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On Thursday, December 31, 2020, 10:55:20 AM PST, David Eckhardt <davearea51a@...> wrote:

What you have constructed is not quite a common mode choke (CMC).? It is a
current choke or current "balun".? Technically, it isn't even a true
current "balun".? It works by dening current flow on the outside of the
coaxial braid.? A true balun would accomplish this function in addition to
assuring the currents on the two conductors - the inside of the coax braid
and the outside of the inner conductor - are of equal amplitude and
opposite phase.? But the second requirement is pretty much assured using
coax cable as what occurs on the inside of the coax is not (greatly -
depending on the integrity of the braid) influenced by what goes on outside
the cable.

I, as well, have constructed and measured several (maybe as many as 15 or
so) true CMCs on 31, 43, and 75 material.? These consist of, for the most
part, 10 to 18 turns of bifilar wound heavy stranded and insulated copper
conductor (#14 and the last on 43 material, #10).? The last one on 43
material uses two stacked 3" OD cores.? The others are wound on two stacked
2.3" OD cores.? The single 31 material is wound on 5 stacked cores of 2.3"
OD.? Since I can run the full legal limit to my parallel conductor feeders
(no coax), I don't want to sense any or absolutely minimal heating in
either the cores or the conductors.? I'll attach the results of my
measurements.? I cheated and used the HP 8753C for the measurements as it
has most of the required conversions built in.

Note the 31 material shines on 160 and good on 75.? However, the last one,
(BRN) which is wound of #10 stranded (11-turns, bifilar) and insulated wire
on two stacked 3" OD cores of 43 material is pretty much the winner for
general use on 75 through 10-meters.

With a house filled with new appliances the chokes not only transform CM to
DM for my parallel conductor feeders, but keeps the SMPS noises from the
appliances (with love, fromChina) out of the feedline and antenna.

Dave -W?LEV

On Thu, Dec 31, 2020 at 3:19 PM Torbj?rn Toreson <torbjorn.toreson@...>
wrote:

Hello,

I have made (several) CMC-filters by wrapping RG58 on a FT240 mix 31
toroid, the result is very good. I have a special instrument to check the
current on the coax outside shield and that goes to zero when I connect the
choke on the transmitter side.

When I started to read K9YC "A hams guide to RFI, Ferrites, Baluns .. etc"
I wanted to measure the impedance, both resistance and reactance and not
only the attenuation as "LOGMAG". I have used to measure LOGMAG via CH0 and
CH1 and I get very probable results such as -35 dB at 5 MHz and -30 dB at
20 MHz. The connections is coax-shield on one side to CH0 and coax-shield
on the other side to CH1 (to center conductors).

I thought that I would see the impedance of the filter by using another
trace (CH1) and asign it the format Resistance to start with. The result
however is around 50 ohms when I expected some kohms. Measuring a 560 ohm
resistor also gives around 50 ohm. SOLT calibration for 2-30 MHz is
performed. What is wrong with my thinking?

If I measure the choke (of course always only the shield) only by CH0 I
get a credible result of e.g. 1,3 kohm at 5 MHz, but the resistance is
getting lower by increasing frequency, so this is probably not the right
way to measure the filters resistance.

Why can I not measure the impedance correct when the CMC-filter is
connected between CH0 and CH1?

I have tried with both a NanoVNA-H4 and a SAA2-N with similar results.

73/Torbjorn/SM6AYM

--
*Dave - W?LEV*
*Just Let Darwin Work*