Shield effectiveness calculator — transfer impedance

The calculator grades the result from the formula SE(dB) ≈ 20 × log10(100 / (Zt × L)) with Zt in mΩ/m and L in m. Below 20 dB is weak, 20–40 dB moderate, and above 40 dB good — a 1 mΩ/m shield over 1 m reads about 40 dB. Real braid coverage and aperture leakage often pull a high-frequency measurement several dB below the estimate.

Transfer impedance Zt is the voltage a 1 A interference current induces on the inner conductor across 1 m of shield — the figure of merit for a shield. Lower Zt means less noise couples in. Typical values at 1 MHz used here are about 10 mΩ/m for foil, 1.5 for 80% braid, 0.5 for 95% braid and 0.1 for solid shield.

Longer runs pick up more noise. The shield voltage drop for a 1 A noise current is V = Zt × L, so the coupled-noise reduction falls as length grows — doubling the length halves the shield effectiveness margin in the SE formula. Grounding strategy (one end for low frequency, both ends for RF) changes how well that shielding is realised.