Common Mode Choke :: My homepage

Common mode choke

Recently I assembled a Common mode choke from HFkits to use with my vertical antenna that, due to space restrictions, do not have radials as counterpoise. This common mode choke should cover the HF band from 3MHz to 30MHz. A common mode choke should prevent the outer conductor from radiate. And it may prevent noise picked up from the outer conductor to reach the radio. This is how the kit after I opened the package. The assembly was straight forward and well documented.

This is how the fully assembled choke looks like.

After building the kit I connected one end of it to an antenna analyzer and the other to a 50 Ohms dummy load and performed a S11 analysis of the choke. This may not be the most interesting test to perform but it should tell us something about how the choke will alter the impedance of the antenna system. Connected to a long cable the result will look different due to impedance transformation and loss. For the test I used a RigExpert AA-30.ZERO. This is how the test setup looked like.

I did not find a small dummy load so a 100W was used. The software for the analyser AntScope runs well under Wine so it was used. It can also quite easily be run directly from the serial line. In AntScope we can export the data as either CVS or a Touchstone file. We could have used a CSV file here and done all the calculations to get the return loss, VSWR, Smith chart or whatever we wanted and then plotted it. We can also import the Touchstone file with the Python package scikit-rf which can do all the calculations and plotting for us, so that is what I will do here. This is how to import the package and the Touchstone file.

import skrf as rf
cmc = rf.Network('commonmodechoke1.s1p')

Now we can easily plot the data.

cmc.plot_s_db(label='Reflection loss')
cmc.plot_s_vswr(label='VSWR')
cmc.plot_s_smith(draw_labels=True, draw_vswr=True, label='Common mode choke')

We can see that the impedance is close to 50 Ohms at lower frequencies and that as the frequency increases the impedance increases slightly due to its inductive part. That there is an inductive part should come as no surprise.