What is a directional coupler used for in radio systems?

A directional coupler is a crucial component in radio systems, primarily used for allowing controlled signal division and measurement. This device works by coupling a fraction of the input signal into another line, enabling the observation of the signal's characteristics without significantly affecting its main path. This is particularly useful for monitoring power levels, ensuring proper signal flow, and preventing interference.

In many applications, directional couplers help maintain signal integrity by allowing engineers to measure the strength of the transmitted signal as well as reflections that can occur due to impedance mismatches. This capability aids in tuning and troubleshooting radio systems, ensuring effective transmission and minimal signal loss.

The other options do not accurately describe the primary function of a directional coupler. Amplification is achieved through amplifiers rather than couplers, filtering is performed by filter circuits designed to remove unwanted frequencies, and connecting multiple antennas in parallel is more typically done through combiners or multiplexer systems. Thus, the role of a directional coupler in controlled signal division and measurement is what establishes it as the correct answer.