T-Junction Power Divider

Overview

The simplest RF power divider using only transmission line impedance transformations. Provides matched input port but no isolation between outputs. Ideal for applications where isolation is not required and high power handling is needed.

Topology

                    ┌──[Z₂, λ/4]── Port 2
                    │
Input ──[Z₀, λ/4]───┤
     (Port 1)       │
                    └──[Z₃, λ/4]── Port 3

Basic structure:

  • Input λ/4 line at Z₀

  • Two output λ/4 branches at Z₂, Z₃

  • Physical T-junction connection point

For unequal split, additional matching:

                    ┌──[Z₂, λ/4]──[Z₂ₘ, λ/4]── Port 2
                    │
Input ──[Z₀, λ/4]───┤
                    │
                    └──[Z₃, λ/4]──[Z₃ₘ, λ/4]── Port 3

Design Equations

Power Split Ratio

K = P₂/P₃ = (Port 2 power / Port 3 power)

Branch Impedances (First Stage)

\[Z₂ = Z₀ × (K + 1) (to higher power port) Z₃ = Z₀ × (K + 1) / K (to lower power port)\]

Matching Sections (Unequal Split Only)

For K ≠ 1, add λ/4 matching transformers:

\[Z₂ₘ = √(2 × Z₀² × (K + 1)) Z₃ₘ = √(2 × Z₀² × (K + 1) / K)\]

Equal Split (K = 1)

Z₂ = Z₃ = 2 × Z₀

No additional matching needed

For Z₀ = 50 Ω:

Z₂ = Z₃ = 100 Ω

Critical: Ports 2 and 3 are directly connected through the junction, so there’s no isolation between them.

Advantages

  1. Simplest design: Only transmission lines, no resistors

  2. Low insertion loss: Only conductor/dielectric losses

  3. Small size: Compact layout (equal split)

  4. Low cost: No isolation resistors to specify/purchase

Limitations

  1. No isolation: S₂₃ ≈ 0 dB

    • Signals at Port 2 leak directly to Port 3

    • Impedance at one output affects the other

  2. Impedance mismatch

Comparison with Wilkinson

Feature

T-Junction

Wilkinson

Isolation

None (~0 dB)

Excellent (>20 dB)

Components

TL only

TL + Resistor

Power handling

Very high

Limited by R

Insertion loss

Very low

Low

Complexity

Simplest

Simple

Bandwidth

Narrow (10-15%)

Medium (20-40%)

Combining

Cannot combine

Can combine

Cost

Lowest

Low

References

[1] Pozar, D. M. (2012). Microwave Engineering (4th ed.), Section 7.3. Wiley.

See Also