[TowerTalk] Test Fixture for Common Mode Chokes

[Brian Beezley]

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