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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Sat 15, 2019 10:39 pm 

Joined: Feb Mon 18, 2008 8:34 pm
Posts: 87
Location: Asheville NC
Jthorusen wrote:
Greetings to Phil and the Forum:

Your diagram states they are equal.

I quote verbatim from the text block under the right hand side illustration in my drawing:

Transmission line is electrically THREE wire line; only currents in center conductor and shield inner surface are NEARLY equal and opposite and cancel. Current in outside surface of shield does not have a counter-current and is therefore equivalent to an additional piece of wire connected to the antenna.
This is why it is named (correctly) as UNBALANCED line.

(emphasis added for clarity)

Note that this applies only to the case where the load is balanced; an unbalanced load such as a coaxial dummy load has no current on the outside of the shield.

And what about the claim that standing waves cause common mode currents.

I never said that either. The term "common mode" is not applicable here. The current on the outside of the shield is unique and not in common with any other current present in the transmission line. What I did say is that standing waves change the MAGNITUDE of the already present unbalanced current on the outside of the coaxial line. When the VSWR is severe, so is the radiation from the line. When the antenna is a perfect match, as the ARRL Antenna Book suggests, the current on the outside of the line can be neglected. As my own experience has proved to my painful satisfaction, when the VSWR is high, this is most certainly NOT the case.

I have now broken my promise to the Forum to shut up twice. Please do not tempt me to break it again.

Thank You,

So in the case of a dummy load, exactly what keeps current from flowing back to the source on the outside of the coax shield?

After all, the coax is 3 wires , right?


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