Directional couplers are widely used in RF systems for signal sampling, power monitoring, VSWR detection, and PIM testing. While these components are passive by nature, their performance is highly dependent on how each port is terminated—especially the coupled port.
In real-world applications, improper termination of the coupled port—either by leaving it open, using poorly matched devices, or low-quality cables—can severely affect the performance of the coupler and even compromise overall system accuracy.
In this article, we explore:
Which key performance indicators are affected when the coupled port of a directional coupler is mismatched—and why it matters.
A typical directional coupler has four ports:
Port 1 (Input): Where the main RF signal enters
Port 2 (Output): Where the main signal exits
Port 3 (Coupled): Taps a small portion of the forward signal
Port 4 (Isolated): Ideally outputs no signal; usually terminated with 50Ω
The coupled port (Port 3) provides a low-power replica of the mainline signal for measurement or feedback. However, if the port is not properly terminated, the reflected signal can travel back into the system and interfere with accurate signal extraction.
Some frequent mistakes in field setups include:
Leaving the coupled port open
Connecting a device with poor impedance match
Using poor-quality connectors or test cables
All of the above can cause signal reflection at the coupled port, affecting system accuracy and introducing measurement errors.
The coupling value (e.g., 10 dB, 20 dB) is designed to be fixed. However, a mismatched coupled port can introduce reflection that disturbs the internal coupling path, causing unintended fluctuations in coupled power output.
Impact: Power monitoring becomes inaccurate, and feedback loops may malfunction.
Directivity measures how well the coupler distinguishes forward from reverse signals. It is defined as:
Directivity (dB) = Coupling (forward) - Coupling (reverse)
A mismatch on the coupled port causes reflected signals to mix with reverse signals, lowering directivity. This makes it harder to differentiate between incident and reflected signals.
Impact: Lower accuracy in reflection measurements, affecting VSWR or PIM analysis.
Although directional couplers are designed for low return loss, reflected waves from the mismatched coupled port can introduce internal standing waves and alter the overall match of the system.
Impact: Higher VSWR may reflect power back to amplifiers or test equipment, causing instability.
Directional couplers are typically very low loss (e.g., <0.2 dB). However, internal reflections from the coupled port can slightly affect the mainline energy flow, increasing insertion loss.
Impact: System efficiency decreases; more power is lost in the passive path.
To prevent degradation of coupler performance:
Always terminate the coupled port with a 50Ω load when not in use
Use low PIM, precision termination loads rated for the operating frequency and power
Avoid frequent connector plugging/unplugging
Clean connectors regularly and use high-quality RF cables
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