A line coupler, commonly called an in-line coupler, is a device that connects two transmission lines without a large amount of additional bulk. While not a requirement, most of these couplers allow two cables of the same size and sex to connect. A common example of a line coupler is the barrel connector used when connecting coaxial cables. The barrel will connect two male coaxial lines of the same size to one another, and the coupler is almost entirely contained within the end caps of the cables.
A transmission line is any cable that transfers a signal from one location to another. These include cable television, telephone and power lines, meaning the developed world is nearly blanketed in them. A coupler is a device that connects two of these lines together, forming a single, longer, line. There are a wide range of types and styles of coupler; in fact, there are often multiple designs for a single connection style.
When a device is in-line, that means that a cable plugs into one end and another cable continues out the other. Essentially, the device is part of the transmission line. In addition to its technical definition, an in-line coupler will generally be as close to the size of the connected cables as possible. This allows the device to exist within the system, but not interfere with movements and access to the system.
Another common aspect of a line coupler that isn’t required is same sex connection types. These couplers will allow two cables that have the same end to connect to one another by providing two ports of the opposite sex. This is especially common on smaller couplers.
Generally, the smaller the line coupler, the less it does. Very small couplers, such as the cable barrel mentioned above, don’t do anything more than connect the cables together. Larger types of line coupler could clean or boost the signal, or provide a lossless coupling. A passive coupler, one that doesn’t do anything beyond couple, will generally be totally self-sufficient. An active coupler, like a signal booster, will often require an external power connection.
These devices have a single in and a single out. When a line coupler has multiple connections, it is more commonly called a line splitter. These will take a single transmission line and split it to multiple outputs or several inputs to a single output. Splitters are rarely powered, any system that modifies the signals of various lines is generally more complex than a powered line coupler.