TNC Connector

- details and information about the TNC connector, - TNC socket and plug with links to suppliers.

The TNC connector is very similar to the BNC connector although it is not nearly as widely used. The main difference between then is that the TNC connector has a screw fitting instead of the bayonet. The screw fitting means that the RF connection of the TNC connector is generally more robust and accordingly it can operate more reliably at higher frequencies.

TNC cennector development

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.

TNC connector performance

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 are able to operate to 18 GHz.

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 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 BNC connectors also come in a number of flavours. The very basic BNC 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.

By Ian Poole

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