An F coupler is a device with two female F-connectors, which are frequently, but incorrectly, referred to as "coaxial connectors." These coaxial cables are typically used with radio frequency cables, such as those used for cable television or satellite dishes. The primary purpose of the F coupler is to join two shorter cables together to make a single but longer cable.
Looking much like a double-headed bullet with a hexagonal nut in the middle, an F coupler typically has two screw-on female F-connectors connected back to back. Low-cost cables with simple push-on male F-connectors can simply be pushed onto each end of the F coupler to join them. Better quality cables will have make F-connectors with screw threads and either a knurled ring or a hexagonal nut. These cables will need to be screwed onto the coupler to generate a connection that is not only physically tight but also unlikely to allow interference in the signal path.
The most common use of an F coupler is to join two coaxial cables with F-connectors together to create a longer cable. Joining two cables together is not without its problems, though. Using a coupler to do this creates two problems at once. The first is that signals get weaker when they are sent over longer cables, and the second is that signals degrade when they pass through a connection point. As a result, although an F coupler is an excellent tool for shorter runs with strong signals, using them can make a marginal signal unusable at the other end of the extended cable.
Those who work with systems that use F couplers should be careful to use the right kind. Although they all typically look the same, other than their different finish colors, there are actually different types. The 75-ohm coaxial cables, which users join together can have different ratings as to their construction, impedance, and the bandwidth of data that they can reliably carry. For the best performance, two cables made with Radio Guide (RG) 6 cabling should be joined with an RG-6 rated, F coupler, while an RG-59 coupler would be best with RG-59 cables.