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RF connectors are designed to to enable coaxial cable feeder to be connected together in a way that enables the connections to be easily made and unmade.
As such RF connectors need to provide the required radio frequency performance on top of providing a secure and reliable DC connection.
As a result, RF connectors have a number of specifications that relate to their RF performance as well as specifications that relate to their ability to provide a reliable low frequency connection.
Understanding RF connector specifications enables the optimum connector to be obtained, balancing performance, size and cost. Selecting the different RF connector types including BNC, N-type, TNC, SMA or one of the many other types can have an impact of performance and usability.
RF connectors are used in many areas. One of the most obvious is in connecting RF between different units, or connecting antenna feeders to receivers and transmitters, etc. These applications are the most common, but RF connectors are also used within equipment as well. Here they enable the equipment to be serviceable - enabling boards or modules to be extracted or inserted without the need to solder connections to be made. Without RF connectors, accessing equipment would be virtually impossible.
For reach application, different specifications will apply - both mechanical and electrical, so choosing the right RF connector for any application requires an understanding of the applicable specifications.
Key RF connector specifications
Like all forms of components and the non-RF forms of connector, there are many specifications that are applicable to RF connectors.
Many of the basic specifications for general connectors can be applicable to RF connectors as well, but often they may be expressed in a different way that reflects the applications in which they will be used.
The list below contains many of the key parameters with explanations to enable the various RF connector specifications to be understood.
- Connector impedance specification: RF connectors form part of the feed system when they are used for RF. As the feeder has a specific impedance, the connector should also have a defined impedance. A very few connectors like the UHF connector (SO239 & PL259 connectors) are a non-constant impedance connector, and as a result of this they are generally only used for lower frequencies (up to 300 MHz or sometimes 500 MHz). Other connectors like the BNC, TNC, N-type, SMA and many more have a constant impedance.
Most connectors have a 50Ω impedance, as this is the most widely used impedance coaxial feeder for commercial RF applications. 75Ω is used for domestic television, so occasionally 75Ω connectors will be seen.
- Plating / finish: In many respects this may sound like a trivial connector specification, but it is far from this as it affects the performance to a very great degree, possibly more than many other aspects of their manufacture.
The reason for this is the skin effect. As the frequency rises, so the skin effect becomes more apparent. The skin effect is a phenomenon whereby as the frequency increases, so the current tends to concentrate more towards the outside or the skin of the conductor - in fact it is at a peak at the outside of the conductor reducing exponentially with distance from the surface of the conductor. This results from the magnetic inductance effect of the current flow which forces the current towards the surface of the conductor.
As any heat loss in the connector will be caused by the resistance of the conductors, it is found that as the frequency increases, so the cross sectional area over which the current flows reduces, and this means that the ohmic losses increase.
At microwave frequencies it is found that the major part of the current flows at a depth of approximately 3δ - about 95% of the current flows within a depth equal to 3 skin depths. This has a major impact on the connector specification and its performance.
At 1000 MHz in a copper conductor, the skin depth is approximately 2µm, gold it is about 2.4µm, and silver it is 2.0µm.
Nickel, as a result of its magnetic properties has a skin depth of only 0.17µm. From this it can be seen that nickel, whilst it does not tarnish, will give very poor microwave performance. Even at 10 MHz the skin depth is only 1.7µm. Added to this, its resistivity is much higher than copper and other popular metals. This means that nickel is a very poor choice of plating material for any RF connector - sometimes other metals are added to it to make it look gold coloured. As plated gold has a slightly matt finish, and nickel gives a nice shiny surface, it can be said that any connectors that are shiny are of no use whatsoever at RF.
- Power handling: For many applications the power handling capacitor of an RF connector is not an issue. However for transmitter systems it can be of major significance.
One of the main RF connector areas that affects its power handling is the centre pin and the mating receptacle. This is much smaller than the outer and as such it has a much high level of current density. This means that the design of the pin and mating receptacle of the RF connector are key to the connector power specification specification.
For proper operation the pin and receptacle must be perfectly aligned and there must be no damage or dirt between them. Also the plating of the mating halves is essential to provide the required RF connector specification. With skin depths measured in micro-metres, the conductivity within this area of the connector is key.
For high power levels the plating of the contacts has a major impact upon the performance. As previously mentioned nickel is useless, but good gold and silver plating of a suitable thickness enable the connectors to reach the highest level of performance.
- Coax cable compatibility: Most coaxial connectors are designed to be used with a particular type of cable. The reason for this is that the different types of coax have different diameters and dimensions. If the coaxial connector is to accept these, then it needs to be designed to match these.
This means that when selecting the right connector, one key connector specification is the type of coax cable that will be used. Normally this is not a major issue as the connector manufacturers design connectors for the most widely used forms of coax. It just means that a key specification for the coax connector is the coax cable type. However if there is a wide discrepancy between the connector and cable, it will not be possible fit the coax and connector together well.
- Voltage: Another important RF connector specification is the voltage withstand capability. In some instances the connector will see high levels of voltage. Obviously for high power levels there will be accompanying high levels of voltage. If there are high levels of reflected power, this can result in higher voltages.
It is necessary to ensure that the connector can withstand the anticipated voltages with a good margin. Remember that the environmental conditions may reduce the voltage capability.
- Frequency range: Some connectors are able to carry frequencies much higher than others. Obviously some forms of connector operate at much higher frequencies than others:- BNC connectors are able to operate at much higher frequencies than UHF connectors (PL259 & SO239). However, not all BNC connectors are the same. Some are manufactured to much higher specifications than others and can work happily at 10 GHz, whereas others may struggle to get much above 1 GHz.
When buying connectors, always buy from a known reputable source and check the specification for the particular part being bought.
When buying on the open market, be very careful, because many cheap connectors are just that and will not perform well. Always check the specification and don't think that because it is an N-type connector that its performance will extend to 18 GHz. It only will if it is manufactured to that specification.
Even many reputable manufacturers run a variety of different quality levels of connectors, so don't assume that just because it comes from a reputable manufacturer, that it will have the full performance. Essentially, think that you get what you pay for. Top quality, full specification connectors are not cheap . unfortunately.
- Straight and right-angled: Although the decision about whether a straight or right-angled free connector is used will be determined by the mechanics of an item of equipment. Right angled connectors are often used within items of equipment. They enable the coax cable to be routed away from the assembly they are connecting to without the need for a loop of cable and therefore reduce the space required. However they often offer a slightly lower specification - often their return loss performance is not as good. For most cases this should not be an issue, but at very high frequencies and where performance is key, it is an aspect with noting. Check the connector specification within the datasheets.
There are many different RF connector specifications. Some are applicable in some applications whereas others will be more appropriate in others. However understanding the different specifications and why they arise will always be of benefit when selecting the right connector for a given application.