The fashion for measuring HF broadband common mode chokes for antenna systems is to use the s21 series through measurement technique, the basis for which is specious as discussed elsewhere on this blog.
Let’s look at an example common mode choke, this time for suppression of ham transmitter ingress to a VDSL2 line.
The spectrum of interest is 1.8 to 10.2MHz, this is the overlap between VDSL2 spectrum and ham bands above 1MHz.
Somewhat arbitrarily, a design specification was drawn up for a prototype choke which would be tested for effectiveness. The draft specification was:
- use of standard modular telephone cable with RJ12 ends for compatibility with the standard VDSL2 line hardware;
- no significant effect on differential mode (assisted by the above); and
- |Zcm|>2000Ω from 1.8 to 10.2MHz.
A prototype ferrite cored choke was designed and fabricated.
Above is the measurement of |Zcm| of the choke. The pink area denotes the limits of the specification, the measured trace must lie above the pink are to comply.
So, the prototype choke complies with the draft specification.
We are not interested in the maximum value of |Zcm|, the draft specification is written in terms of minimum |Zcm| from 1.8 to 10.2MHz.
So, our measurement concern is not about the accuracy of the trace at 5000Ω, but whether its accuracy at up to 2000Ω is sufficient for the purpose at hand (keeping in mind the very considerable tolerance of ferrite cores).
The common obsession with measuring the maximum value of |Zcm| when the minimum value of |Zcm| in the intended operating range by be much less seems quite illogical.
Whilst this article discusses a common mode choke for a VDSL2 line, the issues are not significantly different to a choke for an antenna system and the design might be well suited to an antenna system application over the same frequency range by substituting coax for the modular cable.
Above is measurement of |Zcm| of an antenna common mode choke described in this blog. In this case measured |Zcm|>2000Ω from 2.3 to greater than 30MHz, I have no interest in the maximum value (which occurs at about 6MHz).
Of course a direct better measure is one of Icm with the balun deployed in the target antenna system.
If you are a buyer perusing graphs of |Zcm| (if you find them), don’t be seduced by maximum |Zcm| (which might not even be within a band of interest), focus on the minimum |Zcm| in bands of interest.