Jim Brown wrote:
> On Tue, 05 May 2009 19:27:54 -0700, jimlux wrote:
>
>>> An important difference between coax and ladder line is that coax
>>> can be CHOKED to kill common mode current, but ladder line cannot.
>
>> Just wondering out loud here.. If I put a twisted pair (unshielded) or
>> ladder line through the center of a ferrite toroid, would it not choke
>> the common mode currents? I can see this being somewhat impractical for
>> ladder line, although with some sort of appropriate spacers to make sure
>> it's perfectly centered, and the toroid would have to be big enough that
>> it didn't intercept much of the differential mode field.
>
> Recall that common mode chokes work by adding a high resistive impedance in
> series with the common mode circuit. The primary problem is that there is a
> LOT of leakage flux from the differential field in virtually any bifilar
> winding. Not a problem on receive, but enough to cause SERIOUS heating on
> transmit. Note that saying a line has a lot of leakage flux is VERY
> different from saying that it radiates. There is leakage flux in the very
> near field of the line. In the very FAR field, the net signal is zero, but
> in the near field it is not.
>
>> (hmm, one could design a common mode choke for a balanced transmission
>> line. they're pretty common for switch mode power supply inputs, so the
>> challenge is designing one for HF.)
>
> Certainly worth the attempt, but I don't see a way to do it for QRO.
>
>
I suppose you get trapped in the flux problem. A classic common mode
choke relies on the differential mode flux from one conductor
essentially cancelling the differential mode flux from the other,
leaving just the common mode flux for the inductance (or loss) to act on.
So what you're building is basically a transformer with two coupled
windings, so the core has to carry the whole flux. Yep, that's a
problem with QRO.
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