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Quote (Martin): " More than one choke and different construction may be
required in order to achieve satisfactory operation over a wide frequency
range."

I certainly can confirm that statement from practice. My antenna matching
network is always a work in progress, including the input and output CM
chokes.

More data coming as I wound yet another CM choke yesterday using 3 stacked
2.5" (6.4 cm) #43 material cores with 18 bifilar turns of AWG #12 solid
copper conductor. That took some time with that large gauge conductor
(and less obvious fingerprints as a result)!

That came about after considering my output CM choke on the matching
network (L-Network). I had chosen for that position in the antenna feed
setup a choke (all home made and measured on the HP 8753C) that gave the
best CM rejection from the home appliance SMPS's (with love from China).
I'm in the country and don't even see any city lights at night, so most of
the extraneous noise, other than the usual atmospheric and celestial noise,
is due to the house. Remember, in the good 'ol USofA, FCC excludes home
appliances from any and all regulations addressing conducted and radiated
emissions (good lobby in Congress, I guess). The choke I had in place
measured 500 and 2k ohms over the frequency range of 1.8 to 14.5 MHz.
While that was pretty good, I had the stack of 2.5" 43 material in the
parts cabinet which I had never wound. So, with this discussion, it was
ripe for winding and improvement on the final CM choke in the antenna feed
system. That writeup is coming once I determine and am happy with all the
measurements I can possibly make on that new choke.

Oh, Yes, your suggestion, dahhhh........ (on my part), of passing both
conductors through a common core as a balance monitor for currents in the
two conductors of my open wire feeders will be undertaken and reported on
as well!

Hey, guys and gals, all this would not be possible without the advent of
the NANOVNA's and the moderators having set up this www site (I once worked
with Dave Daniels). THANK YOU!!!

Dave - W?LEV


On Mon, Nov 9, 2020 at 11:08 AM Martin via groups.io <martin_ehrenfried=
[email protected]> wrote:

On Sun, Nov 8, 2020 at 05:24 PM, David Eckhardt wrote:

I could measure total
current on each conductor, and ultimately 'sum' the results to obtain the
expected zero with no CM current.

Hi Dave,

Great stuff, I'm pleased that you have been motivated to make the
measurements, and I'm keen to discover the outcome.

Ideally you would be able to sum the individual amplitudes and phases of
the two conductors forming the transmission line, but it is often easier to
pass both conductors through the same RF current transformer in order to
ensure that you are seeing the true picture.

One of the big Fair-Rite clip-on ferrites is ideal for this purpose.

It has been pointed out to me privately that many of the diagrams that can
be found on-line, showing how common mode current flows and how a choke
balun works, are actually flawed, as the common mode current is shown as
only flowing on one side of the choke, which is impossible. I think this is
simply an illustrative error, made in order to try and simplify the
diagrams, but it can lead to further confusion.

A choke will impede the overall flow of common mode current (on both sides
of the choke) and its reactive component may also lead to phase changes
occurring, which will modify the field radiated by the conductor it is
placed on. So it's not a totally predictable mix, as there are lots of
different interactions going on, especially when the reactance of the choke
is likely to change with frequency, and the conductor common mode standing
wave pattern will also vary with frequency. Which is why it's a good idea
to attempt to quantify the effectiveness of chokes when placed in circuit
at the required frequency(s) of operation. More than one choke and
different construction may be required in order to achieve satisfactory
operation over a wide frequency range.

--
Regards,

Martin - G8JNJ

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