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

 To: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN "Jim Thomson" Fri, 31 Dec 2010 07:11:41 -0800 mailto:towertalk@contesting.com>
 ```Date: Wed, 29 Dec 2010 20:52:22 -0800 From: Jim Brown Subject: Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN 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. ### OK... I'll buy that. OK, now what exactly happens IF the coax is exactly 1/4 wave long.....or 3/4 wave long.....or 5/4 wave long ???? ## W8JI sez if you use a 1/4 or 3/4 wave of coax... on a yagi... and bond the braid to the top of the tower.... you will end up with a HIGH Z at the DE. [he's talking abt several thousand ohms]. If the coax is taped along the boom, then you may well have to factor in the C..between the outer braid and the boom. That stray C could easily be measured with any LCR meter. It could easily be considerable, and MAY affect the overall length of the 1/4 or 3/4 wave of coax. Your .98 VF may have to be tweaked slightly... if applied in the above case... with coax taped to boom...and braid bonded to top of tower. ## The only easy way to see if his concept works..would be to use a clamp on RF ammeter... and run it along the coax.... BELOW the top of the tower. I'd also bond the braid to the base of the tower. W8JI sez that using a balun in this case..[with a 1/4 wave of coax..with braid bonded to top of tower] will make matter's worse..and will not supplement the high Z of the 1/4 wave of coax. I find that hard to believe, I think they would supplement each other. It seems to me his concept hinged on the actual height of the tower VS the band in question. Later.... Jim VE7RF 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 ```
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