One last thing. Common-mode choke impedance is sensitive to capacitance
across the choke. (The Y21 method suppresses shunt port capacitance, not
port-to-port capacitance.) Proximity to anything conductive can affect
results. So can proximity to a dielectric, especially a lossy one. A
wooden table is not good. Placing the choke on a thick piece of
styrofoam should isolate it quite well. Styrofoam is mostly air along
with a little polystyrene, a low-loss dielectric with a relatively low
dielectric constant. The figures I use for styrofoam type 103.7 at 10
MHz are a dielectric constant of 1.03 and a loss tangent of 0.000023.
That's getting pretty close to air. I don't know of anything better
except hanging the choke in air itself. That's actually not a bad idea
if you can do it in a stable way.
Depending on the accuracy you need, you may be able to skip these
precautions. First measure with them in place. Then relax things and see
how much results change.
Brian
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