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..

This RF coax cable conenctors reference is split into several pages each of which addresses different aspects of RF coax cable connector operation and technology:

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.

The drawback of the UHF or Amphenol connector is that it has a non-constant impedance. This limits their use to frequencies of up to 300 MHz, but despite this these UHF connectors provide a low cost connector suitable for many applications, provided that the frequencies do not rise. Also very low cost versions are available for applications such as CB operation, and these are not suitable for operation much above 30 MHz. In view of their non-constant impedance, these connectors are now rarely used for many professional applications, being generally limited to CB, amateur radio and some video and public address systems.

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.

This RF connector has a threaded coupling interface to ensure that it mates correctly. It is available in either 50 ohm or 75 ohm versions. These two versions have subtle mechanical differences that do not allow the two types to mate. The connector is able to withstand relatively high powers when compared to the BNC or TNC connectors. The standard versions are specified for operation up to 11 GHz, although precision versions are available for operation to 18GHz.

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.

BNC connector

The BNC coax connector is widely used in professional circles being used on most oscilloscopes and many other laboratory instruments. The BNC connector is also widely used when RF connections need to be made. 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.

Electrically the BNC coax cable connector is designed to present a constant impedance and it is most common in its 50 ohm version, although 75 ohm ones can be obtained. It is recommended for operation at frequencies up to 4 GHz and it can be used up to 10 GHz provided the special top quality versions specified to that frequency are used.

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.

SMA connector

This sub-miniature RF coaxial cable connector takes its name from the words Sub-Miniature A connector. It finds many applications for providing connectivity for RF assemblies within equipments. 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 SMA connector was originally designed in the 1960s for use with 141 semi-rigid coax cable. Here the centre of the coax forms the centre pin for the connection, removing the necessity for a transition between the coax centre conductor and a special connector centre pin. However its use extended to other flexible cables, and connectors with centre pins were introduced.

SMA connectors are regularly used for frequencies well into the microwave region, and some versions may be used at frequencies up to 26.5 GHz. For flexible cables, the frequency limit is normally determined by the cable and not the 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 perform well under moderate vibration only and the 50 ohm versions are often specified to 4 GHz. 75 ohm versions of the SMB coaxial connector are also available, but there are often not specified up to the same frequencies, often only about 2GHz.

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.

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.

Overview

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.

Further pages from this tutorial
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