What are the main sources of mismatch in a coax-to-microstrip transition?

Four primary discontinuities create reflections. First, the field distribution change from radial (coax) to fringing (microstrip) creates a reactive discontinuity, typically capacitive at the launch point where the center pin is exposed above the ground plane. Second, the coax center pin length between the connector body and the microstrip solder joint acts as a short transmission line section, often with impedance different from 50 ohms due to the changed ground plane distance. Third, the ground path discontinuity where the coax outer conductor transitions to the PCB ground plane through solder joints or press-fit contacts introduces inductance (typically 0.1 to 0.5 nH). Fourth, the microstrip trace width may not match the connector center pin diameter, creating a step impedance discontinuity. Mitigation techniques include minimizing the center pin stub length (less than 0.5 mm for Ka-band), using anti-pad (ground plane clearance) optimization around the pin, adding matching via stubs, and using tapered transition sections. Full-wave simulation is essential above 10 GHz.

How does end-launch compare to vertical launch?

End-launch connectors mount at the PCB edge with the coax axis parallel to the board surface. The center pin contacts the microstrip trace directly, and ground lugs solder to top-side ground pads connected to the ground plane through via rows. This design minimizes the center pin stub length and provides excellent ground continuity, achieving the best electrical performance (return loss below -20 dB to 40-67 GHz depending on connector type). The limitation is that it requires board-edge access and occupies significant edge real estate. Vertical launches pass through the PCB, with the center pin going through a plated via hole to the microstrip layer. This allows connector placement anywhere on the board (not just edges) and is standard for panel-mount applications. However, the through-via creates a long stub that resonates at a frequency where the stub length equals lambda/4, typically 15 to 25 GHz for standard PCB thicknesses (1.5 mm). Back-drilling the via stub reduces this resonance to acceptable levels. Vertical launches typically achieve return loss below -15 dB to 20-30 GHz, inferior to end-launch.

How do you design a coax-to-microstrip transition for mmWave?

Above 30 GHz, every geometric detail matters. Start with the connector selection: 1.85 mm connectors support DC to 67 GHz, 1.0 mm connectors support DC to 110 GHz. Use precision end-launch connectors with controlled center pin protrusion (0.1 to 0.3 mm tolerance). Choose a low-loss substrate (Rogers RO3003, Isola Astra, or liquid crystal polymer) with tightly controlled thickness and dielectric constant. The microstrip width and ground clearance around the connector pad must be optimized using 3D EM simulation (HFSS or CST). Ground vias must be placed close to the connector ground tabs with spacing less than lambda/8 at the maximum frequency (less than 0.5 mm at 67 GHz). The signal pad should include a chamfer or taper transitioning from the connector pin diameter to the microstrip width. An anti-pad in the ground plane beneath the signal pad reduces the parasitic capacitance at the launch point. For frequencies above 50 GHz, mode suppression vias in the ground plane around the transition suppress the parallel-plate mode that can cause resonances and radiation.

Transmission Lines

Coax-to-Microstrip Transition

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A coax-to-microstrip transition connects coaxial cables to PCB microstrip lines. Center conductor solders to the trace; outer conductor connects to the ground plane. End-launch connectors (Southwest, Amphenol SV) achieve return loss < -20 dB to 40 GHz (2.92 mm) or 67 GHz (1.85 mm). Vertical through-board launches allow mid-board placement but have stub resonances at 15 to 25 GHz requiring back-drilling. The most common RF interconnect in PCB electronics.

Category: Transmission Lines

Return loss: < -20 dB

Bandwidth: DC to 67+ GHz

Understanding Coax-to-Microstrip Transitions

Virtually every RF PCB requires at least one coax-to-microstrip transition: the interface where the external coaxial cable connects to the board's microstrip circuitry. This seemingly simple connection is one of the most performance-critical elements in an RF system. A poorly designed launch can limit the entire system's bandwidth, create resonances that cause oscillation in amplifier circuits, and introduce insertion loss that degrades noise figure. Yet a well-designed launch is nearly transparent, introducing less than 0.1 dB loss and -25 dB return loss across the full operating band.

The electromagnetic challenge is the mode conversion from the rotationally symmetric TEM mode of coax to the inhomogeneous quasi-TEM mode of microstrip. In coax, the electric field is purely radial and confined between inner and outer conductors; in microstrip, the field partially exists in the substrate dielectric and partially in the air above the trace. The transition region where one field pattern transforms into the other necessarily creates parasitic reactances. The art of transition design is managing these reactances through geometry optimization: minimizing the center pin stub length (the exposed pin section above the ground plane), optimizing the anti-pad clearance in the ground plane, placing ground vias close to the connector ground tabs, and adding compensation structures (matching stubs or stepped impedance sections) when needed.

Transition Design Parameters

Microstrip Impedance:
Z₀ ≈ (87/√(ε_r+1.41)) × ln(5.98h/(0.8W+t)) (Ω)

Via Stub Resonance:
f_res = c/(4L_stub√ε_r)

Ground Via Spacing:
p < λ_eff/8 = c/(8f_max√ε_eff)

Where h = substrate height, W = trace width, t = copper thickness, L_stub = via stub length below the signal layer. For 1.5 mm FR-4 stub (ε_r=4.2): f_res = 24.4 GHz. Back-drilling to 0.3 mm stub: f_res = 122 GHz.

Coax-to-Microstrip Launch Comparison

Launch Type	Connector	Usable BW	Return Loss	Best For
Edge-mount SMA	SMA (3.5 mm)	DC to 18 GHz	< -18 dB	General PCB test
End-launch 2.92 mm	2.92 mm (K)	DC to 40 GHz	< -20 dB	Ka-band modules
End-launch 1.85 mm	1.85 mm (V)	DC to 67 GHz	< -18 dB	V-band/E-band
Vertical through-board	SMA / N-type	DC to 18 GHz	< -15 dB	Panel mount, mid-board
Vertical (back-drilled)	2.92 mm	DC to 30 GHz	< -18 dB	Backplane, mid-board

Common Questions

Frequently Asked Questions

What causes mismatch in the transition?

Four sources: (1) field distribution change (radial to fringing, typically capacitive), (2) center pin stub impedance (different Z₀ in exposed region), (3) ground path inductance (0.1 to 0.5 nH from solder joints), (4) pin-to-trace width step discontinuity. Fix: minimize pin stub (<0.5 mm for Ka-band), optimize anti-pad, tight ground vias, tapered transition. EM simulation essential above 10 GHz.

End-launch vs vertical launch?

End-launch: board edge, pin parallel to surface, minimal stub, best performance (< -20 dB to 40-67 GHz), but needs edge access. Vertical: through-board via, anywhere on PCB, but via stub resonates at λ/4 (15 to 25 GHz for 1.5 mm boards). Back-drilling reduces stub to extend bandwidth. Vertical typically -15 dB to 20-30 GHz.

How to design for mmWave?

Use 1.85 mm (67 GHz) or 1.0 mm (110 GHz) connectors. Low-loss substrate (Rogers RO3003, LCP). Ground vias < λ/8 spacing (<0.5 mm at 67 GHz). Signal pad chamfer/taper from pin to trace width. Anti-pad in ground to reduce launch capacitance. Mode suppression vias around transition above 50 GHz. Full 3D EM simulation mandatory.

Related Terms

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