Topband: Common mode chokes

[Pete Smith]

At 09:23 PM 7/4/2009, Jim Brown wrote:
>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.

I wasn't aware that W2DU had developed the "center-tapped" approach, 
as opposed to the basic idea of the bead-type current balun.  As 
explained on the lab notebook page, I tested with and without the 
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.

>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.

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

73, Pete

73, Pete N4ZR