There are a number of commonly available RF connectors that are used with coax cable to provide screened connections. These connectors are used in a number of areas whether to carry RF or radio frequency signals or just to provide a much higher level of screening than would be possible if more ordinary "open" connectors were used.
RF connectors are used in many areas. Naturally their main uses are associated with RF applications. Everything from domestic television, through CB and ham radio to the large number of commercial and industrial applications. However RF or coaxial connectors are also used in areas where screening is one of the major priorities rather than the fundamental RF properties. Coaxial connectors are widely used with a variety of test instruments. For example RF connectors are used on oscilloscopes. These and many other applications all use RF connectors.
RF connector basics
Many connectors, such as the D-type connectors and many other multiway connectors consist of a series of pins with connections in parallel to each other. RF coaxial connectors need to retain the coaxial nature of the cable they are used with. As a result they consist of a central pin for the inner of the coax cable, and then an outer connection around the inner for the outer conductor on the cable. This makes these RF connectors very different to other, more "traditional" connectors.
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.
RF connector types
There is a large host of different types of RF connector. Some are in widespread use whereas others are les widely used. Some of those in widespread use and likely to be encountered in the standard electronics laboratory or by the hobbyist are:
- BNC   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 as an RF connector, being used on RF test equipment, transmitters, receivers and almost any RF equipment. The BNC connector has a bayonet fixing to prevent accidental disconnection while being easy to disconnect when necessary.
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.
- N-type The N-type connector is a high performance RF coaxial connector used in many RF applications. The N-type RF connector is larger than the BNC connector and it 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 connector. The standard versions are specified for operation up to 11 GHz, although precision versions are available for operation to 18GHz.
- UHF connector (SO239 / PL259): The UHF connector is also sometimes known as the Amphenol coaxial connector. The plug may be referred to as a PL259 coaxial connector, and the socket as an SO239 connector as these were their original military part numbers. The 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.
The BNC, N-type and UHF (S)239 / PL259) connectors are possibly the most widely used types of connector in many circles. Although many other types of RF connector exist, they tend to be found more in specialised areas of RF technology or in higher cost applications.
Other RF connector types
There are very many other types, but these are not as likely to be encountered in more everyday usage, although some are still used in very large quantities. These include:
RF connectors are widely used in many areas of electronics, covering frequencies from DC up to many GHz. They are an essential part of many electronics equipment, enabling screened connections to be made tot e quality required. The performance required of these RF connectors means that costs are not always cheap. While sometimes it is possible to buy cheap RF connectors, these may be of inferior quality, and they may prove to be a false economy.
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