Dave, NU8A, and Jim Lux, thanks for filling in where I am unable. I do
detect a very slight difference connecting both conductors vs. measuring
only a single conductor, but your explanation makes it clear why. BTW:
The impedance of my lines on the toroids is between 80 and 100 ohms.
Fortunately, I have a good supply of carbon resistors to 'experiment' in
that respect. The line impedance is lower with the Teflon coated
conductor than with the DavisRF 'antenna' wire,likely due to closer
conductor spacing.
Again, thank you both for your inputs. Life should be a continuous
learning process! These NANO groups are great in that respect.
Dave - W?LEV
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On Thu, Feb 11, 2021 at 12:20 AM Jim Lux <jim@...> wrote:
On 2/10/21 9:05 AM, David Eckhardt wrote:
Thank you for the great summary and information about measure CMC CM
attenuation. I have a couple of follow up questions for you and/or the
group, if I may. I notice the process is for bifilar chokes. And, it
looks
like you only connect one wire of the bifilar turns to the VNA. You also
said you can connect both wires if you short both ends. I am still trying
to make sense of shorting both wires and why their wouldn't be a
difference
in CM impedance compared to only connecting one wire. To me, it would
seem
that connecting both ends together would create a parallel path and would
not be a true measurement of impedance (i.e. due to wires in parallel,
similar to two resistors in parallel). But, I know we are dealing RF and
ferrite toroids ... my intuition can't make that jump, yet.
* I have actually tried one vs. both wires in parallel with the VNA in
measuring CM attenuation.. There is a very minor difference. The
largest
practical effect is to reduce resistance (not so much the ¡ÀjX portion). *
* Considering each wire of the bifilar pair contributes
equal
inductance by itself, so the total connected in parallel will be half
that
of each wire alone. This ignores mutual coupling. A 3 dB difference in
30 dB of total *
the mutual L of two parallel conductors is very close to the inductance
of a single conductor. There will be essentially no reduction in the L
from paralleling.
(this is why flat strap has lower AC resistance, but very close to the
same inductance, as a round wire with the same cross section)
--
*Dave - W?LEV*
*Just Let Darwin Work*
