Why does stripline give higher directivity than microstrip?

Directivity in a coupled-line coupler depends on the equality of even and odd mode phase velocities. In microstrip, the inhomogeneous dielectric (part substrate, part air) causes the even mode to see higher effective permittivity than the odd mode, creating a velocity difference that degrades directivity to typically 15 to 20 dB. In stripline, the homogeneous dielectric surrounds both conductors symmetrically, so even and odd modes experience the same permittivity and propagate at the same velocity. This gives theoretical infinite directivity at center frequency, with practical directivity of 30 to 40 dB limited by fabrication tolerances and connector transitions.

How is the coupling coefficient controlled?

In broadside-coupled stripline, coupling is controlled by three parameters: the dielectric thickness between the two strip conductors (thinner equals tighter coupling), the strip width (wider equals tighter coupling), and the offset between strips (less overlap equals looser coupling). For a 3 dB coupler in a homogeneous dielectric with epsilon_r of 3.5, typical dimensions might be 0.2 mm strip separation, 0.5 mm strip width, and full overlap. The ground planes are typically 1.5 to 2.0 mm apart total, with the strips centered in the stack. EM simulation is essential for final design because fringing fields and finite strip thickness affect the impedances.

What are the fabrication challenges?

The main challenge is achieving precise registration (alignment) between the two strip conductor layers. Misregistration by even 25 micrometers changes the coupling and degrades amplitude balance. Standard multilayer PCB registration is plus or minus 50 to 75 micrometers, which may limit performance for tight couplers. High-precision lamination processes or LTCC achieve plus or minus 10 to 25 micrometers. Another challenge is controlling the dielectric thickness between strips, which directly sets the coupling: a 10 percent thickness variation causes approximately 0.5 dB coupling change. Thin prepreg layers (50 to 100 micrometers) needed for tight coupling can be difficult to process uniformly in standard PCB fabrication.

Transmission Lines

Broadside-Coupled Stripline

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A coupled transmission line configuration in which two strip conductors are stacked vertically between two ground planes in a homogeneous dielectric environment. Because stripline's fully enclosed dielectric ensures equal even and odd mode phase velocities, broadside-coupled stripline inherently achieves high directivity (> 30 dB practical) and tight 3 dB coupling for directional couplers, baluns, and broadband power dividers without the mode velocity compensation needed in microstrip.

Category: Transmission Lines

Directivity: > 30 dB (practical)

Coupling: 3 to 20 dB

Understanding Broadside-Coupled Stripline

Stripline consists of a flat conductor centered between two parallel ground planes, completely surrounded by a homogeneous dielectric. This symmetry means that both even mode (where both strips carry currents in the same direction) and odd mode (opposite direction currents) propagate through the same effective dielectric, experiencing identical phase velocities. This is the critical advantage over microstrip, where the air-dielectric boundary causes unequal mode velocities that degrade directivity.

In the broadside configuration, the two strips are on adjacent internal layers of a multilayer PCB, aligned vertically. Coupling is controlled by the dielectric thickness between strips (thinner = tighter coupling), strip width (wider = tighter), and overlap amount. For a 3 dB coupler at 3 GHz in Rogers RO4003C (ε_r = 3.38), typical dimensions are 0.2 mm strip separation, 0.5 mm width, 1.8 mm total ground spacing. The resulting coupler achieves 3 ± 0.3 dB coupling and > 30 dB directivity over octave bandwidth.

Design Parameters

Coupling Coefficient:
k = (Z_0e − Z_0o) / (Z_0e + Z_0o)
For 3 dB: k = 0.707, Z_0e ≈ 120 Ω, Z_0o ≈ 21 Ω (50 Ω system)

Directivity (equal velocities):
D = ∞ at center frequency (theoretical)
D > 30 dB practical (limited by fabrication tolerance)

Registration Sensitivity:
ΔC ≈ ± 0.5 dB per 50 µm misregistration (typical)

Stripline vs Microstrip Coupling

Parameter	Broadside Stripline	Broadside Microstrip	Edge Stripline	Edge Microstrip
Max Coupling	3 dB	3 dB	6 dB	8 dB
Mode Velocity	Equal	Unequal	Equal	Unequal
Directivity	> 30 dB	10 to 20 dB	> 25 dB	15 to 20 dB
Fabrication	Internal layers	Multilayer (exposed)	Internal layers	Single layer
Probing Access	Difficult (buried)	Top access	Difficult	Easy

Common Questions

Frequently Asked Questions

Why higher directivity than microstrip?

Homogeneous dielectric gives equal even/odd mode velocities. In microstrip, the air-substrate boundary creates velocity mismatch degrading directivity to 15 to 20 dB. Stripline achieves theoretical infinite directivity at center frequency, with practical limits of 30 to 40 dB from fabrication tolerances and connector transitions.

How is coupling controlled?

Three parameters: dielectric thickness between strips (thinner = tighter), strip width (wider = tighter), and overlap amount. For 3 dB in ε_r = 3.5: ~0.2 mm separation, 0.5 mm width, full overlap. EM simulation is essential; fringing fields and finite strip thickness affect impedances.

What are fabrication challenges?

Layer registration: ±50 to 75 µm standard PCB causes ~0.5 dB coupling change per 50 µm. LTCC achieves ±10 to 25 µm. Dielectric thickness control: 10% variation causes ~0.5 dB coupling change. Thin prepreg (50 to 100 µm) for tight coupling can be difficult to laminate uniformly.

Related Terms

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