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The nature of common mode current
Common mode current is a component of current flowing on an RF transmission line. It is a component of each line current in the case of two wire open line, and in the case of a coaxial line, it flows entirely and exclusively on the outer surface of the outer conductor.
In both cases, common mode current forms a standing wave on the transmission line, it varies in amplitude and phase along the line.
Because it forms a standing wave, measurement at a single point does not guarantee that a significant current will be observed even though there is significant common mode current, you may just be measuring at or near a current minimum.
Is a current minimum at the shack as good as no current? No, not at all because associated with a current minimum is a voltage maximum, very high electric field intensity exists in the region of a voltage maximum (or current minimum). This is what gives rise to RF burns to the lips from microphone grilles!
Read the full article
Common mode current is a component of current flowing on an RF transmission line. It is a component of each line current in the case of two wire open line, and in the case of a coaxial line, it flows entirely and exclusively on the outer surface of the outer conductor.
In both cases, common mode current forms a standing wave on the transmission line, it varies in amplitude and phase along the line.
Because it forms a standing wave, measurement at a single point does not guarantee that a significant current will be observed even though there is significant common mode current, you may just be measuring at or near a current minimum.
Is a current minimum at the shack as good as no current? No, not at all because associated with a current minimum is a voltage maximum, very high electric field intensity exists in the region of a voltage maximum (or current minimum). This is what gives rise to RF burns to the lips from microphone grilles!
Read the full article
Measuring common mode current
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