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
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