[TowerTalk] Measuring the resistance of ferrite

[Jim Lux]

On 4/28/15 6:56 AM, Bob Good wrote:
> Other than using the sales / technical literature,  how can I measure
> the resistance of ferritecore or beads to ensure that I have the
> recommended 5000 ohms resistance?

If you have a R/X meter or VNA or antenna analyzer or similar.. you can 
wind a few turns of wire through the core and measure the R and X vs 
frequency.

Jim's RFI-Ham pdf has some examples.

#31 is very good for lots of HF applications.  But it is not available 
in as many physical form factors (I use 2.4" diameter toroidal cores for 
everything, and they're definitely available in #31)

Every manufacturer has their own numbering/coding scheme, although a lot 
of people use Fair-Rite's numbers (73, 31, etc.) are FairRite numbers.


Of course, there's no guarantee that a #50 core from Mfr A is going to 
be the same as a #50 core from Mfr B.  If you are faced with a box of 
cores of unknown origin (e.g. no data sheet with them), then you're 
really stuck unless you just measure them.


There are an enormous number of ferrite cores out there intended for RFI 
management at VHF frequencies from the consumer electronics industry, 
and they show up surplus all the time.



  I gather that the resistance is
> frequency dependent and rises as frequency increases.  Also I gather
> that the best overall mix for HF is #73.  I assume this designation
> is universally recognized regardless of the supplier?  Can you help
> shed some light for this Old old timer?  Thanks,  Regards,  Bob K4BG
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