[RFI] Common mode chokes
jim at audiosystemsgroup.com
Fri Mar 19 00:04:18 PDT 2010
On Fri, 19 Mar 2010 00:31:43 -0600, Larry Benko wrote:
>I beg to differ with you. Effective common mode choking is about
>impedance and not just resistance. I know that if the impedance is
>nearly all due to inductance, external capacitances can render the choke
That's exactly the point.
>but there are plenty of common mode chokes made for
>narrow band usage that are parallel resonant.
But those chokes are much less effective, because they tend to have a much
lower impedance than a multi-turn choke on #31 in its resonant region. AND
those simple chokes can be exquisitely sensitive to feedline length. In
other words, those simple inductive chokes you describe are quite primative
in concept. The resonant ferrite choke is a far more sophisticated animal.
Indeed, the reason that a typical ferrite bead works well at VHF but not at
HF is that it is resistive at VHF, but simply a small inductor at HF.
>You can hardly design a
>ferrite choke with enough turn to be effective at the low end of the HF
>range that does not go through the parallel resonant frequency within
>the HF range.
No, but if you know what you're doing, you can use two or more chokes in
series, each tuned to a different part of the spectrum to cover a rather
wide bandwidth, just as we used to stagger-tune an IF. Thanks to a Q at
resonance of about 0.4, chokes wound on #31 material, have dominant
resistive impedance over a bit more than three harmonically related ham
bands. That means that a single choke, if its resonance is carefully
placed, can cover 160-40M, and a second in series can extend the coverage
The importance of resistance is not new (I've found references to it in 50+
year old material from EU ferrite mfrs), but it IS forgotten by nearly
everyone. So much so that I had to discover it for myself, published it in
2004 in an AES paper, and didn't find the older references until a year
See measured data and a detailed development of this in my tutorial.
>Jim Brown wrote:
>> You're missing the point of how ferrite chokes work. IT'S NOT ABOUT
>> INDUCTANCE, IT'S RESISTANCE! An effective ferrite choke is a VERY low Q
>> parallel resonant circuit tuned to the frequency you want to suppress.
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