Why does stripline support pure TEM mode while microstrip does not?

In stripline, the conductor is completely surrounded by a uniform dielectric material between two ground planes. The electromagnetic field is entirely contained within one homogeneous medium. This satisfies the conditions for a transverse electromagnetic (TEM) mode: the electric and magnetic fields are purely transverse to the direction of propagation, with no longitudinal component. The phase velocity is constant at all frequencies: v = c / sqrt(Er). In microstrip, the conductor sits on top of the dielectric with air above. The field exists partly in the dielectric and partly in air, two different media with different phase velocities. This makes a pure TEM mode impossible, and the actual mode (quasi-TEM) has frequency-dependent effective permittivity, causing dispersion. Stripline's pure TEM mode means zero dispersion, which preserves signal integrity for wideband and pulsed signals.

When should I use stripline instead of microstrip?

Use stripline when you need complete shielding (no radiation from the trace), zero dispersion (wideband or pulse applications), or when routing RF signals on inner layers of a multi-layer PCB to avoid crosstalk with components on the outer layers. Stripline is preferred for corporate feed networks in phased arrays (where radiation from the feed network would corrupt the antenna pattern), for high-density RF modules where multiple signal layers must coexist without coupling, and for applications requiring tight impedance control across wide bandwidth. Use microstrip when you need to mount surface-mount components directly on the trace, when you need lower loss (microstrip has lower dielectric loss because part of the field is in air), or when you only have a two-layer PCB.

How does stripline impedance depend on geometry?

The characteristic impedance of stripline depends on the trace width (w), the dielectric thickness between the ground planes (b), the trace thickness (t), and the dielectric constant (Er). An approximate formula is: Z0 = (60 / sqrt(Er)) times ln(4b / (0.67 pi times (0.8w + t))). For a 50-ohm stripline in Rogers 4003C (Er = 3.55) with b = 1.524 mm (60 mil total, 30 mil each side) and t = 35 micrometers (1 oz copper): w approximately equals 0.37 mm (14.6 mil). This is narrower than the equivalent microstrip trace because the stripline conductor is surrounded by dielectric on all sides, increasing the capacitance per unit length. The narrower trace increases conductor loss, which is why stripline typically has 20 to 40 percent higher total loss than microstrip at the same frequency.

Transmission Lines

Stripline

A phased array antenna with 256 elements needs a corporate feed network that distributes the transmit signal to every element with controlled amplitude and phase. If this feed network is routed as microstrip on the PCB surface, the traces radiate, corrupting the antenna pattern with unwanted sidelobes from the feed itself. Move the feed network to an inner stripline layer, sandwiched between two copper ground planes: the fields are completely contained, zero radiation escapes, and the antenna pattern is determined solely by the element excitations. The feed network becomes electromagnetically invisible. This is stripline's defining advantage: a fully shielded, non-radiating transmission line built into the PCB stack-up.

Category: Transmission Lines

Mode: Pure TEM (zero dispersion)

Shielding: Complete (between ground planes)

Stripline vs. Microstrip Comparison

Property	Stripline	Microstrip
Mode	Pure TEM	Quasi-TEM
Dispersion	Zero	Frequency-dependent ε_eff
Shielding	Complete	Partial (open top)
Radiation	None	Increases with frequency
Component mounting	Not possible (inner layer)	SMD on trace
Loss (at same freq)	20 to 40% higher	Reference
50 Ω trace width (RO4003C)	0.37 mm (narrower)	0.84 mm

Stripline impedance (approximate):
Z₀ = (60/√ε_r) × ln(4b / (0.67π(0.8w + t)))
b = ground spacing, w = trace width, t = trace thickness

Propagation velocity:
v = c / √ε_r (constant at all frequencies, pure TEM)

50 Ω design example (RO4003C, ε_r = 3.55):
b = 1.524 mm, t = 35 μm: w ≈ 0.37 mm (14.6 mil)

Coupled stripline (differential):
Z_diff = 2 × Z_odd
Used for differential signaling on inner layers. Coupling controlled by trace-to-trace gap. Typical 100 Ω differential pair: two 0.15 mm traces with 0.2 mm gap in the same dielectric layer. Edge-coupled stripline maintains better impedance control than broadside-coupled because layer registration tolerances are tighter than via alignment.

Common Questions

Frequently Asked Questions

Why pure TEM in stripline?

Uniform dielectric surrounds the conductor. All fields in one medium = constant phase velocity at all frequencies = zero dispersion. Microstrip: air above, dielectric below = two media = quasi-TEM = frequency-dependent ε_eff = dispersion.

When to choose stripline?

Complete shielding needed (phased array feeds). Zero dispersion (wideband/pulse). Inner-layer routing to avoid crosstalk. Choose microstrip for: SMD component mounting, lower loss, simpler 2-layer PCB.

Impedance geometry?

Z₀ depends on w, b, t, and ε_r. Narrower trace than microstrip (higher capacitance from full dielectric surround). Narrower trace = higher conductor loss: 20 to 40% more than microstrip. Trade-off: shielding vs. loss.

Related Terms

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PCB Design

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