Topband: Common mode chokes

[Jim Brown]

On Sat, 04 Jul 2009 16:33:27 -0400, Pete Smith wrote:

>These are fair points, Jim.  One of the purposes if this series of 
>tests is to see if Clifton Labs' super-duper center-tapped choke, 
>which combines 2 different wound cores on each side of the center 
>ground, will give significantly better performance for 160-10 
>meters.  That's coming up.

Good move. I haven't seen the Clifton thing, but it sounds like a 
good idea, for the reason noted in the previous post. 

>The reason I used beads (aside from having ON4UN's design to go by) 
>was that I had a piece of RG-59 and 100 #73 beads, and I figured it 
>would be a lot easier to use the beads and wrap them in tape than to 
>figure out how to weatherproof two cores and a bunch of windings.

Using what you have in the back room can be a useful starting point.  
ON4UN's design is simply echoing W2DU's 30-year old work, which was a 
landmark when he did it. Wound chokes are simply a better way (number 
of turns squared rather than X number of beads) of getting higher 
choking impedance in the frequency range of interest. 

Assessing the value of common mode chokes of various sorts in a 
circuit can yield misleading results if you only evaluate them in a 
circuit, because their performance will depend on all the impedances 
in the circuit to which you're adding them. A more meaningful test is 
to first measure the impedance vs frequency of the choke, THEN see 
how it performs in a given circuit. 

As I've noted, those measurements are not easy to do, and most folks 
who attempt them with inappropriate test methods (i.e., an antenna 
analyzer or network analyzer) get very wrong answers. 

73,

Jim K9YC