// RF Passives › Directional Couplers
Designing Directional Couplers
Step-by-step design guide for three essential RF hybrid couplers — the directional coupler, branch-line 90° hybrid and rat-race 180° hybrid. Enter your frequency and substrate, follow the steps, and get exact PCB trace widths and lengths. Beginners welcome — every concept explained from first principles.
// What are Directional Couplers?
The Essential RF Signal-Routing Components
A directional coupler is a four-port network that samples or splits RF power in a controlled, direction-dependent way. Unlike the Wilkinson divider (which splits power equally between matched ports), couplers control both the amplitude ratio and the phase relationship between output ports.
Three types are covered here:
✦ Directional Coupler — samples a small fraction of power (e.g. −10 dB, −20 dB) from a transmission line without disturbing it. Used for power monitoring, levelling loops and antenna VSWR sensing.
✦ Branch-Line Coupler (90° Hybrid) — splits power equally (−3 dB) with a 90° phase difference between the two output ports. Used in balanced amplifiers, IQ mixers, phased array antenna feeds and QPSK modulators.
✦ Rat-Race Coupler (180° Hybrid) — splits power equally (−3 dB) with either 0° or 180° phase difference. Used in balanced mixers, push-pull amplifiers, antenna feeds requiring sum/difference (Σ/Δ) ports and Butler matrices.
All three work on the same principle: quarter-wavelength transmission line sections create controlled interference between signal paths. The dimensions determine how much power exits each port and with what phase.
Three types are covered here:
✦ Directional Coupler — samples a small fraction of power (e.g. −10 dB, −20 dB) from a transmission line without disturbing it. Used for power monitoring, levelling loops and antenna VSWR sensing.
✦ Branch-Line Coupler (90° Hybrid) — splits power equally (−3 dB) with a 90° phase difference between the two output ports. Used in balanced amplifiers, IQ mixers, phased array antenna feeds and QPSK modulators.
✦ Rat-Race Coupler (180° Hybrid) — splits power equally (−3 dB) with either 0° or 180° phase difference. Used in balanced mixers, push-pull amplifiers, antenna feeds requiring sum/difference (Σ/Δ) ports and Butler matrices.
All three work on the same principle: quarter-wavelength transmission line sections create controlled interference between signal paths. The dimensions determine how much power exits each port and with what phase.
Key difference from Wilkinson divider: A Wilkinson divider's outputs are always in phase. Hybrid couplers give you a phase relationship between outputs — 90° for branch-line, 0°/180° for rat-race. This phase control is what makes them essential in IQ mixers, balanced amplifiers and phased arrays.
// PCB Substrate — shared across all three coupler designs
Presets: FR4 (εr=4.4, h=1.6mm, tan δ=0.020) · Rogers 4350B (εr=3.66, h=0.762mm, tan δ=0.0037) · Rogers 5880 (εr=2.2, h=0.787mm, tan δ=0.0009) · RO4003C (εr=3.55, h=0.813mm, tan δ=0.0027)
Uses the Hammerstad-Jensen closed-form model — same as the Microstrip Calculator. Accuracy within 1–2% of full-wave EM simulation.
Uses the Hammerstad-Jensen closed-form model — same as the Microstrip Calculator. Accuracy within 1–2% of full-wave EM simulation.
// Select coupler type to design
// Directional Coupler — Circuit Schematic
Main line (Through path)
Coupled line
Electric field coupling
Coupled output port
// Step 01 — What is a Directional Coupler?
Concept and Port Functions
A directional coupler has four ports. Two parallel transmission lines run close together for a quarter wavelength — energy couples from one line to the other through the electromagnetic field in the gap between them.
| Port | Name | Power | Phase vs Port 1 | Use |
|---|---|---|---|---|
| Port 1 | Input | 0 dB ref | 0° | Signal source |
| Port 2 | Through | ≈ 0 dB (−0.1 to −0.5 dB) | 0° | Main signal continues |
| Port 3 | Coupled | −C dB (e.g. −10, −20 dB) | −90° | Sampled signal for monitoring |
| Port 4 | Isolated | ≈ −∞ dB (terminated) | — | 50 Ω termination |
Key insight: The directional coupler is not a power splitter — it's a power sampler. The through port (Port 2) carries almost all the power with negligible insertion loss. Port 3 (coupled port) takes only a tiny fraction — 1% for a 20 dB coupler, 10% for a 10 dB coupler. This makes it ideal for monitoring transmitter output power without interrupting it.
// Step 02 — Enter Design Specifications
Frequency and Coupling Factor
// Inputs
MHz
Ω
dB
C = 3 dB → 50% of power coupled (this is a 3 dB hybrid — same as branch-line)
C = 10 dB → 10% coupled, 90% through — common for power monitoring
C = 20 dB → 1% coupled, 99% through — common for directional power meters
C = 30 dB → 0.1% coupled — used in high-power transmitters to sample signal safely
Choosing coupling factor: For power monitoring, use 20 dB (easy to detect 1% of power without expensive attenuators). For antenna VSWR sensing, use 10–20 dB. For balanced amplifiers or IQ mixers, use 3 dB (branch-line design in Tab ②).
// Step 03 — Calculate Even and Odd Mode Impedances
Z₀e and Z₀o — The Two Key Parameters
A coupled-line directional coupler works because two modes propagate in the coupled region — the even mode (both lines in phase) and the odd mode (lines out of phase). Each mode sees a different impedance.
C_linear = 10^(−|C|/20) (convert dB coupling to linear)
Z₀e = Z₀ × √((1 + C_linear) / (1 − C_linear))
Z₀o = Z₀ × √((1 − C_linear) / (1 + C_linear))
Note: Z₀e × Z₀o = Z₀² always (geometric mean equals system impedance squared).
C_linear = 10^(−|C|/20) (convert dB coupling to linear)
Z₀e = Z₀ × √((1 + C_linear) / (1 − C_linear))
Z₀o = Z₀ × √((1 − C_linear) / (1 + C_linear))
Note: Z₀e × Z₀o = Z₀² always (geometric mean equals system impedance squared).
// Results
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Physical meaning of Z₀e and Z₀o: Z₀e > Z₀ and Z₀o < Z₀ always. The larger the coupling (smaller C dB number), the wider the gap between Z₀e and Z₀o — and the harder it is to fabricate a tight gap on PCB. For C = 3 dB on FR4, the gap can be as small as 0.05 mm — at the edge of standard PCB manufacturing. For C = 10 dB the gap is typically 0.2–0.5 mm — easily fabricated.
// Step 04 — PCB Trace Gap and Width
Converting Z₀e and Z₀o to Physical Dimensions
The even and odd mode impedances determine the trace width (W) and the gap between the two traces (s). Exact synthesis requires the Garg-Bahl coupled-line formulas. The key insight is:
A narrower gap → stronger coupling (lower C dB, more power transferred)
A wider gap → weaker coupling (higher C dB, less power transferred)
The Microstrip Calculator does not directly compute coupled-line parameters, but you can use it to find the single-line widths as a starting point, then adjust the gap in your EM simulator.
A narrower gap → stronger coupling (lower C dB, more power transferred)
A wider gap → weaker coupling (higher C dB, less power transferred)
The Microstrip Calculator does not directly compute coupled-line parameters, but you can use it to find the single-line widths as a starting point, then adjust the gap in your EM simulator.
// Approximate PCB Dimensions on your substrate
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Gap accuracy: The gap formula used here is an approximation (Kirschning-Jansen). For tight coupling (<15 dB), always verify with Sonnet Lite, OpenEMS or ADS Momentum before fabricating. Even small errors in gap (0.05 mm at 3 GHz) cause significant coupling variation. For >15 dB coupling, standard PCB tolerances (±0.05 mm) are acceptable.
⚡ Microstrip → Z₀e reference width
⚡ Microstrip → Z₀o reference width
// Step 05 — Performance and Layout Rules
Expected Performance and PCB Layout
Expected performance at f₀:
• Through port insertion loss: 0.1–0.5 dB (depends on substrate, copper roughness)
• Coupling: C ± 0.5 dB at f₀, degrades ≈1–2 dB at band edges
• Directivity: typically 15–25 dB for microstrip (better with stripline)
• Isolation (Port 1 to Port 4): ≈ Coupling + Directivity dB
• Bandwidth: typically 20–30% around f₀ for <1 dB coupling ripple
Layout rules:
① Both coupled traces must be identical in length. Any length asymmetry reduces directivity and introduces phase error at the coupled port.
② Keep the gap s uniform along the entire coupled length. Any taper or variation in gap changes the coupling continuously and distorts the frequency response.
③ Terminate Port 4 with a precision 50 Ω chip resistor (0402 or 0201) directly at the port pad. A poor termination on Port 4 dramatically reduces directivity — the coupler relies on Port 4 being matched.
④ Avoid bends inside the coupled region. The coupling is defined by the straight parallel section. If you must bend, do it outside the coupled region, with both lines bending identically.
⑤ Use ground vias alongside both traces at intervals of λ/20 to suppress parallel-plate modes on multi-layer boards.
• Through port insertion loss: 0.1–0.5 dB (depends on substrate, copper roughness)
• Coupling: C ± 0.5 dB at f₀, degrades ≈1–2 dB at band edges
• Directivity: typically 15–25 dB for microstrip (better with stripline)
• Isolation (Port 1 to Port 4): ≈ Coupling + Directivity dB
• Bandwidth: typically 20–30% around f₀ for <1 dB coupling ripple
Layout rules:
① Both coupled traces must be identical in length. Any length asymmetry reduces directivity and introduces phase error at the coupled port.
② Keep the gap s uniform along the entire coupled length. Any taper or variation in gap changes the coupling continuously and distorts the frequency response.
③ Terminate Port 4 with a precision 50 Ω chip resistor (0402 or 0201) directly at the port pad. A poor termination on Port 4 dramatically reduces directivity — the coupler relies on Port 4 being matched.
④ Avoid bends inside the coupled region. The coupling is defined by the straight parallel section. If you must bend, do it outside the coupled region, with both lines bending identically.
⑤ Use ground vias alongside both traces at intervals of λ/20 to suppress parallel-plate modes on multi-layer boards.
Microstrip vs Stripline: Microstrip directional couplers typically achieve 15–20 dB directivity limited by the unequal even/odd mode velocities. Stripline couplers achieve 35–40 dB directivity because the modes travel at equal velocities (fully TEM medium). If you need high directivity (>25 dB), use stripline or add a compensation capacitor at the coupled port end.
// Complete PCB Dimensions — on your substrate
| Element | Dimension | Value | Notes |
|---|---|---|---|
| 50 Ω port traces | Width | — | All 4 port feed lines |
| Coupled region traces | Width W | — | Both traces identical |
| Trace gap | Gap s | — | Uniform along full length |
| Coupled section | Length ℓ = λ/4 | — | Electrical: 90° at f₀ |
| Port 4 termination | Resistor | 50 Ω | 0402 chip, at port pad |
Enter substrate parameters above to see PCB dimensions.
// Design Complete
Your Directional Coupler Summary
// Branch-Line Coupler (90° Hybrid) — Circuit Schematic
Z₁ = Z₀/√2 arms (horizontal)
Z₂ = Z₀ arms (vertical)
Port feed lines (50 Ω)
// Step 01 — What is a Branch-Line Coupler?
Concept, Ports and Phase Relationships
The branch-line coupler is a square ring of four λ/4 transmission line sections. It is a 3 dB directional coupler — it splits power equally between two output ports, with a 90° phase difference between them.
| Port | Name | Power (from Port 1) | Phase | Common use |
|---|---|---|---|---|
| Port 1 | Input | 0 dB ref | 0° | Signal in |
| Port 2 | Through | −3 dB | −90° | Main output |
| Port 3 | Coupled | −3 dB | −180° | Quadrature output |
| Port 4 | Isolated | ≈ −30 dB | — | 50 Ω termination |
Why 90° phase difference? In a balanced amplifier, two identical amplifiers are connected between Port 1→Port 2 and Port 3→Port 4 of two branch-line couplers. The 90° phase shift cancels reflections from the amplifiers at the input — meaning any impedance mismatch at the amplifier input reflects back but combines destructively at Port 1 and constructively at Port 4 (the termination), giving a perfect match at Port 1 regardless of how well-matched the amplifiers are individually.
// Step 02 — Enter Design Specifications
Frequency and Impedance
// Inputs
MHz
Ω
// Step 03 — Calculate Line Impedances
Z₁ (Horizontal) and Z₂ (Vertical) Arms
The branch-line coupler has two pairs of λ/4 arms with different impedances:
Z₁ (horizontal, shunt arms) = Z₀ / √2
Z₂ (vertical, series arms) = Z₀
For Z₀ = 50 Ω: Z₁ = 50/√2 = 35.36 Ω and Z₂ = 50 Ω
Physical meaning:
Z₁ = Z₀/√2 — the horizontal (shunt) arms are lower impedance, meaning wider traces than the 50 Ω port lines.
Z₂ = Z₀ — the vertical (series) arms are exactly 50 Ω — same width as the port lines, making layout simpler.
Z₁ (horizontal, shunt arms) = Z₀ / √2
Z₂ (vertical, series arms) = Z₀
For Z₀ = 50 Ω: Z₁ = 50/√2 = 35.36 Ω and Z₂ = 50 Ω
Physical meaning:
Z₁ = Z₀/√2 — the horizontal (shunt) arms are lower impedance, meaning wider traces than the 50 Ω port lines.
Z₂ = Z₀ — the vertical (series) arms are exactly 50 Ω — same width as the port lines, making layout simpler.
// Electrical Results
35.36 Ω
50.00 Ω
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Why Z₁ = Z₀/√2? Even/odd mode analysis of the square ring shows that for equal power split (−3 dB) and perfect isolation, the shunt arms must have impedance Z₀/√2. This sets up the correct interference pattern: power combines in phase at Ports 2 and 3, and cancels at Port 4.
// Step 04 — PCB Trace Widths and Lengths
Physical Dimensions on Your Substrate
// Step 05 — Layout Rules
How to Place the Branch-Line Coupler on PCB
The branch-line coupler is a square ring — the layout is straightforward but the symmetry is critical.
① The ring must be a perfect square electrically. All four arms must be λ/4 at f₀. The horizontal arms are shorter physically than the vertical arms (because Z₁ < Z₂, so the microstrip is wider and εeff is slightly different).
② The four corner junctions must be clean T-junctions. Use right-angle bends (mitered at 45°) or curved bends at the corners. Sharp right angles add parasitic inductance that shifts the operating frequency.
③ Terminate Port 4 with a precision 50 Ω chip resistor. In a balanced amplifier, Port 4 feeds one amplifier input and Port 1 feeds the other — neither port has a physical termination resistor. In a simple power splitter application, Port 4 must be terminated.
④ The coupler is inherently narrowband (20–30% BW). For wider bandwidth, use a multi-section branch-line coupler with 3 or more sections — doubles the bandwidth at the cost of physical size.
⑤ Keep the ring away from ground vias and copper pours — especially the inner area of the ring. Copper fill inside the ring changes the effective permittivity and shifts the resonant frequency.
① The ring must be a perfect square electrically. All four arms must be λ/4 at f₀. The horizontal arms are shorter physically than the vertical arms (because Z₁ < Z₂, so the microstrip is wider and εeff is slightly different).
② The four corner junctions must be clean T-junctions. Use right-angle bends (mitered at 45°) or curved bends at the corners. Sharp right angles add parasitic inductance that shifts the operating frequency.
③ Terminate Port 4 with a precision 50 Ω chip resistor. In a balanced amplifier, Port 4 feeds one amplifier input and Port 1 feeds the other — neither port has a physical termination resistor. In a simple power splitter application, Port 4 must be terminated.
④ The coupler is inherently narrowband (20–30% BW). For wider bandwidth, use a multi-section branch-line coupler with 3 or more sections — doubles the bandwidth at the cost of physical size.
⑤ Keep the ring away from ground vias and copper pours — especially the inner area of the ring. Copper fill inside the ring changes the effective permittivity and shifts the resonant frequency.
Balanced amplifier connection: Connect Port 1 of coupler 1 → amplifier A input and Port 3 → amplifier B input. Amplifier A output → Port 1 of coupler 2, amplifier B output → Port 3 of coupler 2. Port 2 of coupler 2 is the combined output. Ports 4 of both couplers terminate in 50 Ω and absorb the reflected power from amplifier mismatches.
// Complete PCB Dimensions — on your substrate
| Element | Width | Length | Notes |
|---|---|---|---|
| 50 Ω port traces | — | As needed | All 4 port feed lines |
| Z₁ horizontal arms (×2) | — | — | Z₀/√2 — wider than 50 Ω |
| Z₂ vertical arms (×2) | — | — | Z₀ — same as port traces |
| Overall ring size (est.) | — | Square footprint approx. | |
Enter substrate parameters above to see PCB dimensions.
// Design Complete
Branch-Line Coupler Summary
Expected: −3 dB at both output ports ± 0.5 dB, isolation >20 dB at f₀, bandwidth 20–30%.
// Rat-Race Coupler (180° Hybrid) — Circuit Schematic
Ring: Z = Z₀√2, total 3λ/2
Port 1 — Σ (sum input)
Port 3 — Δ (difference, 180° shifted)
Port 4 — Isolated
// Step 01 — What is a Rat-Race Coupler?
Concept, Ring Structure and Port Functions
The rat-race coupler (also called a ring hybrid) is a circular ring of transmission line with total circumference 3λ/2. Four ports connect to the ring at specific points spaced λ/4 apart (except one gap which is 3λ/4).
The unequal spacing creates a 180° phase difference between the two output ports. Unlike the branch-line coupler (which can only produce 90° split), the rat-race gives you a choice of 0° or 180° phase depending on which input port you use.
The unequal spacing creates a 180° phase difference between the two output ports. Unlike the branch-line coupler (which can only produce 90° split), the rat-race gives you a choice of 0° or 180° phase depending on which input port you use.
| Input Port | Port 2 output | Port 3 output | Port 4 | Application |
|---|---|---|---|---|
| Port 1 (Σ) | −3 dB, 0° | −3 dB, 180° | Isolated | Sum-difference network |
| Port 4 (Δ) | −3 dB, 0° | −3 dB, 0° | Isolated | In-phase combiner |
Key advantage over branch-line: The rat-race has a Σ (sum) port and a Δ (difference) port. In a monopulse radar antenna, two feeds are combined with a rat-race: the Σ port gives maximum gain for target detection, the Δ port gives a null in the centre for precise angular tracking. In a balanced mixer, LO enters Port 1 (Σ) and the two diodes see opposite-phase LO — enabling excellent LO-RF isolation.
// Step 02 — Enter Design Specifications
Frequency and Impedance
The rat-race coupler design is even simpler than the branch-line — there is only one line impedance throughout the entire ring.
// Inputs
MHz
Ω
// Step 03 — Ring Impedance and Dimensions
The Single Design Formula
The rat-race ring has a single characteristic impedance throughout:
Z_ring = Z₀ × √2
For Z₀ = 50 Ω: Z_ring = 50 × 1.414 = 70.71 Ω
Ring dimensions:
Total circumference = 3λ/2 at f₀
Port 1 → Port 2: λ/4 clockwise
Port 2 → Port 3: λ/4 clockwise
Port 3 → Port 4: λ/4 clockwise
Port 4 → Port 1: 3λ/4 clockwise (the long way around — this is what gives 180° phase shift)
Z_ring = Z₀ × √2
For Z₀ = 50 Ω: Z_ring = 50 × 1.414 = 70.71 Ω
Ring dimensions:
Total circumference = 3λ/2 at f₀
Port 1 → Port 2: λ/4 clockwise
Port 2 → Port 3: λ/4 clockwise
Port 3 → Port 4: λ/4 clockwise
Port 4 → Port 1: 3λ/4 clockwise (the long way around — this is what gives 180° phase shift)
// Electrical Results
70.71 Ω
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Why Z₀√2? Same reason as the Wilkinson divider — the ring impedance must transform 50 Ω at each port to 50 Ω looking into the ring in both directions simultaneously. The √2 factor comes from the two parallel paths around the ring each presenting 2×Z₀ load, which the ring transforms to Z₀.
// Step 04 — PCB Trace Dimensions
Physical Trace Width and Ring Size
// PCB Trace Results — on your substrate
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Circular vs square layout: The rat-race is naturally laid out as a circle — all sections have the same impedance so it's physically a ring of constant-width trace. A square layout is also possible (fold the ring into a rectangle) and is sometimes preferred for dense PCB layouts. The ring radius is the total ring circumference divided by 2π.
// Step 05 — Layout Rules
PCB Layout for the Rat-Race Coupler
① The ring is a single continuous trace of constant width. Unlike the branch-line, there are no impedance transitions inside the ring — every section has the same width (Z₀√2). This makes fabrication very straightforward.
② Port positions must be accurately placed. Ports 1, 2, 3 are spaced λ/4 apart around the ring (90° arc each). Port 4 is λ/4 from Port 3 going the short way (or equivalently 3λ/4 from Port 1 going the long way). Any error in port position causes amplitude or phase imbalance.
③ Use curved bends — avoid sharp corners. The ring is naturally circular. If you use rectangular segments (a square ring), miter the corners. Sharp bends add parasitic reactance and shift the resonant frequency.
④ The long section (3λ/4) makes the layout large. The rat-race ring is physically bigger than a branch-line coupler of the same frequency because of the 3λ/2 circumference. At 2.4 GHz on FR4, the ring radius is approximately 15–18 mm, making it a 30–36 mm diameter component.
⑤ Bandwidth is inherently limited (20–25%). The 3λ/4 section disperses more than the λ/4 sections at off-centre frequencies. For wider bandwidth, use a modified Gysel coupler or multi-section design.
② Port positions must be accurately placed. Ports 1, 2, 3 are spaced λ/4 apart around the ring (90° arc each). Port 4 is λ/4 from Port 3 going the short way (or equivalently 3λ/4 from Port 1 going the long way). Any error in port position causes amplitude or phase imbalance.
③ Use curved bends — avoid sharp corners. The ring is naturally circular. If you use rectangular segments (a square ring), miter the corners. Sharp bends add parasitic reactance and shift the resonant frequency.
④ The long section (3λ/4) makes the layout large. The rat-race ring is physically bigger than a branch-line coupler of the same frequency because of the 3λ/2 circumference. At 2.4 GHz on FR4, the ring radius is approximately 15–18 mm, making it a 30–36 mm diameter component.
⑤ Bandwidth is inherently limited (20–25%). The 3λ/4 section disperses more than the λ/4 sections at off-centre frequencies. For wider bandwidth, use a modified Gysel coupler or multi-section design.
Practical size concern: At low frequencies (below 1 GHz), the rat-race ring becomes very large (diameter >70 mm at 900 MHz on FR4). At these frequencies, use lumped-element equivalents or the branch-line coupler (which can be miniaturised with high-permittivity substrate or lumped capacitors).
// Complete PCB Dimensions — on your substrate
| Element | Width | Length | Notes |
|---|---|---|---|
| 50 Ω port traces | — | As needed | All 4 port feed lines |
| Ring trace (entire ring) | — | — | Constant width = Z₀√2 |
| λ/4 arc sections (×3) | — | — | Port 1↔2, 2↔3, 3↔4 |
| 3λ/4 arc section (×1) | — | — | Port 4↔1 (long section) |
| Ring radius (circular) | — | Circumference / 2π | |
Enter substrate parameters above to see PCB dimensions.
// Design Complete
Rat-Race Coupler Summary
Expected: −3 dB at Ports 2 & 3, isolation >20 dB at f₀, bandwidth 20–25%.
About Directional Couplers
Directional couplers are four-port passive networks used throughout RF and microwave engineering to sample, split, or combine signals in a controlled, direction-dependent manner. All three types covered here — the directional coupler, branch-line coupler and rat-race coupler — are based on quarter-wavelength transmission line sections implemented as microstrip traces on PCB.
Directional Coupler
The directional coupler uses two parallel coupled transmission lines to extract a small fraction of power from a main line. The coupling factor C determines how much power is sampled — 10 dB takes 10% of power, 20 dB takes only 1%. The even and odd mode impedances Z0e and Z0o set the physical dimensions of the coupled region. Directional couplers are used in power detectors, levelling loops, reflectometers and automatic gain control circuits.
Branch-Line Coupler (90° Hybrid)
The branch-line coupler is a square ring of four quarter-wavelength lines with two impedance values: Z0/√2 for the horizontal shunt arms and Z0 for the vertical series arms. It splits power equally between two output ports with a 90° phase difference. This 90° quadrature relationship is essential for IQ modulators and demodulators, balanced amplifiers, and phased array antenna feeds.
Rat-Race Coupler (180° Hybrid)
The rat-race coupler is a ring with circumference 3λ/2 and uniform impedance Z0√2. The unequal port spacing creates a 180° phase difference between outputs. It can function as either a power splitter (0° or 180° output phase depending on which port is driven) or as a sum/difference network. Rat-race couplers are widely used in balanced mixers, push-pull amplifiers, and monopulse radar antenna feeds.