KB6NU's Ham Radio Blog

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Choosing a core for an RF choke

Posted: 10 Jul 2020 04:26 PM PDT
http://feedproxy.google.com/~r/kb6nu...m_medium=email


While reading about how to configure my solar system for Field Day,Â* I came
across a recommendation to use an RF filter on the input of the solar
charge controller. As you can see from the photo at right, its just an RF
choke formed by winding the input cable through the core a couple of
times.Â*I have a bunch of cores, so I figured that instead of spending $5 on
this core, Id just use one I already have.

So, I emailed the company and asked about the core. Rob, the DIYSolarforU
proprietor, who is also a ham, informed me that it was a Mix 77 ferrite
core. I looked through my box of cores and couldnt come up with a 77, but I
did have a Mix 61 core that I used.

Truthfully, I dont know if my homemade choke did all that much. Its
supposed to protect the controller from strong, nearby transmitters. Since
I was only running 5W, Im not sure it was needed at all. Certainly the
controller didnt produce a lot of RF noise. I could hear a little hash once
in a while, but it was so low that it really didnt bother me.

After Field Day, I emailed Rob and described my experience. He said that my
choice of the Mix 61 core really wasnt a very good choice. So, I did a
little googling and foundÂ*this Palomar Engineers article on ferrite mix
selection. I learned that there are actually two different types of ferrite
cores: nickel-zinc (NiZn) and manganese-zinc (MnZn). According to the
article:

The NiZn ferrite cores (mix 43, 52, 61) have low permeability, exhibit high
volume resistivity, moderate temperature stability and high ‘Q’ factors for
the 500 KHz to 100 MHz frequency range. They are well suited for low power,
high inductance resonant circuits. Their low permeability factors also make
them useful for wide band transformer applications. Nickel-zincÂ* ferrites
have a higher resistivity and are used at frequencies from 2 MHz to several
hundred megahertz.Â*The exception is common mode inductors where the
impedance ofÂ* NiZn material is recommended from 70 MHz to several hundred
GHz.

The MnZn ferrite cores (Mix 31, 73, 75, 77) have high permeabilities above
800 µ, have fairly low volume resistivity and moderate saturation flux
density. They offer high ‘Q’ factors for the 1 KHz to 1 MHz frequency
range. Cores from this group of materials are widely used for switched mode
power conversion transformers operating in the 20 KHz to 100 KHz frequency
range. These cores are also very useful for the attenuation of unwanted RF
noise signals in the frequency range of 2 MHz to 250 MHz. Manganese-zinc
ferrites are generally used in inductor applications where the operating
frequency is less than 5 MHz. The exception is common mode inductors where
the impedance of MnZn material makes it the best choice up to 10 MHz.

I dont have a Mix 77 core in my collection, so I may have to buy one.
Another thing that I might try is to make choke with some of the iron
powder cores that I have. In either case, I think testing the chokes will
be a good application for my nanoVNA.



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