Topband: QRe: Common mode chokes

[Jim Brown]

On Sun, 05 Jul 2009 05:17:45 -0400, Pete Smith wrote:


>I wasn't aware that W2DU had developed the "center-tapped" approach,

He did not. He originated the concept of a string of beads common mode 
choke to decouple the feedline from the antenna. 
  
>center tap and compared the results, and there was a huge improvement 
>with the ground, so I've tentatively concluded that it is the whole 
>"circuit" that does the trick.

It makes complete sense that the ground should help a lot by forming a 
voltage divider in the common mode circuit. 

>Jack, K8ZOA at <www.Cliftonlaboratories.com> has the equipment and 
>the education/experience to do the measurement properly.  He has done 
>them but does not appear to have posted them on his web page.  Since 
>I got them in a private e-mail, it would not be appropriate to go 
>into a lot of detail, but his basic conclusion was the same as mine - 
>wound chokes provide a lot more inductance, 

WRONG!  Common mode chokes wound on ferrite cores work by providing 
RESISTIVE IMPEDANCE. They are a VERY LOW Q PARALLEL RESONANT CIRCUIT. 
Magnetic coupling couples resistance from the core to provide the low 
Q, and stray capacitance combines with the inductance of the winding to 
provide the resonance. Ferrite materials like #31 and #43 typically 
provide a Q on the order of 0.4 at HF.  See my tutorial. 

>and the center tap ground greatly imp[roves the attenuation.  

Right.  

>My approach is bass-ackwards - 
>substitute chokes in a working system, changing nothing else, and 
>look at the practical outcome.  I hope to swap his choke into my 
>feedline today and compare.  

That's certainly a good approach. What is "his choke?" 

>It may be that at 160 meters there will 
>not be a huge advantage, given how little apparent room for 
>improvement there is, but performance may be much better at higher 
>and lower frequencies.  I can look at slices of spectrum from about 
>25 KHz on up to 30 MHz.

To predict (or understand) performance of a circuit like this, we must 
analyze the common mode circuit as a classic filter network, where the 
components of the common mode circuit are the common mode impedances of 
the source and load and the common mode path. Filters are fundamentally 
voltage dividers, with a series impedance that blocks current and a 
shunt impedance that takes current to ground rather than to the load. 
The choke provides the series impedance, the ground provides a 
resistive shunt. 

73,

Jim Brown K9YC