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.
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📐 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:
- Losses are lowest when the strip takes up about half the shield width.
- Mount the strip on PTFE supports — metal pillars short the field and change the impedance.
- Round the edges of the strip: at power a sharp edge starts corona.
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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.