Free Coaxial-to-Microstrip Launch Tool
Model RF connector launch parasitic inductance and pad capacitance, calculate return loss ($S_{11}$), and size compensation tapers for edge-launch SMA and K-connectors.
📡 Connector Launch & PCB Specs
📊 Discontinuity Parasitics & Return Loss
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The Physics of Coaxial-to-Microstrip Connector Launches
Transitioning from a cylindrical TEM coaxial transmission line to a quasi-TEM planar microstrip trace introduces geometric discontinuities that severely degrade high-frequency performance:
- Step Discontinuity: The cylindrical center pin sits above the ground plane, introducing an unshielded series inductance ((L_{disc})).
- Soldering Pad Excess Capacitance: The wide copper footprint beneath the connector pin generates excess shunt capacitance ((C_{pad})) to the internal ground plane.
Return Loss Approximation
For small reactive perturbations in a (50Omega) system, the input reflection coefficient is:
$$Gamma(f) approx rac{j omega (L_{disc} - C_{pad} Z_0^2)}{2 Z_0 + j omega (L_{disc} + C_{pad} Z_0^2)}$$
Notice that if (L_{disc} = C_{pad} cdot Z_0^2), the first-order reactive reflection cancels out! Microwave engineers exploit this by narrowing the trace right beneath the launch pin (adding inductive series reactance) or cutting a ground relief window directly beneath the soldering pad to minimize (C_{pad}).
Frequently Asked Questions
What is ground cut-out relief under the SMA launch pad?
Removing a rectangular window in the top ground plane beneath the connector solder pad reduces excess shunt capacitance C_pad, restoring the local impedance to 50Ω and preventing capacitive dips on Time-Domain Reflectometry (TDR).
How close should ground vias be stitched to the transition?
Ground via stitching around edge-launch connectors should have a center-to-center pitch of no more than λ/10 at the maximum operating frequency to prevent substrate resonance cavity modes.