Interesting article on Common Mode currents ( The Nature of Common Mode current )

[VK2RK]

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

Measuring common mode current

owenduffy.net

 

[BillC]

Can you use a variable inductance in the station earth lead to move the standing voltage away so as to place a node or null point in the vicinity of the transmitter and mic. lead etc ? I think that could be a simple trick to resonate the offending field to a different position on the line.

 

[VK3YNV]

Can you use a variable inductance in the station earth lead to move the standing voltage away so as to place a node or null point in the vicinity of the transmitter and mic. lead etc ? I think that could be a simple trick to resonate the offending field to a different position on the line.

Hi Bill, You might achieve the same effect by changing the coax feedline length.

But in any event, one of the functions of the balun is to minimize the common mode current in the coax shield, so you don't have the problem of the feedline radiating, as well as, in this case to act as an rf transformer and do a 4:1 impedance transformation.

Its an interesting topic. Baluns are deceptively simple looking devices that hide a lot of complexity.

 

[BillC]

Thanks Ray, I have had the idea for years that a lots of co-ax fed dipole antennas could easily be working as a top loaded vertical ,hard to tell whats going on aye ".