TNC Connector

The TNC connector is used in many areas where a medium sized connector with a screw fit is required to provide secure mating.

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The TNC connector is a medium sized form of RF connector. In overall form it is very similar to the very popular BNC connector, but rather than having a bayonet connection it uses a screw fit.

The outer sleeve of the TNC RF connector screws onto the mating half and this provides a secure and reliable mated connection.

Typically the TNC connector is likely to be used in applications where the connector needs to survive vibration, and provide reliable performance at high frequencies and the convenience of the bayonet connection is not required.

TNC connector beginnings

Although the BNC connector proved to be very useful, the TNC connector was developed to overcome one of the major shortcomings - that of variations in resistance and connection with the outer sleeve during vibration. As the bayonet fixing moved slightly there were small variations in resistance that gave rise to noise.

The TNC connector overcame this issue by having a much firmer screw or threaded arrangement for the outer part of the connector.In fact the TNC connector gains its name from the threaded arrangement used: Threaded Neill Concelman.

TNC connector performance

The TNC connector is essentially an RF conenctor that is a threaded version of its bayonet cousin. It possesses many of the same properties, although in view of the much more satisfactory connection on the outer, its frequency limit is extended. Most TNC connectors are specified to 11 GHz, and some are able to operate to 18 GHz. This assumes that thees RF connectors are obtained from reputable suppliers and are not some of the cheaper variants that are available. That said, many of the much cheaper TNC connectors still provide useful performance, but not to the same frequencies.

Basic TNC Specifications
Parameter Specification
Cable Type Coaxial
Securing Screw thread
Typical operating frequency range 0 - 11 GHz
Diameter (Male) 0.590 in (15.0 mm)
Diameter (Female) 0.378 in (9.6 mm)
Thread 7/16-28 UNEF

TNC connector types

TNC connectors come in a variety of formats. Not only are there plugs and sockets but there are also adapters and also other items such as attenuators.

TNC plugs are designed not only for the required impedance, but also to accept a particular coax cable format. In this way all the internal piece parts are compatible with the coaxial cable used. It is therefore necessary to specify the TNC plug for use the cable to be used. Although there is some latitude, it is naturally best to select the correct cable format.

In addition to this there are straight and right angled variants. Of these the straight connectors are the most widely used, although right angled connectors where the cable leaves the plug at right angles to the centre of the connector centre line are also available. These are ideal in many applications where the cables need to leave the connector in this manner to ensure cables are in a tidy fashion, or where space is at a premium. Unfortunately right-angled connectors have a marginally higher level of loss than their straight through counterparts. This may not be significant for most applications, but at frequencies near the operational limit of the connector there may be a small difference.

The sockets or female TNC connectors also come in a number of flavours. The very basic TNC connector consists of a panel mounting assembly with a single connection for the coax centre. The earthing is then accomplished via the panel to which the connector is bolted using a single nut. Large washers can be used to provide an earth connection directly to the connector.

Some of these connectors may also use four nuts and bolts to fix them to the panel. These arrangements are only suitable for low frequency applications, and not for RF. Where impedance matching and full screening is required. Bulkhead mounting connectors where coaxial cable entry is provided are available for this. Again these are available for a variety of cable dimensions and the correct type should be used.

There are two main variants of the TNC connector assembly method:

  • Compression gland type
  • Crimp type

The compression gland type has the centre pin of the connector which is usually a solder pin and the braid and sheath of the cable are held by an expanding compression gland fixed by a nut at the rear of the connector. This type of connector by its nature can cope with a (limited) range of cable sizes and requires no specialised tooling to assemble. This makes it ideal for small quantity production, either for one off cables for laboratory use of for limited production runs.

The crimp TNC connector has the centre pin which is normally crimped to the centre conductor. This crimped pin is then pushed into position through an inner ferrule which separates the inner insulation sheath and the braid of the cable. An outer ferrule is then crimped over the braid and outer insulation which fixes the cable to the connector. Greater accuracy is required for the crimp style connectors and therefore the correct connector variant must be chosen for the cable being used. This may result in a crimp style connector not being practicable for some cable types.

In addition to this the assembly requires the use of the correct crimping tools to ensure that the connector is correctly crimped. While these connectors are usually preferred for large production runs because they are faster to assemble, it is not possible for them to be reworked for obvious reasons.

For both styles of TNC connector it is essential that the exact amount of insulation is stripped from each section to ensure accurate and successful assembly.

The sockets or female TNC connectors also come in a number of flavours. In view of the fact that TNC connectors are normally used for RF applications, bulkhead mounting connectors where coaxial cable entry is provided are normally used. Again these are available for a variety of cable dimensions and the correct type should be used.

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