After a fair amount of time using the 2643251002-based common mode choke (CMC) (current balun?), I got curious about how much attenuation it presents to common mode currents. I cobbled together a test jig, and connected it to my nanoVNA.
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| The 2643251002-based common mode choke (CMC), with 3D printed spider, hot melt adhesive, and self-amalgamating tape. |
Steve G3TXQ has published work on various choke constructions.
- 2012 work can be found here: http://www.karinya.net/g3txq/chokes
- 2015 work can be found here: https://gm3sek.com/wp-content/uploads/2019/01/G3TXQ-RC.pdf
In his 2015 article, he claims that a good common mode impedance to aim for is 4000Ω or more. In his 2012 work he also suggests operating in the CMC's area where its resistance is larger than its reactance.
First thing, let's sense check the calibration of the VNA. Usual SLOT calibration was performed. A BNC joiner is used for the 0Ω test link:
- Yellow (insertion loss, dB): 0dB
- Blue (resistance, Ω): ~0Ω
- Green (reactance, Ω): ~0Ω
All seems sensible. Now let's add the CMC into the jig:
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| Peak insertion loss is -39dB at 16.8 MHz |
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| Resonance is at 14.9 MHz, where its resistance is 8.7kΩ. |
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| On the low-end of the band, resistance > reactance, of 3.4kΩ, from 5.0 MHz. |
I neglected to check where resistance > reactance on the high-end of the band. But from the measurements taken, this CMC does seem to exhibit good common mode attenuation for at least 60m, 40m, 30m, and 20m bands.
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