radio frequency, RF, energy from one place to another using a transmission line. The most convenient, and hence the most commonly used form of transmission line is coaxial cable which consists of two concentric conductors, an inner conductor and an outer conductor, often called the screen. Between these two conductors there is an insulating dielectric.

Coaxial cable has a number of properties, one of which is the characteristic impedance. In order that the maximum power transfer takes place from the source to the load, the characteristic impedances of both should match. Thus the characteristic impedance of a feeder is of great importance. Any mismatch will result in power being reflected back towards the source.

It is also important that RF coaxial cable connectors have a characteristic impedance that matches that of the cable. If not, a discontinuity is introduced and losses may result.

There is a variety of connectors that are used for RF applications. Impedance, frequency range, power handling, physical size and a number of other parameters including cost will determine the best type for a given applications.

UHF connector

The UHF connector, also sometimes known as the Amphenol coaxial connector was designed in the 1930s by a designer in the Amphenol company for use in the radio industry. The plug may be referred to as a PL259 coaxial connector, and the socket as an SO239 connector. These are their original military part numbers

These coaxial connectors have a threaded coupling, and this prevents them from being removed accidentally. It also enables them to be tightened sufficiently to enable a good low resistance connection to be made between the two halves. Read more about the UHF connector . . . .

N-type connector

The N-type connector is a high performance RF coaxial connector used in many RF applications. This coax connector was designed by Paul Neill of Bell Laboratories, and it gained its name from the first letter of his surname.

The N-type coaxial connector is used for many radio frequency applications including broadcast and communications equipment where its power handling capability enables it to be used for medium power transmitters, however it is also used for many receivers and general RF applications. Read more about the N-type connector . . . .

BNC connector

The BNC coax connector is widely used in professional circles being used on most oscilloscopes and many other laboratory instruments, although it is widely used for many other RF applications. The BNC connector has a bayonet fixing to prevent accidental disconnection while being easy to disconnect when necessary. This RF connector was developed in the late 1940s and it gains its name from a combination of the fact that it has a bayonet fixing and from the names of the designers, the letters BNC standing for Bayonet Neill Concelman. In fact the BNC connector is essentially a miniature version of the C connector which was a bayonet version of the N-type connector. Read more about the BNC connector . . . .

TNC connector

The TNC connector is very similar to the BNC connector. The main difference is that it has a screw fitting instead of the bayonet one. The TNC connector was developed originally to overcome problems during vibration. As the bayonet fixing moved slightly there were small changes to the resistance of the connections and this introduced noise. To solve the problem a screw fixing was used and the TNC coax cable connector gains its name from the words Threaded Neill Concelman.

Like the BNC connector, the TNC connector has a constant impedance, and in view of the threaded connection, its frequency limit can be extended. Most TNC connectors are specified to 11 GHz, and some may be able to operate to 18 GHz. Read more about the TNC connector . . . .

SMA connector

This sub-miniature RF and microwave coaxial cable connector takes its name from the words Sub-Miniature A connector. It finds many applications for providing connectivity for RF assemblies within equipment. 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. The SMA connector was originally designed for use with 141 semi-rigid coax cable. However its use extended to other flexible cables, and connectors with centre pins were introduced. Read more about the SMA connector . . . .

SMB connector

The SMB connector derives its name as it is termed a Sub-Miniature B connector. It was developed as a result of the need for a connector that was able to connect and disconnect swiftly. It does not require nuts to be tightened when two connectors are mated. Instead the connectors are brought together and they snap fit together. Additionally the connector utilizes an inner contact and overlapping dielectric insulator structures to ensure good connectivity and a constant impedance.

SMB coaxial connectors are not as widely used as their SMA counterparts. They are used for inter board or assembly connections within equipment, although they are not widely used for purchased microwave assemblies in view of their inferior performance. Read more about the SMB connector . . . .

SMC connector

A third SM type connector is not surprisingly the Sub Miniature C or SMC coaxial cable connector. It is similar to the SMB connector, but it uses a threaded coupling interface rather than the snap-on connection. This provides a far superior interface for the connection and as a result, SMC coaxial cable connectors are normally specified to operate at frequencies up to 10 GHz.

SMC coaxial cable connectors provide a good combination of small size and performance. They may also be used in environments where vibration is anticipated. In view of their performance they find applications in microwave equipment, although they are not normally used for military applications where SMA connectors tend to be preferred.

MCX connector

A number of mico-miniatiure RF connectors have been developed by a variety of manufacturers to meet the growing demand for cost effective, high quality smaller connectors. These are finding high levels of use, for example in the cellular phone industry, where size, cost and performance are all important. In fact the MCX is about 30% smaller in both size and weight than an SMB connector to which it has many similarities.

One connector that falls into this category is the MCX (MicroCoaX) coax connector. This was developed in the 1980s by Huber and Suhner of which MCX is a trade name. The MCX connector has many similarities with the construction of the SMB connector using a quick snap-on interface, and utilising an inner contact and an overlapping dielectric insulator structure.

The MCX connector is normally specified for operation up to 6 GHz, and it finds applications in a variety of arenas including equipment for cellular telecommunications, data telemetry, Global positioning (GPS) and other applications where size and weight are important and frequencies are generally below 5 GHz.

MMCX connector

Another connector which is being widely used is the MMCX connector. Being some 45% smaller than an SMB connector, the MMCX is ideal where a low profile outline is a key element. It is therefore ideal for applications where board height is limited, including applications where boards may be stacked. As such it is being widely used in many cellular telecommunications applications.

The connector provides a snap fitting and also utilises a slot-less design to minimise leakage.

There is a great variety of RF coaxial cable connectors in use today. The list above describes some of the more popular types of RF connector, but there are nevertheless more varieties available. When choosing a coaxial cable connector, the requirements should be carefully matched to the available options to see which RF connector will provide the best choice. In this way the best compromise between size, weight, performance and cost can be achieved.

By Ian Poole

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