EMC Question of the Week: August 13, 2018
The impedance of a 5-cm ground strap or bonding jumper at 30 MHz is generally determined by its
- resistance
- inductance
- capacitance
- insulation
Answer
The correct answer is "b". While it is true that this question did not specify the strap material or the connection geometry, it would be hard to come up with a realistic situation where the inductance would not dominate.
The resistance of a 5-cm strap made of a good conductor like copper would likely be well below 1 mΩ. The resistance of the connections at each end of the strap would likely add a few mΩ. And while the inductance of a strap cannot be calculated (because inductance is a property of current loops); any connection made using a 5-cm strap would likely form a loop with an approximate circumference of around 10 cm, with an inductance greater than 10 nH. At 30 MHz, 10 nH of inductance has an impedance of almost 2 Ω. In other words, the impedance of a connection made using this strap is dominated by the connection inductance at MHz frequencies.
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