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Stripline in a rectangular shield calculator

A strip instead of a round conductor

A flat conductor in a rectangular shield has more surface than a round one of the same cross-section: lower loss at high frequencies thanks to the skin effect, and easier mounting. Lines like this sit in amplifier pi-networks and in VHF filters.

a 30 mmb 20 mmW 20 mmt 1 mmε 1W ≤ 0.8·a, t ≤ 0.3·b — the limits of the formula
a × b — inner cross-section of the shield, W × t — cross-section of the strip, ε — permittivity of the filling
Frequency (MHz)
Fill material
ε of the fill
Inner shield width a (mm)
Inner shield height b (mm)
Strip width W (mm)
Strip thickness t (mm)
Required characteristic impedance Z₀ (Ω)
Result
Characteristic impedance Z₀
Quarter-wave section (λg/4)

📐 The formulas​

Forward problem — impedance from the geometry:

Z₀ = (59.952 / √ε) × ln((1 + a/b) / (W/b + t/b))

Inverse problem — the strip width for a wanted impedance:

W = ((1 + a/b) / e^(Z₀ × √ε / 59.952) − t/b) × b

Symbols:​

  • a — inner width of the shield [mm]
  • b — inner height of the shield [mm]
  • W — strip width [mm]
  • t — strip thickness [mm]
  • ε — permittivity of the filling
Where this calculator stops:
  • W ≤ 0.8 × a — a wider strip comes too close to the side walls
  • t ≤ 0.3 × b — a thicker strip is no longer “flat” as far as this formula is concerned
  • The strip must sit centrally, parallel to the top and bottom walls
  • The calculation is for the TEM mode and ignores losses

🎯 In practice​

  • Pi-networks and filters in VHF amplifiers, where the strip is cut from sheet copper.
  • Matching sections inside screened enclosures.
  • Replacing a round conductor where the assembly has to be shallower.
Practical advice:
  1. Losses are lowest when the strip takes up about half the shield width.
  2. Mount the strip on PTFE supports — metal pillars short the field and change the impedance.
  3. Round the edges of the strip: at power a sharp edge starts corona.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Stripline in a rectangular shield calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/lines/coax_strip (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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