Its easy to make a common mode choke for this application. I used a ferrite core and wound as many turns as possible through it.
The core to use is the Fair-Rite Toroid Core FT240-31. like the one shown on the right (Yes I am an Amazon Affiliate). Again, making the common mode choke is not critical – other than the ferrite core material needs to be type 31. Using this type should attenuate EMI up to 400 mHz. If you are interested in the specifications of Fair-Rite’s 31 material, you can view the specifications here. Then I put it between my solar panel and the solar charge controller. Next I put a 0.1 uF capacitor from the negative side of the charge controller that was labeled Solar to earth ground.
Here is a photo showing the coil and the capacitor installed on my Windy Nation charge controller.
I also tried adding a 0.1 Uf capacitor to the other terminals on the charge controller but the only one that was effective is the one on the negative terminal of the solar panel input. As they say, your mileage may vary so its worth a try adding a capacitor on the other terminals as well. Like I stated earlier, using the AM/FM radio to see the additional capacitors will make a difference for you is a great tool.
In conclusion, I am pleased with the results of eliminating EMI from my solar PV system. My ham radio station has no RFI interference on 80 through 10 meters. I also have no interference on 2 meters.
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I will try your rfi choke for my solar charging system on my sailboat this fall.
Thanks for posting your work.