Directional couplers are key passive components in microwave/RF systems, coupling a portion of power from a main transmission line to a secondary line (coupled port) at a specific ratio while suppressing reverse power (isolated port). They find wide use in power monitoring, signal distribution, and mixer design. Below is an overview of their core properties, advantages, and disadvantages.
Core Properties
1. Coupling: The power ratio (in dB) between the input and coupled ports, indicating the proportion of power tapped. Common values include 3dB (equal split), 6dB, 10dB, and 20dB, chosen by system needs. A 10dB coupler, for example, diverts 10% of input power to the coupled port.
2. Isolation: The power ratio (in dB) between the input and isolated ports, measuring reverse power suppression. Higher values mean less leakage; ideally, the isolated port carries no power.
3. Directivity: The difference (in dB) between isolation and coupling, reflecting directional performance—coupling forward power while rejecting reverse signals. Practically >20dB (ideally infinite).
4. Insertion Loss: Power loss (in dB) between input and through ports, caused by conductor/dielectric losses. Ideally <1dB.
5. Frequency Range: Parameter stability is limited to specific bands. Microstrip/stripline couplers suit mid-low frequencies (hundreds of MHz to tens of GHz); waveguides work for >10GHz; coaxial types cover mid-high frequencies.
6. Power Capacity: Maximum sustainable power (peak/average), determined by materials and structure. Waveguide/coaxial types handle kilowatts (radars, base stations); microstrip versions manage watts (phones, receivers).
7. VSWR: Voltage standing wave ratio should approach 1 for optimal matching, minimizing signal reflection.
Advantages
lPassive operation: No external power required, ensuring high reliability in harsh environments (e.g., high temperatures, vibrations).
lFlexible functionality: Enables power splitting (e.g., 3dB couplers for equal division) and monitoring (e.g., 20dB couplers extracting 1% power) via coupling selection, adapting to diverse system needs.
lBroadband capability: Some structures (e.g., branch-line, Lange couplers) cover multi-octave ranges (e.g., 1-10GHz), meeting demands of 5G and satellite communication.
lEasy integration: Microstrip/stripline types integrate with filters/amplifiers on the same substrate, reducing system size and cost, especially for mass-produced consumer electronics.
lLow interference: High directivity design suppresses reverse coupled signals, reducing interference to main lines and other ports, ensuring signal purity.
Disadvantages
nInevitable insertion loss: Even ideal models lose power due to coupling; real-world devices suffer additional losses from conductor resistance and dielectrics, requiring compensation in system design.
nLimited directivity: Typically 20-40dB (not infinite), leading to reverse power leakage through isolated ports, which may interfere with sensitive components like receiver front-ends.
nRestricted power capacity: Microstrip/planar structures, limited by dielectric breakdown and heat dissipation, cannot handle high-power scenarios (e.g., radar transmitters), requiring larger, costlier waveguides or coaxial types.
nFrequency band fluctuations: Coupling and directivity vary significantly at band edges (e.g., ±1dB from nominal), needing complex designs (e.g., multi-section couplers) that increase development difficulty.
nSensitivity to manufacturing precision: Errors in microstrip line width/spacing severely affect performance at high frequencies, demanding high-precision processes (e.g., photolithography) that raise costs.
Directional couplers are vital in microwave/RF systems. Selection depends on frequency, power, size, and cost, with design optimizations addressing their limitations.