Transmission Lines, Cables, and Interconnects Microstrip and Stripline Informational

What is the difference between microstrip, stripline, coplanar waveguide, and grounded coplanar waveguide?

These four planar transmission line types are the fundamental building blocks of RF PCB and MMIC design, each with different characteristics suited to different applications: (1) Microstrip: a conductor trace on top of a dielectric substrate with a ground plane on the bottom. The electromagnetic field exists partly in the substrate and partly in the air above, making it a quasi-TEM mode. Advantages: easy fabrication (single-sided), easy to mount components, and accessible for probing and tuning. Disadvantages: radiation at discontinuities (bends, junctions), the mixed-dielectric medium causes frequency-dependent effective permittivity, and coupling to adjacent traces can cause crosstalk. Typical applications: general RF circuits, matching networks, and microwave circuits below 30 GHz. (2) Stripline: a conductor trace embedded between two ground planes in a homogeneous dielectric. Supports a pure TEM mode (no dispersion). Advantages: no radiation (completely shielded by the ground planes), pure TEM mode provides better broadband performance, and excellent isolation between circuits on different layers. Disadvantages: inner-layer fabrication is more complex, components cannot be mounted on the signal layer (requires via transitions), and thermal management is more difficult (the heat must pass through the dielectric to reach the ground planes). Typical applications: high-isolation feed networks, broadband couplers, and multilayer PCB interconnects. (3) Coplanar waveguide (CPW): a center conductor with ground planes on both sides, all on the same surface of the substrate. The electromagnetic field is concentrated in the gaps between the center conductor and the ground planes. Advantages: no via holes needed for grounding (ground is on the same surface), easy integration with flip-chip and die attach (bond wires connect directly to the ground), lower dispersion than microstrip at mmWave frequencies, and convenient for on-wafer probing. Disadvantages: the ground plane width must be finite (typically 3-5 times the gap width for good performance), and the two ground planes must be connected (with air bridges or via stitching) to suppress the parasitic slotline mode. Typical applications: MMIC circuits, mmWave designs above 40 GHz, and on-wafer measurements. (4) Grounded CPW (GCPW): a CPW with an additional ground plane on the bottom of the substrate, combining the CPW and microstrip characteristics. Advantages: combines the easy grounding of CPW with the mechanical support of a bottom ground plane, suppresses substrate modes (the bottom ground confines the fields), and provides better heat sinking than standard CPW (the bottom ground acts as a heat spreader). Disadvantages: more complex mode structure (coupled CPW and microstrip modes), and requires via stitching between the top and bottom ground planes. Typical applications: high-frequency PCB designs (5G, radar), mmWave transitions, and high-density packaging.

Category: Transmission Lines, Cables, and Interconnects

Updated: April 2026

Product Tie-In: PCB Substrates, Connectors, Cable Assemblies

Planar Transmission Line Comparison

Selecting the right transmission line type is one of the first and most impactful decisions in any RF PCB or MMIC design.

Impedance Equations

(1) Microstrip: Z0 depends on the trace width W, substrate height h, and dielectric constant epsilon_r. Approximate formula (for W/h > 1): Z0 = (120*pi) / (sqrt(epsilon_eff) * (W/h + 1.393 + 0.667*ln(W/h + 1.444))). Where epsilon_eff = (epsilon_r + 1)/2 + (epsilon_r - 1)/2 * (1 + 12*h/W)^(-0.5). For 50 ohms on Rogers RO4003C (epsilon_r = 3.55, h = 0.2 mm): W ≈ 0.45 mm. (2) Stripline: Z0 = (60/sqrt(epsilon_r)) * ln(4*b / (pi*d*0.67*(0.8 + t/d))). Where b = ground plane spacing, d = trace width, and t = trace thickness. For 50 ohms on FR-4 (epsilon_r = 4.2, b = 0.5 mm): d ≈ 0.2 mm. (3) CPW: Z0 depends on the center conductor width S, the gap G, and epsilon_r. Z0 = 30*pi / (sqrt(epsilon_eff) * K(k)/K'(k)). Where k = S/(S + 2G), K is the complete elliptic integral, and epsilon_eff ≈ (epsilon_r + 1)/2 for thick substrates. For 50 ohms on alumina (epsilon_r = 9.9): S = 60 um, G = 40 um is typical. (4) GCPW: the impedance is a function of both the CPW gap dimensions and the substrate height. The bottom ground plane lowers the impedance compared to standard CPW. Design tools (TX-Line, ADS LineCalc) are essential for accurate GCPW impedance calculation.

Selection Guide

(1) Frequency < 6 GHz: microstrip is the default choice. Simple, well-understood, low cost. (2) Frequency 6-40 GHz: microstrip or GCPW. GCPW provides better mode control and easier grounding at higher frequencies. (3) Frequency > 40 GHz: CPW or GCPW is preferred. Microstrip suffers from increased radiation and dispersion at mmWave. (4) High isolation required: stripline. The shielded structure provides 20+ dB more isolation than microstrip between adjacent circuits. (5) MMIC/on-wafer: CPW for probing compatibility. Most MMIC foundry PDKs include CPW and microstrip models.

Transmission Line Formulas

Microstrip: Z₀ = f(W, h, εᵣ), quasi-TEM
Stripline: Z₀ = f(d, b, εᵣ), pure TEM
CPW: Z₀ = f(S, G, εᵣ), quasi-TEM
GCPW: CPW + bottom ground plane
50Ω RO4003C microstrip: W≈0.45mm on 0.2mm

Common Questions

Frequently Asked Questions

Which has the lowest loss?

At low frequencies (< 10 GHz): stripline has the lowest loss (homogeneous dielectric, no radiation). At high frequencies (> 30 GHz): CPW can have lower loss than microstrip because the current return path is closer (narrower gap means lower conductor loss from the ground return current). The loss ranking depends strongly on the substrate material: on low-loss substrates (Rogers, alumina): all types have similar conductor-dominated loss. On high-loss substrates (FR-4): the dielectric loss dominates and is similar for all types.

Can I mix transmission line types on one PCB?

Yes, and this is very common. A typical RF PCB might use: microstrip on the top layer (for component mounting and matching networks), stripline on inner layers (for routing signals between sections with high isolation), and GCPW at connectors (for controlled impedance at the board edge). Transitions between types require careful design: microstrip-to-stripline via transitions, microstrip-to-GCPW tapers, and each transition adds a small reflection (S11 > -25 dB if well designed).

What is the substrate mode problem?

Substrate modes are electromagnetic modes that propagate through the dielectric substrate, bypassing the intended transmission line. This causes: unexpected coupling between distant circuits (long-range crosstalk), resonances in the substrate that create gain/loss ripple, and radiation leakage. Substrate modes are more problematic for: thick substrates (h > lambda/10), high dielectric constant materials (epsilon_r > 6), and higher frequencies (where lambda is smaller). Mitigation: use thin substrates, add ground vias around transmission lines (creating a cage that blocks substrate mode propagation), or use GCPW (the bottom ground plane suppresses substrate modes).

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