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

To: towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] Cutting a FT-240 type 33...in half
From: Steve Hunt <steve@karinya.net>
Date: Thu, 12 Feb 2015 19:21:52 +0000
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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)



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