EMC calculators
Interactive tool
Six calculations that come up constantly in EMC work, each using the formula derived in the matching guide rather than a fitted approximation. Every figure updates as you type. Nothing is sent anywhere: the arithmetic runs in your browser.
Decibel converter
Enter one value; the others follow. The dBm column needs an impedance, because a field has none.
Field strength from a receiver reading
E = V + AF + cable loss - preamplifier gain, all in decibels.
Field strength: -
Distance scaling
Far-field inverse-distance law: 20 log10(r1/r2). For comparing limit sets, not for predicting a site.
Scaled level: -
Skin depth
Depth at which current density falls to 1/e. One ounce copper is 34.8 um.
Skin depth: -
Aperture leakage
SE = 20 log10(lambda / 2L) for a slot shorter than half a wavelength. Leakage follows the longest dimension, not the area.
Shielding effectiveness: -
Conductor inductance and impedance
L = (mu0 l / 2pi)[ln(2l/r) - 0.75], then Z = 2 pi f L. This is what a pigtail costs.
L: -
What each one is for, and where it comes from
Section titled “What each one is for, and where it comes from”Decibel converter. dB(uV), volts and dBm, with the impedance exposed because the dBm column is meaningless without it. The offset everyone remembers as 107 is computed from the impedance you enter rather than assumed, so changing it away from 50 ohms gives the right answer instead of a wrong one. Derivation in decibels for EMC.
Field strength from a receiver reading. The measurement chain as a sum: reading plus antenna factor plus cable loss minus preamplifier gain. Losses add and gains subtract because they were divisions and multiplications before the logarithm. A sign error here offsets an entire scan by a constant.
Distance scaling. The far-field inverse-distance law, 20 log10(r1/r2), which is how a 3 m limit and a 10 m limit are put on the same axis. Use it to compare limit sets, not to predict what a site will measure: ground reflection and near-field behaviour do not follow it.
Skin depth. How deep current runs in a conductor, and how that compares with one ounce copper at 34.8 um. Above about 3.6 MHz the skin depth is thinner than the copper, so width still helps return impedance and thickness no longer does. Context in return current paths.
Aperture leakage. The number that decides real shielding performance, 20 log10(lambda / 2L). Try a 10 cm seam at 1 GHz and compare it with the several hundred decibels the metal itself offers. Context in shielding theory.
Conductor inductance and impedance. What a length of wire actually presents, from L = (mu0 l / 2pi)[ln(2l/r) - 0.75], with the impedance at your chosen frequency and the quarter-wave resonance. This is the calculation that prices a pigtail. Context in grounding for EMC.
Two cautions
Section titled “Two cautions”These are design and triage tools, not compliance evidence. A formal result comes from an accredited laboratory following the method in the applicable standard, with a calibrated chain and a stated uncertainty. Nothing here carries an uncertainty budget.
The formulas are idealisations. The distance law assumes far field and free space, the aperture expression assumes a slot shorter than half a wavelength and ignores depth, and the inductance formula assumes a straight round conductor much longer than its radius. Where an input leaves the range in which a formula holds, the tool says so rather than returning a confident number.