A directional coupler is a key passive RF component used to sample a small portion of the power from the main transmission path and direct it to one or more coupled ports. This device is designed with high isolation between the primary signal path and the coupled port, ensuring minimal interference. There are two main types: standard directional couplers, which have one coupled port and one terminated port, and dual directional couplers, which provide both forward and reverse coupling. Some variations include forward and reverse couplers, depending on which direction the coupling occurs.
The schematic representation of a directional coupler typically shows four ports: the input (or through) port, the output (or transmitted) port, the coupled port, and the terminated port. The amount of coupling affects the insertion loss in the main path—lower coupling results in lower insertion loss. However, the power rating of the coupled port is usually lower than that of the main path. If the power difference exceeds the coupled port’s capacity, damage may occur.
The termination at the isolated port plays a critical role in performance. When matched to the transmission line impedance (usually 50 ohms), it minimizes reflections. Mismatched or open/closed terminations can cause reflections back into the main path, potentially leading to signal distortion or even damage. It's essential to ensure the termination can handle the expected power levels.
Directional couplers are widely used in test and measurement applications. For instance, they help measure input and reflected power in a transmission line, enabling calculations of the voltage standing wave ratio (VSWR). They’re also used for signal sampling, injection, and power monitoring, where the coupler’s own losses must be accounted for to maintain accuracy.
Isolation between ports is another important parameter. Even with good design, some leakage exists, known as isolation. Directionality, the ratio of isolation to coupling, is a key performance metric. These parameters vary with frequency, so designers must consider frequency response and manufacturing tolerances when making precise measurements.
Most directional couplers do not allow DC current due to grounding at the ports. However, some models are designed for DC pass-through. In such cases, it’s crucial to keep the current within rated limits to avoid overheating or performance degradation. All ports of a dual-directional coupler should be properly grounded and matched to the system impedance.
Lastly, a 90-degree or 180-degree bridge is sometimes called a "coupler," but it operates differently from a traditional directional coupler. While their physical designs may look similar, they function based on different principles. Using them as low-coupling directional couplers can lead to unintended consequences, so it's important to understand their distinct behavior.
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