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## Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

 To: towertalk@contesting.com Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN Steve Hunt Fri, 31 Dec 2010 20:05:57 +0000 mailto:towertalk@contesting.com>
 I have some modelled examples on my web site which illustrate the effect Jim is talking about. Take a look at the section "Why reactive chokes are undesirable" on this page: http://www.karinya.net/g3txq/chokes/ The section finishes: "We conclude that a high value Resistive choke is the safe option for all scenarios. We also conclude that Rules-of-Thumb which equate the required choke impedance to some multiple of the differential-mode load impedance are unsound." 73, Steve G3TXQ On 30/12/2010 04:52, Jim Brown wrote: > On 12/29/2010 7:35 PM, NPAlex@aol.com wrote: >> I think a more accurate description of several coiled turns of coaxial >> cable would be a RF Choke, serving to limit common mode currents on the >> outside >> of the coax cable. > That's an Alice In Wonderland view of things. Yes, the coil of coax is > an inductance in the common mode circuit, but the common mode circuit > also includes the series impedance of the transmission line. If that > line is less than 1/4 wave, it is a CAPACITIVE REACTANCE, so the > inductive reactance CANCELS all or part of it and the common mode > current INCREASES rather than decreases. The same thing happens if the > line is between 1/2 and 3/4 wavelengths. Further, the electrical length > is based on a Vf of 0.98, NOT the Vf of the coax (because the common > mode current flows on the outside of the shield). The 0.98 factor > accounts for the slight shortening effect of the outer jacket. > > The only effective common mode choke is a very lossy parallel resonant > circuit, which is what you get if you wind multiple turns of coax around > a lossy ferrite core (like #31 or #43). All you have to do is use the > number of turns that puts the very broad resonance where it covers the > bands you work. > > Study http://audiosystemsgroup.com/RFI-Ham.pdf > > 73, Jim K9YC > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk
 Current Thread [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN, NPAlex Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN, Jim Brown Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN, Steve Hunt <= Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN, Bob Alexander