A 90-degree hybrid coupler is a passive microwave component designed to split or combine radio frequency (RF) signals at a 90-degree phase difference between its output ports, widely used in RF and microwave systems such as radar, telecommunications, and satellite communications. This coupler operates by dividing an input signal into two equal-amplitude outputs with a 90° phase shift, or combining two signals into a single output while maintaining isolation between the input ports.

The basic structure of a 90-degree hybrid coupler consists of four ports: an input port (P1), two output ports (P2 and P3), and an isolated port (P4). When a signal is applied to P1, it splits equally between P2 and P3, with P3’s output lagging P2’s by 90 degrees. The isolated port (P4) is designed to have minimal signal leakage, typically providing 20 dB or more isolation from the input, preventing interference between components connected to different ports. This isolation is critical in systems like transceiver modules, where it prevents transmitted signals from overwhelming sensitive receiver circuits.

Common implementations include microstrip, stripline, and waveguide designs, each suited to specific frequency ranges. Microstrip couplers, etched on printed circuit boards (PCBs), are cost-effective and work well for frequencies between 1 GHz and 20 GHz, making them ideal for cellular base stations. Waveguide couplers, used for frequencies above 18 GHz (e.g., in 5G mmWave systems), offer lower insertion loss (typically <0.5 dB) and higher power handling (up to several kilowatts), though they are larger and more expensive.

Performance parameters include coupling ratio, phase balance, and bandwidth. A 3-dB coupling ratio is standard, meaning each output port receives half the input power (minus losses). Phase balance within ±2 degrees of 90° ensures accurate signal combination in applications like phased-array antennas, where phase precision is critical for beamforming. Bandwidth varies by design, with some couplers operating over a narrow band (e.g., 10% of center frequency) for precision applications, while others cover broad bands (e.g., 1–18 GHz) for multi-band systems.

Isolation between ports is a key specification, typically 20–30 dB, ensuring that signals at the output ports do not interfere with each other or the input. This is particularly important in receiver systems, where strong signals at one output port could otherwise desensitize the other. Return loss, a measure of how much signal is reflected at the input port, is typically better than 15 dB, ensuring efficient power transfer into the coupler.

Materials and manufacturing processes depend on the frequency range and application. Microstrip couplers use PCB substrates like Rogers 4350 with copper traces, while waveguide couplers are machined from aluminum or brass with gold plating to minimize RF losses. For high-power applications, such as radar transmitters, couplers may use heat sinks or cooling channels to dissipate excess power.

 a 90-degree hybrid coupler is a versatile RF component that enables precise signal splitting and combining with controlled phase relationships, supporting the reliable operation of high-frequency systems in telecommunications, defense, and aerospace industries.

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