UHF connector, including the PL259 plug, and SO239 socket

- details and information about the UHF RF connector, including the SO239 UHF socket and PL259 plug, UHF connector adapters and links to suppliers.

The UHF connector is a coaxial RF connector that is used in low cost applications for frequencies often in the HF and the bottom end of the VHF spectrum. Although it does not offer a particularly high level of performance, this RF connector is nevertheless satisfactory for many applications where cost may be an issue.

The UHF RF connector was designed in the 1930s by E. Clark Quackenbush, a design engineer working for the Amphenol company. This RF connector design was aimed to cover frequencies in the range 0.6 to 300 MHz and it was aimed at use within the radio industry. In view of the fact that the frequency of operation for the connector originally extended to 300MHz - the bottom of the UHF band of frequencies, it was given the name of UHF connector.

Connector designations

In view of the fact that the UHF connector was designed in the Amphenol company, it is also sometimes referred to as the Amphenol connector. In addition to this the plugs and sockets may be referred to by different designations. The plug may be referred to as a PL259 coaxial connector, and the socket as an SO239 connector. These numbers arise from the original military numbers given to the UHF connectors

Basic description

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 basic RF connector (PL259) is has a relatively large threaded hole through which the coax cable enters. This is suitable for large, low loss cable, and also make the connector suitable for relatively high power applications. However where smaller cable is to be used, versions with a thinner hole are available or a reducer sleeve can be used. This reducer sleeve screws into the threaded coax cable entry hole may be used to make the cable entry diameter suite the size of the cable being used.

Basic UHF Connector Specification Summary
Parameter Specification
Cable Type Coaxial
Securing screw mating
Typical operating frequency range 0 - 100 MHz
Diameter 18.0 mm / 0.71 in (typical)
Male connector type designation PL259
Female connector type designation SO239
Impedance Non-constant
Max operating voltage Up to 500V for high quality versions
Weatherproofing Non-weatherproofed


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 100 MHz or so, despite the fact they were originally designed for use to 300MHz. Today's requirements are more exacting and therefore it is not advisable to use them much beyond 100 MHz, although some applications still see them used on equipment with frequencies extending to 150 MHz and more. Also there are some very low cost versions of the UHF connectors intended for applications like CB and these can really only be used for frequencies up to about 30 MHz. In view of their non-constant impedance, these RF connectors, of any quality, are now rarely used for many professional applications, being generally limited to CB, amateur radio and some video and public address systems.

Soldering PL259 connectors

Soldering PL259 connectors is not always easy. Start by stripping back about 1.5 inches (35mm) of the outer coating or sheath of the cable, taking care not to cut too deeply and score any of the fibres of the conductive braid. Leave around 0.5 inch (13mm) of the copper braid or shielding in place and then remove about 0.5 inch (13mm) of the plastic core.

Tin the exposed central copper core. To do this, heat the core with the soldering iron and apply a thin even coating of solder to it. Take care not to keep the soldering iron on the conductor for too long otherwise the dielectric spacing between the outer and inner conductors of the coax will melt. Once the cable has cooled slide the inner part of the PL259 plug over the cable with a screwing action until the copper core appears at the end of the centre pin. The trimmed shield will have become trapped between the core and the inside of the PL259. The outer sheath or covering or covering of the coax cable will ensure a snug fit and any protruding shielding should be removed with the sharp knife.

The UHF connector has been available for many years, and it is likely to remain in use for many years to come. These days it is not normally used in many professional applications as other connector types are generally more suitable. Nevertheless the UHF connector is still used in some more niche applications where the highest performance is not required, and where its rugged mechanical construction and low cost are important.

By Ian Poole

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