[TowerTalk] [Bulk] Cutting a FT-240 type 33...in half

[Steve Hunt]

Yes - that's exactly what happens with the common-mode current in a choke!

I could just as easily have wound the toroid with coax, had a 
differential-mode signal flowing in the coax, and driven common-mode 
current along the outside surface of the braid. The result would have 
been exactly the same because the DM signal has no effect on the core flux.

Imagine a conventional 1:1 flux-coupled transformer with a 10 Ohm 
resistive load connected across its secondary. We would expect to 
measure a 10 Ohm resistive impedance at the primary; and if we drive a 
current of 1A into the primary we would expect to dissipate 10W 
somewhere. But to think that the 10W is being dissipated in the primary 
wire - because we measured a resistance of 10 Ohm there - is plain 
silly; it's obviously being dissipated in the secondary load. In other 
words the dissipation does *not* take place in the "coupled" resistance.

A similar thing is happening with the CM choke - we measure a resistive 
component in the winding impedance because of the properties of the 
ferrite. If we pass current into that winding, the power is primarily 
dissipated in the physical component which generated the "coupled" 
resistance; that's the ferrite - not the wire forming the winding!

Anyone who has had a core overheat, or even shatter, will know it was 
because of high flux levels causing heat generation within the ferrite, 
*not* because of dissipation in the coax braid and subsequent heat 
transfer to the core. If it were the latter, we could simply provide a 
good thermal barrier between the coax and the core and never have to 
worry about core dissipation again!

Steve G3TXQ



On 12/02/2015 17:48, Roger (K8RI) on TT wrote:
> Sounds more like you inductively coupled the RF to the cores using 
> them as a load, rather than as an in line chole
>
> 73
>
> Roger (K8RI)
>
>