Grounding Strap Inductance Explained — Impedance vs. Resistance in RF Bonding

A "good ground" at DC can be a terrible one at RF

Measure a grounding strap or bonding jumper with a multimeter and it will read a fraction of an ohm — by DC resistance alone, it looks like an excellent connection. But EMC problems rarely live at DC. At any real frequency, what matters is impedance, not resistance, and a straight conductor's impedance is dominated by its self-inductance: Z = 2π·f·L. Because that grows directly with frequency, a strap that's essentially a short circuit at 60Hz can present tens of ohms at 100MHz — enough to completely defeat the purpose of the bond.

Where a straight conductor's inductance comes from

Even a straight length of wire or strap, with no coil or loop in sight, has self-inductance — the classical Grover/Terman result for a straight conductor's external inductance depends on its length and its cross-sectional geometry: a flat strap follows L(nH) = 0.2·l·[ln(2l/(w+t)) + 0.2235·(w+t)/l + 0.5] and a round wire follows L(nH) = 0.2·l·[ln(4l/d) − 1] (all dimensions in mm). Both share the same dominant behavior — inductance scales roughly with length and only logarithmically with cross-section — which is the key insight for grounding practice: length matters far more than how thick the conductor is.

Why straps beat wires

For the same length, a wide flat strap has meaningfully lower inductance than a round wire — because the logarithmic term depends on the conductor's width-plus-thickness rather than just its diameter, a strap with the same cross-sectional area as a wire, but spread wide and thin instead of round, presents noticeably less impedance. This is the physical basis for one of the most repeated pieces of EMC grounding advice: replace a wire "pigtail" with a flat strap wherever possible, and keep it as short as the installation allows.

Why length dominates, and a bonding-strap design checklist

How much strap width actually helps (and where it stops helping), and a practical checklist for specifying an RF bonding strap.

Width helps, but with diminishing returns

Because the strap's width and thickness only appear inside a logarithm, doubling a strap's width doesn't halve its inductance — it reduces it by a much smaller amount. Going from a 5mm-wide strap to a 20mm-wide strap (4×) typically only cuts inductance by roughly a third, not by a factor of four. That doesn't mean width is pointless — every bit of impedance reduction matters at RF — but it does mean chasing an ever-wider strap has rapidly diminishing returns, and length reduction is almost always the more effective lever if the installation allows it.

What this calculator doesn't include

This is the strap's own self-inductance only. A real bonding connection's total impedance also includes the loop it forms with whatever return path the current actually takes (which can dominate over the strap's own inductance if the loop area is large), contact/joint resistance at each end, and proximity effects from nearby conductors. Treat this number as one input to a bonding decision, not the complete picture of a real installation's RF performance.

Checklist

Keep the strap as short as the physical installation allows — length has the strongest effect on inductance.
Use a flat strap, not a round-wire pigtail, wherever the installation permits it.
Check impedance (not just inductance) at your actual frequency of concern — the same strap that's fine at 1MHz may not be at 100MHz.
Remember this covers the strap's own self-inductance only — a large return-current loop area can dominate the total bonding impedance regardless of strap geometry.
For genuinely low-impedance RF bonding, consider multiple parallel straps or a solid conductive panel/gasket rather than relying on strap width alone.

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Grounding Strap Inductance: Why Impedance, Not Resistance, Rules RF Bonding