Dual-Junction RF Circulator: Principle and Types

Introduction

Dual-junction RF circulators connect two ferrite junctions in series to achieve stronger non-reciprocity, higher isolation, and better tolerance to mismatch than a single-junction device. They are a pragmatic way to keep transmitters safe from reflected power while holding insertion loss and VSWR to tight budgets in base stations, phased-array radar, satcom terminals, and test systems.

Two Topologies at a Glance

• T-Type: Tx–Rx arranged linearly; antenna at one end; a matched resistor terminates the isolation port.
• Y-Type: Star-like split; antenna at the top; both isolation ports are resistively terminated.

T-Type Dual-Junction Circulator

The T-type places the antenna and the two active junctions along a single path. The isolation port is resistively terminated so that reflected energy from the antenna is absorbed rather than re-entering the transmitter. This geometry is common in compact radios where the RF chain is arranged end-to-end on a microstrip or stripline layout.

When it shines

• Space-constrained layouts and low BOM count.
• Systems prioritizing lowest insertion loss between Tx and ANT.
• Moderate bandwidth with predictable biasing and simple thermal path.

Things to watch

• Asymmetry can make Rx protection slightly less forgiving under severe antenna detuning.
• Thermal hotspot near the termination if antenna mismatch is frequent.

Y-Type Dual-Junction Circulator

The Y-type forms a three-way node: the antenna sits at the top branch while transmitter and receiver occupy the lower branches. Each isolation path ends with a matched load. The symmetry helps maintain balance over wider bandwidths and reduces sensitivity to PCB tolerances.

Where it excels

• Wider bandwidth or multi-band operation with better port-to-port balance.
• Arrays and TDD systems that encounter fast-changing VSWR conditions.
• Thermal spreading thanks to two smaller terminations instead of one hot load.

Design cautions

• Slightly higher insertion loss compared to a well-tuned T-type.
• More complex routing and matching at the central node.

Types of Dual-Junction Circulators by Structure

Beyond topology (T-type vs Y-type), dual-junction circulators are delivered in multiple structural implementations to fit different power, bandwidth, and integration needs.

Microstrip (PCB / SMD)

• Best for: compact radios, modules, and arrays where size and cost matter.
• Pros: smallest footprint, easy routing, good for multi-channel layouts.
• Limits: lower power than drop-in/coaxial; board stack-up strongly affects bandwidth.

Drop-in

• Best for: medium-to-high power transmitters and wideband front-ends.
• Pros: robust ferrite block with controlled cavity; solid thermal path; stable over temperature.
• Limits: larger height and mechanical integration required (milled pockets or carriers).

Coaxial

• Best for: lab/test gear, SATCOM feed chains, or systems using coax interconnects.
• Pros: convenient connectors, excellent shielding, easy to swap and test.
• Limits: largest volume and weight; transitions add insertion loss in densely integrated radios.

Performance & Design Trade-offs

• Isolation: dual-junction adds 10–20 dB isolation headroom over a comparable single-junction, depending on ferrite material, bias, and matching.
• Insertion loss: keep total below the link-budget threshold—use low-loss ceramics, short transitions, and tight tolerance magnets.
• VSWR: a balanced layout and accurate 50-Ω terminations keep port VSWR low and protect power amplifiers.
• Power & thermal: size the resistor(s) for mismatch power. In Y-type, two resistors share dissipation; in T-type, plan a heat path for the single load.
• Form factor: microstrip/stripline for compact radios; drop-in or waveguide for higher power.

Selection Checklist

• Band: L/S/C/X/Ku/Ka; target fractional bandwidth
• Power: CW and peak; mismatch survival
• Loss/ISO: Max IL, min isolation, VSWR
• Size: Microstrip, drop-in, coaxial, or waveguide
• Thermal: Termination wattage & heat spread
• Reliability: Temp drift, magnet aging, shock/vibe

Test & Verification

• Use a calibrated VNA (TRL/SOLT) with suitable fixtures; de-embed adapters.
• Measure S₂₁ (Tx→ANT), S₃₂ (ANT→Rx), and isolation S₁₃/S₃₁ across temperature.
• Mismatch testing: sweep antenna VSWR 1.2–3.0 and record power dissipation at terminations.
• Thermal soak and shock to verify magnet stability and bias circuitry.

FAQ

Is a dual-junction always better than a single-junction?

No. If your band is narrow and the antenna is well matched, a quality single-junction may meet targets with lower cost. Dual-junction adds margin for tough VSWR and wider bandwidths.

Which topology should I start with?

Choose T-type for lowest path loss and simple routing; choose Y-type for symmetry, bandwidth, and thermal sharing.

How do I size the resistive terminations?

Estimate worst-case reflected power (PA power × reflection coefficient) and add 2× safety. In Y-type, split the dissipation between two loads.

References

• Pozar, D. M. Microwave Engineering, 4th ed.
• Collin, R. E. Foundations for Microwave Engineering.
• Manufacturer app notes on ferrite circulators and biasing best practices.

© HzBeat · Dual-Junction RF Circulators

Article Tags

dual-junction RF circulatorprincipleT-typeY-typemicrostrip circulatordrop-in circulatorcoaxial circulatorferrite circulatorHzBeatRF isolatormicrowave components